FastSync/RSYNC_COMPAT.md

Rsync Feature Compatibility

This document maps rsync's full feature set to FastSync's current implementation status.

Summary

Status	Count	Description
✅ Parity	83	Reproduces rsync's semantics for this option's scope
⚠️ Caveat	63	Fully wired and tested, but carries a documented behavioral difference from rsync (named in the row and/or the wave notes)
❌ Divergent	4	Rejected, an accepted no-op, or impossible on any portable filesystem call
Total	150	One row per rsync option/feature group; a row may name several spellings

This matrix reports honest rsync parity, not "implemented" as a synonym for "parsed". A ✅ row matches rsync for the option's scope. A ⚠️ row is real and tested but diverges in at least one documented way — FastSync's push-only model, its own wire protocol, the delete timings that approximate rsync's engine modes, the safe-subset privilege model (--super/--copy-as), the stricter xattr/ACL and temp-dir policies, and the output counters that rsync computes on the generator side. An ❌ row is either rejected (--stderr=client, --protocol with any value but the current one), an accepted no-op (-s/--secluded-args), or impossible (-N/--crtimes). The counts are derived from the rows below; update them together with the table.

Recently closed parity gaps (protocol 2.23.0). The rsync-parity wave wired up the short options -r, -b, -L, -B; rsync short-option clustering (-av, -aAX, -rlpt) and attached/inline values (--opt=value, -B1000, -essh, -MOPT); -c now implies the checksum quick-check; --checksum-choice accepts xxh64/xxhash/xxh3/xxh128/md5/auto and rejects md4/sha1/ none by name; --compress-choice accepts zstd/none/auto; --checksum-seed=0 is randomized per transfer; --skip-compress uses rsync's default suffix list; --timeout/--contimeout match rsync's defaults; deletion gained --max-delete partial semantics with exit 25; symlinks are stored verbatim; and --specials recreates sockets. Every one of those still has an entry below with its remaining caveats. See the Rsync-Parity Wave (protocol 2.23.0) section near the end for the full list and the known limitations.

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1. General Options

Flag	Rsync Description	FastSync Status	Notes
-a, --archive	Archive mode is -rlptgoD (rsync includes owner/group)	✅ Parity	Phase 7 Wave A: real rsync archive. -a/--archive now implies --links + the four per-attribute preserve flags (perms/times/owner/group) + --devices + --specials, i.e. -rlptgoD. Owner/group are implied, but their application stays privilege-gated exactly like rsync: a receiver that cannot chown logs a warning and skips it (see the preserve-attribute split note below). FastSync is always recursive, so no -r is needed. It no longer implies compression or multithreading (those moved to -z/-j). The short-option namespace is now rsync-parity (see the Phase 7 note)
-v, --verbose	Increase verbosity	✅ Parity	Sets log_level=DEBUG
-q, --quiet	Suppress non-error messages	✅ Parity	Suppresses client output while preserving errors
--help	Show help	✅ Parity	Prints usage and exits; -h is not accepted
-V, --version	Print version	✅ Parity
--info=FLAGS	Fine-grained info verbosity	⚠️ Caveat	Supports copy, misc, skip, stats, all, and none; explicit flags override --verbose, and none suppresses info output; unsupported names are rejected
--debug=FLAGS	Fine-grained debug verbosity	⚠️ Caveat	io, proto, pack, and util are supported; --debug=help lists flags; other rsync categories are rejected
--stderr=MODE	Change stderr output mode	❌ Divergent	errors (default) and all are supported; client is rejected with a clear error (--stderr=client is not supported) because FastSync has no rsync client-message channel — the rejection itself is the documented behavior (Phase 7 Wave B decision). The modes that exist work; the missing rsync channel cannot be emulated without a wire change
--no-motd	Suppress daemon MOTD	✅ Parity	Client-only display switch (Wave C): the daemon still sends the configured motd file on a host::module/path connection; the client reads and discards the frame without showing it. Without the flag the MOTD is printed to stdout after the config/auth handshake and escaped so control bytes cannot inject terminal sequences
--exclude=PATTERN	Exclude files matching pattern	✅ Parity	Glob matching in scanner
--include=PATTERN	Include files matching pattern	✅ Parity	Glob matching in scanner
-C, --cvs-exclude	Auto-ignore CVS files	✅ Parity	Applies the well-known rsync default exclude set as exclude rules during scanning (RCS SCCS CVS CVS.adm RCSLOG cvslog.* tags TAGS .make.state .nse_depinfo ~ # .#* ,* _* * *.old *.bak *.BAK *.orig .rej .del- *.a *.olb *.o *.obj *.so *.exe *.Z *.elc *.ln core .svn/ .git/ .hg/ .bzr/); .git/-style repo dirs are pruned without descending

2. Modifying Output

Flag	Rsync Description	FastSync Status	Notes
--stats	Give transfer stats	⚠️ Caveat	Prints file/byte counts. Divergence: the receiver-only counters rsync derives during its generator pass (matched/unchanged data, file-list bytes, deleted-entry count) are reported as 0 by FastSync, and the byte total counts source bytes actually sent rather than the post-delta/post-compression wire volume. Counts that FastSync can observe locally (files, bytes, timing) are accurate
-h, --human-readable	Human-readable numbers	✅ Parity	Formats transfer byte and rate counts using rsync's decimal (base-1000) units, matching rsync -h (e.g. 1.23M), not binary units
-i, --itemize-changes	Per-file change summary	✅ Parity	Prints rsync-style >f+++++++++ lines to stdout only for files actually sent (also under -j/--threads); unchanged files print nothing, matching single--i behavior
--progress	Show progress	⚠️ Caveat	Prints a periodic aggregate transfer line (bytes sent and current rate), not rsync's per-file progress block. With -P the partial-file retention behavior is fully implemented; only the progress presentation differs
-P	Same as --partial --progress	⚠️ Caveat	Phase 7 Wave B: -P parses to --partial + --progress. On a failed/interrupted write the receiver now retains the already-written temp file at the destination path (best-effort rename instead of unlink when configured), so a later --append/--append-verify run can resume it; --partial-dir still stages completed files under the confined partial dir and installs them atomically. The retention never runs when --partial is off, when no data was actually written, or under --ignore-existing/--existing (the destination is not ours to overwrite), and it only ever renames the already-written temp (never a corrupt blend; a failed rename falls back to the normal unlink). See the -S/--sparse interplay note (a retained sparse temp has full logical size)
--out-format=FORMAT	Custom output format	⚠️ Caveat	Per-transfer template on stdout; tokens %f %n %l %b %M %% (%b is the source length, always == %l; post-compression/delta wire bytes are not counted); unknown escapes preserved
--log-file=FILE	Log to file	✅ Parity	log_file config field
--log-file-format=FMT	Log format	✅ Parity	Requires --log-file; writes one template line per transferred file using the same token set as --out-format (including %b == source length)
--8-bit-output, -8	Leave high-bit chars unescaped	✅ Parity	Applies to displayed paths and protocol debug output
--list-only	List files instead of copying	✅ Parity	ls -l-style listing of files that would be transferred; scans the source only, contacts no server, writes nothing; also works with -n

3. File Selection

Flag	Rsync Description	FastSync Status	Notes
--exclude-from=FILE	Read exclude patterns from file	✅ Parity	Reads patterns from file
--include-from=FILE	Read include patterns from file	✅ Parity	Reads patterns from file
--filter=RULE	Add file-filtering rule	⚠️ Caveat	Long option only: rsync's short -f conflicts with FastSync sendfile (see FastSync-specific list), so -f is not reassigned. Supported subset: +/- include/exclude, implicit-exclude patterns, include/exclude word forms, a leading / anchor (to the transfer root, or to a .rsync-filter file's directory), and a trailing / for dir-only rules; first match wins with a default of include inside the filter layer. Filters are an independent layer from --exclude/--include (an entry must pass both). Rejected with a clear error (no silent no-ops): merge/dir-merge/hide/show/protect/risk/clear words, rules that begin with :/./! (merge/dir-merge/list-clear shorthands), and include/exclude modifiers other than / (! C s r p x)
--files-from=FILE	Read source file list from file	⚠️ Caveat	Entries are paths relative to the source root (leading ./ stripped, ../absolute entries rejected at parse time, blank lines ignored; NUL-delimited with -0). A listed regular file is transferred; a listed directory transfers its whole subtree (FastSync recursion is always on, unlike rsync's non-recursive default). Non-listed paths and their subtrees are pruned by the scanner. A listed entry that does not exist under the source (and an empty list) is a hard error reported before any transfer, unless --ignore-missing-args / --delete-missing-args is given (see the Safety & Security rows): those flags downgrade the listed-but-missing case to a skip and, for --delete-missing-args, a destination deletion; an empty list stays a hard error in every mode. Listing . (whole tree) and empty listed directories are fine. Scalability note: file_list_affects is O(list size) per scanned entry, so a very large --files-from list against a huge tree is quadratic; lists are typically small enough that this is acceptable, but it is the documented bound. Delete scoping (protocol 2.23.0): the manifest carries the set of synchronized directories, and the extras walk only visits those subtrees, so --delete with a --files-from subset no longer removes destination paths outside the listed directory subtrees (a data-loss fix matching rsync)
-0, --from0	Delimit *-from files with NULs	✅ Parity	--files-from entries become NUL-delimited; the flag may appear before or after --files-from on the command line. NUL mode preserves entry bytes exactly (trailing CR/LF are part of the name; only newline mode trims them)
--max-size=SIZE	Skip files larger than SIZE	✅ Parity	max_size in scanner
--min-size=SIZE	Skip files smaller than SIZE	✅ Parity	min_size in scanner
-I, --ignore-times	Don't skip files matching size+time	✅ Parity	ignore_times config field (crosses the wire). Disables the size+mtime quick-check in the --incremental per-file handshake and the basis-dir quick-match, forcing the file to be transferred rather than skipped as unchanged. Receiver-side policy: match_by_metadata (file_receive.c) is bypassed, so the receiver never replies STATUS_OK for a matching size+mtime. Requires --incremental to have the handshake to act on (rsync does its quick check by default; FastSync's -I/--size-only/--modify-window only take effect under --incremental, exactly like they take effect through the basis check)
--size-only	Skip based on size only	✅ Parity	With --incremental, ignores mtime
-@, --modify-window=NUM	Mod-time comparison accuracy	✅ Parity	Whole-second tolerance with nanosecond-aware comparisons
--existing	Skip creating new files on receiver	✅ Parity	Existing destination files continue through normal update handling
--ignore-existing	Skip updating existing files	⚠️ Caveat	ignore_existing config field (crosses the wire; receiver-side policy). For a destination entry that already exists, the receiver skips the write: in the regular-file path, existing/delay-updates-staged, hardlink-sibling, and special/device handlers all return FILE_SAVE_SKIPPED without overwriting (passed as no_replace to the write engine), and --backup is disabled for skipped files. Note: it is applied at write time, so an existing dest whose size+mtime differ still has its data (or delta) transmitted before the write is discarded — functionally correct, bandwidth-suboptimal vs rsync, which short-circuits earlier. Like rsync, it does not apply to directories/symlinks (those return before the block). Combines with -j/--threads and --delay-updates. See Phase-4/— notes below
--remove-source-files	Sender removes regular files after confirmed transfer	✅ Parity
-x, --one-file-system	Do not cross filesystem boundaries	✅ Parity	Sender scanner captures the root device and does not descend into mount-point crossings (st_dev differs). Protocol 2.23.0 matches rsync's entry emission: the mount-point directory itself is emitted as a payload-less directory entry (so the destination gets an empty directory) while its contents are skipped; previously the crossing subdirectory was dropped entirely
-F	Add the default .rsync-filter rules	⚠️ Caveat	Reads one filter rule per line from each directory's .rsync-filter file during traversal and applies it to that directory's subtree; the current directory's rules are evaluated before its ancestors', so deeper files override shallower ones and per-directory files override the command-line --filter/-C base by default (matching rsync's first-match-wins precedence); .rsync-filter files are never transferred. The rsync -FF behavior (also .cvsignore) is out of scope; unsupported rule types inside the file abort with a clear error

4. Directory Options

Flag	Rsync Description	FastSync Status	Notes
-r, --recursive	Recurse into directories	✅ Parity	Default behavior
-R, --relative	Use relative path names	⚠️ Caveat	Meaningful together with --files-from (FastSync's default full-tree scan always mirrors the full source argument path below the destination root, so -R does not change it). With -R + --files-from each listed entry is transmitted under its bare relative destination path: an entry sub/x.txt lands at <dest>/sub/x.txt (its leading components preserved) instead of under the <dest>/<full source path> mirror. Only the path sent on the wire changes; the client still reads the absolute source path, and the delete manifest derives from the sent (relative) paths so --delete and --remove-source-files stay consistent in both layouts. Works single-threaded and under -j/--threads (including chunk serialization)
--no-implied-dirs	Don't send implied dirs with -R	⚠️ Caveat	Client-side, meaningful only with -R + --files-from. rsync would normally create the ancestor directories implied by a listed file so it can be written; with --no-implied-dirs a listed file whose parent directory is not itself (or via an ancestor) explicitly listed cannot be placed, and FastSync fails the whole run up front with a clear error (--no-implied-dirs: cannot place file '...': parent directory '...' is not explicitly listed). Listing the directory (or an ancestor of it, or the whole tree .) permits the file. In every other mode the option has no effect. FastSync has no per-entry skip channel, so the rsync "omit the file" case is surfaced as a hard pre-transfer error
-d, --dirs, --old-dirs, --old-d	Transfer dirs without recursing	⚠️ Caveat	-d <dir> transmits an explicit directory entry for the source-root directory, so the destination mirror is created empty and nothing is descended into. With --files-from exactly the listed items are transferred: a listed directory is created empty (no descent) and a listed file is transferred with its content; the dest layout follows the same -R rules as plain files. A new wire frame (STATUS_MKDIR) carries each directory entry — the path and, when --preserve/-a (metadata mode) is negotiated, the directory's metadata; the receiver creates it with the same confined mkdir-parent semantics as regular writes, in single-threaded and -j/--threads receivers (chunk serialization carries a per-entry type marker). Directory entries appear in the delete manifest so --delete prunes correctly. Directory TIMES are transmitted (the STATUS_DIR_TIMES frame carries every traversed source directory's captured times, including --dirs entries) and applied by the receiver at the END of the transfer, after all children and the delete/publication phases, so a later child write cannot clobber a directory's mtime (-O/--omit-dir-times skips this application). FastSync divergences: directory modes/ownership are still not applied (only times are), and empty directories are still never created (a STATUS_DIR_TIMES entry is record-only), filter/--exclude rules are not re-applied to the listed dirs mode (there is no descent during which they would apply), and -d never creates the intermediate directories between the destination root and a listed file beyond the usual on-demand parent creation. Under --delay-updates only regular files are staged: directory entries are created immediately, so a delayed run that fails part way can leave the already-created empty directories behind (matching rsync, which also creates directories as it processes the file list and only delays regular-file data)
--mkpath	Create missing path components	✅ Parity	Wire option (client → server). At connection start the server creates the client's destination root directory (and any missing leading components below its own authorized root) when --mkpath is set, failing the connection cleanly if it cannot. Without --mkpath a destination root that does not exist yet is rejected up front (rsync semantics), so the flag is the only way to transfer into a not-yet-created destination directory. Creation is confined by the same secure mkdir walk as file writes (O_NOFOLLOW, no ..)

5. Transfer Modifications

Flag	Rsync Description	FastSync Status	Notes
-u, --update	Skip files newer on receiver	✅ Parity	update config field (crosses the wire; receiver-side policy, implies metadata transmission). Before writing a regular file, the receiver checks file_destination_is_newer_secure() (via stat_is_newer, second-then-nanosecond strict > on the existing destination) and skips the write when the destination is newer than the source (FILE_SAVE_SKIPPED); equal-or-older destination (or a newer source) is transferred normally. Applied at write time on the regular-file, delay-updates-staged, hardlink-sibling, and special/device paths. Only regular destinations can be guarded (the newer-check requires S_ISREG), and like the other write-time policies it does not short-circuit the data transfer for a differing-size dest. --remove-source-files correctly respects the receiver's skip outcome so a skipped source is not removed
--inplace	Update files in-place	✅ Parity	Direct write mode
--append	Append data to shorter files	⚠️ Caveat	Tail-only resume. When an existing destination file is SHORTER than the source, the receiver negotiates a resume offset with the sender and only the tail is transferred; the receiver rebuilds the full file (retained prefix + tail) and installs it through the normal atomic store path, so the result is byte-identical to the source whenever the retained prefix matches. Plain --append does NOT content-verify that prefix (rsync parity): a destination whose prefix differs from the source is resumed anyway, so the result (wrong prefix + correct tail) is NOT byte-identical and the file is effectively left corrupt — the documented rsync-parity risk (use --append-verify when the prefix cannot be trusted). Non-content attributes (permissions/ownership/mtime, via -M) are still applied. Requires the per-file STATUS_CHECK handshake, so it implies --incremental; it takes precedence over block delta for a growing file and falls back to delta/full when the destination is not shorter. Incompatible with -s (chunk serialization) and --whole-file (both rejected up front so the mode never silently degrades to a full transfer). Combines with --inplace, --partial/--partial-dir, and --delay-updates (the reconstructed full file flows through those paths unchanged). Divergence: rsync appends in place; FastSync reconstructs and atomically installs, so an interrupted or failed resume never leaves a half-written file at the destination (no corruption window), and --append is thus safe to use with the normal atomic path — not only with in-place writes
--append-verify	Append with old-data checksum	⚠️ Caveat	Like --append, but the retained prefix IS verified before resuming: the sender transmits the source prefix checksum and the receiver compares it to the xxHash64 of the retained destination prefix; on a match only the tail is transferred, on a MISMATCH the run falls back to a clean full transfer so the result is always a byte-identical source copy (never a corrupt prefix+tail blend). Wire/protocol: the append handshake adds STATUS_APPEND / STATUS_APPEND_SIG / STATUS_APPEND_OK / STATUS_APPEND_DATA frames and PROTOCOL_VERSION was bumped 2.9.0 → 2.10.0 (peers must match, and both must be 2.10.0 or the run fails the version check). Same implications/incompatibilities as --append; when both spellings are given --append-verify wins (the safer semantics). See the Phase-3 append notes below
-W, --whole-file	Copy whole file (no delta)	✅ Parity	whole_file config field. Forces a full (whole-file) copy, disabling the block-level delta machinery: the sender only sends STATUS_NEXT + full data (client_send.c) and the receiver never requests a delta signature/reconstruction — the receiver's try_delta = use_delta && !whole_file && ... short-circuits. whole_file crosses the wire folded into use_delta (the wire carries use_delta && !whole_file), so no separate field/bump is needed. Delta is opt-in (--delta needs --incremental); -W additionally makes --fuzzy inert (no similar-file delta basis). --append/--append-verify are incompatible with -W and rejected up front (both sides). See the delta/append notes below
--block-size=SIZE	Force checksum block-size	✅ Parity	Phase 7 Wave B: --block-size is an alias for --delta-block; both set config->delta_block_size (default DELTA_BLOCK_SIZE_DEFAULT, bounds DELTA_BLOCK_SIZE_MIN..MAX, out-of-range values are rejected with the default kept). The value is genuinely honored by the delta engine end-to-end: delta_signature_create_seeded(old, size, config->delta_block_size, seed) on the sender and receiver, delta_apply(old, ...) with the same size, so a non-default block size changes the block count of every signature the harnesses exchange (verified by unit + integration tests)

6. Destination Handling

Flag	Rsync Description	FastSync Status	Notes
-n, --dry-run	Trial run with no changes	⚠️ Caveat	Server-contacting since protocol 2.21.0. The final routing predicate is dry_run_targets_server() in src/client/client_send.c: any target a real run would reach over the wire selects the server-contacting path — an SSH transport, a daemon host::module destination, an explicit --server-host or --server-port/--port, TLS, or a source-bind --address — and the client handshakes with the receiver, which runs the normal read-only per-file check and answers STATUS_DRY_RUN_TRANSFER/STATUS_OK without mutating anything. A plain local destination (none of those) keeps the original client-side manifest that never dials the default 127.0.0.1:8080. Would-delete reporting for --delete* is deferred (dry-run never deletes).
-b, --backup	Make backups of overwritten files	✅ Parity	Backup before overwrite
--backup-dir=DIR	Backup directory hierarchy	✅ Parity	backup_dir config field
--suffix=SUFFIX	Backup suffix (default ~)	✅ Parity	suffix config field
--delay-updates	Put updated files in place at end	⚠️ Caveat	Successfully received files are staged under a private 0700 .fastsync-stage dir inside the receive root and atomically renamed into their final destinations only after the whole transfer (manifest/delete handling included) succeeds, just before the success/outcome frame is sent. The delete walker deliberately skips the staging dir at the receive root, so --delete removes genuine extras but never the staged files (deletion runs before publication; rsync's delete-after ordering is not implemented). --existing/--ignore-existing/--update decide against the final destination path at stage time; --backup moves the old file aside at publication, and --force is honored at publication (protocol 2.23.0): a staged regular file or symlink may replace a destination directory that blocks it. Incompatible with --inplace and with --backup-dir=.fastsync-stage (the internal staging name is reserved; both are rejected). The staging dir name is fixed, so two simultaneous delayed transfers to the same destination root are serialized with an exclusive advisory lock held for the whole transfer: the second session fails cleanly instead of corrupting the first. Aborting or failing before publication installs nothing and removes the staging tree; a crash between stage and publish leaves staged leftovers that the next delayed run wipes at start (process death releases the lock). A stage→publish failure aborts the transfer (best-effort cleanup of the not-yet-published staged files; already-published files are not rolled back). Works in single-threaded and -j/--threads modes
-T, --temp-dir=DIR	Create temporary files in DIR	⚠️ Caveat	--temp-dir with the rsync short -T (the timeout alias moved to long-only --timeout). Protocol 2.23.0 receiver policy: the scratch dir is confined to the receive root — a relative dir is resolved below it, and an absolute path or one containing .. is rejected by the receiver (an absolute/foreign-filesystem scratch dir was the divergence; rsync's standalone mode would follow an absolute --temp-dir, while its daemon also confines). Temp copies use a unique name there and are atomically renamed into place. On EXDEV (scratch dir and destination on different filesystems) the receiver falls back to a non-atomic copy instead of aborting the transfer, matching rsync. --inplace and --partial-dir writes bypass the scratch dir

7. Deletion

Flag	Rsync Description	FastSync Status	Notes
--delete	Delete extraneous files from dest	⚠️ Caveat	use_delete config field. Deletion is always derived from the transmitted keep-set manifest of the paths the sender sent/keeps (never from unchecked input), runs through the symlink-safe walker bounded by MAX_SERVER_DELETE_COUNT, and skips the .fastsync-stage staging dir under --delay-updates. FastSync's default timing when no timing flag is given is delete-after (extras are removed only once the whole transfer succeeded) — intentionally NOT rsync's --del/delete-during default, to preserve FastSync's commit-style safety. By default the destination mirror of a path the source scan pruned (filter/exclude/size rules) is protected from deletion — matching rsync, which does not delete excluded files under --delete; --delete-excluded opts back into deleting them (see below). Deletion is scoped to the synchronized directories sent in the manifest (protocol 2.23.0), so a --files-from subset no longer deletes untransmitted paths outside the listed directory subtrees. The walk is bounded: a client --max-delete=NUM (or the 100000-entry server bound) makes it partial — entries up to the bound are removed, the rest are skipped, and the client exits 25 (RERR_PARTIAL), matching rsync, rather than failing the transfer. Extraneous destination symlinks are unlinked by name (never followed); a directory still holding a kept/protected entry is left behind rather than failing
--delete-before	Delete before transfer	⚠️ Caveat	Implies --delete. The sender runs a full source pre-scan (paths only) and transmits the keep-set manifest BEFORE any file data; the receiver validates it, removes every destination entry not listed (bounded walk, staging-dir skip, protected prefixes honored), then acks STATUS_OK. The sender only starts streaming after the deletion committed, or aborts if the receiver reported a deletion error. By definition the deletions already happened when a later transfer phase fails — rsync's delete-before is destructive the same way; a subsequent failure does not restore the removed files. Divergence: the keep-set is the pre-scan snapshot, so a file that appears on the source between the pre-scan and the data pass is still transferred but was not protected from deletion
--del, --delete-during	Delete during transfer	⚠️ Caveat	Both spellings accepted; imply --delete. FastSync streams the source in a single directory scan and has no per-directory generator pass, so deletions cannot be interleaved per-directory the way rsync's delete-during does. --delete-during therefore selects the same early engine mode as --delete-before (manifest transmitted before any data, extras removed and acknowledged before data is applied); observable success/failure behaviour equals --delete-before. That is the documented divergence from rsync, where --del is the default meaning of --delete
--delete-delay	Find deletions during, delete after	⚠️ Caveat	Implies --delete. Commit-mode timing: extras are removed only after the whole transfer succeeded. rsync's delete-delay records the deletion list during its scan and applies it at the end; FastSync never snapshots the destination while data flows (the keep-set is the transmitted manifest and the destination is listed only at deletion time), so --delete-delay is implemented as the same end-of-transfer commit as --delete-after with identical safety. That is the documented divergence
--delete-after	Delete after transfer	✅ Parity	Implies --delete. The delete-after timing is also what plain --delete does: the keep-set manifest closes the data stream and the receiver commits the bounded deletion only after the terminal STATUS_FINISHED proves the whole transfer (every data frame received and stored) succeeded. A failed or aborted transfer removes nothing
--delete-excluded	Also delete excluded files	⚠️ Caveat	delete_excluded config field. Under --delete FastSync protects (rsync's default) the destination mirror of paths the sender's source scan pruned by the user-selection rules — the --filter/-F/-C layer and the legacy --exclude/--include layer. The sender transmits those concrete pruned paths as protected prefixes in the delete-manifest frame (see the Phase-3 notes below); the walker never descends into or removes them. --delete-excluded opts back in: the sender sends an empty protected list, so the excluded destination mirrors become ordinary extras and are removed. --max-size/--min-size pruned mirrors are a separate, always-on protection (protocol 2.23.0, rsync parity): size-pruned source mirrors survive --delete even with --delete-excluded. Divergences (documented): protection is derived only from what the source scan actually pruned — a stray destination-only file that happens to match an exclude rule is not protected (FastSync never re-applies rules to the destination, keeping deletion sender-derived)
--max-delete=NUM	Max files to delete	✅ Parity	max_delete config field (default -1 = no client limit; 0 = delete nothing). Protocol 2.23.0 matches rsync's partial semantics: the receiver deletes up to NUM entries (regular files, symlinks and empty directories; each directory removal counts as one) and then stops deleting, skips the rest, and reports the run as partial. The client prints a "deletions stopped due to --max-delete limit" message and exits 25 (rsync's RERR_PARTIAL), not a hard failure — the transfer itself succeeded. NUM only applies together with --delete (it is inert otherwise, matching rsync). A client NUM below the server hard bound MAX_SERVER_DELETE_COUNT (100000) replaces it; a NUM above it never raises that cap. Deleting an entire destination with no limit is still bounded by the server's 100000-entry ceiling. --delete-missing-args exact-path deletions and the ordinary extras walk draw from the same budget, matching rsync
--ignore-errors	Delete even with I/O errors	⚠️ Caveat	Sender-side, client-only config field. rsync suppresses --delete when the transfer had I/O errors; FastSync's equivalent is a source-scan I/O error (an unreadable directory, e.g. EACCES): by default the scan aborts the run so no deletion happens. With --ignore-errors the scan continues past the unreadable directory, the readable tree is transferred and the deletion still runs (the mirror of the unreadable directory is treated as an extra). The run still exits non-zero (the error is reported, matching rsync's error status). Divergence: without the flag FastSync aborts the whole run on the scan error, whereas rsync transfers the rest of the tree and merely skips the deletion; both leave the deletion undone
--force	Force deletion of non-empty dirs	⚠️ Caveat	force_delete receiver config field (crosses the wire). rsync's --force lets an incoming non-directory replace a destination directory; FastSync implements exactly that: when a regular file (or symlink) is written to a path that is currently a (possibly non-empty) destination directory, --force removes that directory tree first — confined to the receive root and symlink-safe (O_NOFOLLOW fd walk, symlinks removed by name, never followed) — so the install can place the file. Protocol 2.23.0 honors --force on the --delay-updates publication path too, not only the immediate-install path. Without --force such a write fails and the run aborts. Gated by the server --allow-delete policy (a client cannot use --force to remove a destination tree on a server that forbids deletion)
-m, --prune-empty-dirs	Prune empty dir chains	✅ Parity	-m/--prune-empty-dirs (Phase 7 Wave A freed the rsync short -m; FastSync multithreading is now -j/--threads). FastSync's recursive transfer records directory times but never CREATES an empty directory (a STATUS_DIR_TIMES entry is record-only, and --dirs empty entries are pruned by this flag), so empty directories are inherently never transferred (which is rsync's -m behavior) and truly-empty destination directory chains are removed by --delete regardless of this flag. The flag's additional real effect is on the --dirs explicit directory-entry generator: a plain -d <empty-dir> run omits the empty source directory's entry, so nothing is created at the destination (no STATUS_MKDIR, no -i/--out-format change line, and an existing empty mirror becomes an extra that --delete prunes). Explicitly --files-from-listed directories always pass through (documented --files-from behavior). A directory that still holds an excluded-but-protected file survives, matching the --delete-excluded default

Deletion-timing implementation notes (Phase 3): the delete flags above are real. Two new config booleans (delete_during, delete_delay) join the already serialized delete_before/delete_after, so the on-the-wire config layout changed and PROTOCOL_VERSION was bumped 2.7.0 → 2.8.0 (peers must match). The STATUS_MANIFEST frame is count-delimited and position-independent: the receiver commits the deletion either when the manifest arrives (early modes: --delete-before/--delete-during, which additionally acknowledge with STATUS_OK before data flows) or after the terminal STATUS_FINISHED proves the whole transfer succeeded (commit modes: plain --delete/--delete-after/ --delete-delay). Timing is chosen purely from the config, so server policy (--allow-delete off) still disables deletion without deadlocking the early manifest ack. --delete-delay and --delete-during are each implemented as the closest safe approximation their engine mode allows; the divergences are noted in the rows above.

Deletion-policy notes (Phase 3, delete-policy wave): this wave made the deletion family real — --delete-excluded, --max-delete, --ignore-errors, --force, --prune-empty-dirs — and, to support them, the STATUS_MANIFEST frame carries the keep-set paths followed by a list of protected prefixes (destination-relative paths the source scan pruned by user-selection rules, which the walker must never delete unless --delete-excluded opted out). Two config booleans were added for the wave: force_delete (crosses the wire; the receiver clears a directory that blocks an incoming file) and ignore_errors (client-only; the sender's scan continues past an unreadable directory). max_delete's default became -1 ("no client limit"). These wire/layout changes bumped PROTOCOL_VERSION 2.8.0 → 2.9.0 (peers must match). All four wire additions — force_delete, delete_excluded, prune_empty_dirs, max_delete — round-trip unchanged and are validated on receive.

Missing-args note (Phase 3, missing-args wave; extended in 2.23.0): --ignore-missing-args and --delete-missing-args are implemented as described in the Safety & Security rows. Wire impact: the STATUS_MANIFEST frame carries a list of destination-relative exact-delete paths (the missing entries' mirrors), and the config frame gained a delete_missing_args boolean (ignore_missing_args stays client-only, exactly like ignore_errors). These wire/layout changes bumped PROTOCOL_VERSION 2.9.0 → 2.10.0 (peers must match). The receiver validates the section identically to the keep-set (non-empty, relative, traversal-free; the shared MAX_MANIFEST_ENTRIES/MAX_MANIFEST_BYTES budget spans every section). On commit the receiver runs the exact-path deletions FIRST (manifest_delete_missing_args: confined per-path unlink/rmdir, deep removal only under --force/--delete, staging/basis protected, never blocked by the protected-prefix list) and then the ordinary extras walk when --delete is active (manifest_delete_all). A client may request the exact-path deletions without --delete; the server's --allow-delete policy gates them exactly like --delete, so an unauthorized server ignores the request while the missing entries are still skipped.

Delete scoping and partial limits (protocol 2.23.0). The STATUS_MANIFEST frame now carries four sections — keep-set, protected prefixes, exact-delete (missing-args) paths, and the set of synchronized directories. The extras walk is scoped to the synchronized directories, so a --files-from subset no longer deletes untransmitted destination paths outside the listed directory subtrees (a data-loss fix, matching rsync). --max-size/--min-size pruned source mirrors are protected independently of --delete-excluded. Extraneous destination symlinks are unlinked by name (never followed). The --max-delete budget is partial: the walker deletes up to the effective bound (a client --max-delete=NUM below the hard bound, else the hard MAX_SERVER_DELETE_COUNT = 100000) and then stops, skips the rest, and reports the run as partial so the client exits 25 (RERR_PARTIAL) exactly like rsync — it is a successful transfer with an incomplete deletion, not a hard failure. The exact-path missing-args removals and the extras walk share that one budget. A directory that still holds entries the walker leaves in place (a protected excluded file, a kept manifest entry, a symlink) is left behind rather than failing the run — matching rsync's "cannot delete non-empty directory" behaviour.

Manifest size: the sender's collections (streaming or early pre-scan) are unbounded, but the receiver rejects a manifest whose aggregate count exceeds MAX_MANIFEST_ENTRIES (1 048 576 entries across ALL sections) or whose aggregate path bytes exceed MAX_MANIFEST_BYTES (16 MB across all sections) as a hard protocol error. A heavily filtered source whose exclusion list grows large thus fails the run cleanly on the receiver (STATUS_ERROR) instead of being silently truncated. In the commit modes this only means the deletion is refused after the data already arrived; in the early modes (--delete-before/--delete-during) the manifest is the first frame, so an oversized manifest aborts the whole transfer BEFORE any data is sent. Keep the source tree small enough for the receiver's manifest caps when using the early timing.

Early-delete ACK wait: after committing a large deletion (up to MAX_SERVER_DELETE_COUNT removals) the receiver's STATUS_OK/STATUS_ERROR reply can legitimately take much longer than a normal round trip, so the sender waits for that single ACK with an extended explicit deadline (1 hour) instead of the default 60 s per-message receive window. A receiver that is genuinely gone still aborts the wait via connection close/error; the extended bound only protects against aborting after the deletion already committed on the receiver.

Flag-conflict policy: unlike rsync's last-one-wins behaviour, every deletion timing flag implies --delete, and combining a timing flag with --no-delete (in either argument order) — or more than one timing flag — is rejected as a configuration error rather than silently resolved. Note the check is order-independent because it runs over the fully parsed config. The deletion POLICY flags (--delete-excluded, --max-delete, --ignore-errors, --force) do NOT imply --delete; without --delete they are inert (matching rsync).

Append-resume notes (Phase 3, append wave): --append and --append-verify are real. Both are negotiated when an existing destination file is found to be shorter than the source during the per-file STATUS_CHECK; the receiver replies with a new STATUS_APPEND frame carrying the resume offset (the prefix length it already holds) instead of STATUS_NEXT/STATUS_DELTA_SIGNATURE. The sender transmits ONLY the tail. For --append-verify it first sends the source's prefix xxHash64 in a STATUS_APPEND_SIG frame; the receiver compares it to the retained prefix and answers STATUS_APPEND_OK (transfer the tail) or STATUS_NEXT (prefix mismatch → the sender falls back to a byte-exact full transfer). The tail arrives in a STATUS_APPEND_DATA frame (compression and metadata still apply). The receiver then rebuilds the full file in memory (prefix + tail) and routes it through the existing atomic store engine, so all of --inplace, --partial/--partial-dir, --delay-updates, --backup, --existing/--ignore-existing/--update and delete-manifest behaviour is unchanged and the result is a byte-identical source copy (given a matching prefix). These new frames changed the wire, so PROTOCOL_VERSION was bumped 2.9.0 → 2.10.0 (peers must match; the pre-existing append/append_verify config booleans already crossed the wire). CLI: both flags imply --incremental (the handshake needs it); they are incompatible with -s (chunk serialization) and --whole-file (both rejected up front, never a silent full transfer); when both spellings are given --append-verify wins. The FastSync divergence from rsync is intentional and safer: rsync appends in place, whereas FastSync reconstructs the whole file and atomically installs it, so an interrupted or failed resume never leaves a partial/corrupt file at the destination — this is why plain --append works on the normal atomic path, not only with --inplace.

8. Metadata Preservation

Flag	Rsync Description	FastSync Status	Notes
--preserve	(FastSync alias, not an rsync flag)	✅ Parity	FastSync-only alias for -p + -t (mode + mtime), long-form only. It is not rsync's --preserve (rsync has no such option); the short -M that used to spell it is now rsync's --remote-option. The wire metadata also carries uid/gid for -o/-g/-a, and ownership is applied via -o/-g, -a, or an explicit identity flag (--numeric-ids/--usermap/--groupmap/--chown/--copy-as)
-p, --perms	Preserve permissions	✅ Parity	Real per-attribute flag (protocol 2.22.0): preserve_perms applies the source mode independently of times/owner/group. Strict rsync parity (protocol 2.23.0): the source mode is copied exactly, including setuid/setgid/sticky and group/other-write bits — there is no masking. Without -p, a new file gets source_mode & ~umask when metadata is present (else the historical fixed 0644); new directories without -p still use FastSync's 0755 creation default, because directory metadata is only applied when a directory attribute is requested. -A/--acls implies -p; --chmod does not imply -p (rsync parity) and applies its own unsanitized changes to the new mode. -X/--xattrs does not imply -p. The SSH port moved to --ssh-port. rsync-parity short form
-o, --owner	Preserve owner	✅ Parity	Real per-attribute flag (preserve_owner): preserve the source uid, resolved on the receiver by name against its own user database with a raw-numeric fallback (only numeric ids cross the wire). --usermap/--chown=USER imply it. Application follows the --super/--no-super policy; a non-opted daemon module applies no ownership (see the Daemon Mode notes)
-g, --group	Preserve group	✅ Parity	Real per-attribute flag (preserve_group): preserve the source gid, resolved by name on the receiver with a raw-numeric fallback. --groupmap/--chown=:GROUP imply it. Same privilege/super-policy gating as -o
-t, --times	Preserve modification times	✅ Parity	Real per-attribute flag (preserve_times): apply the source mtime independently of the other attributes. -O/--omit-dir-times suppresses directories only and -J/--omit-link-times suppresses symlinks only; -U/-N do not imply it. --preserve/-a imply it, and --incremental/--delta auto-enable it unless --no-times/--no-preserve
-E, --executability	Preserve executability	✅ Parity	Preserves executable permission bits (implies metadata preservation)
--chmod=CHMOD	Affect file permissions	✅ Parity	Faithful port of rsync 3.4.1's parse_chmod/tweak_mode: numeric octal and symbolic ugo/rwx changes, D/F directory/file selectors, X (execute only on directories or already-executable files), s/t setuid/setgid/sticky, and append semantics — repeated clauses and repeated --chmod options accumulate in order (joined with commas). The changes are applied to the new mode without sanitization (matching rsync) and --chmod does not imply -p (rsync parity). Applied to files and directories on the receiver
-A, --acls	Preserve ACLs	⚠️ Caveat	Implemented on Linux via the POSIX-ACL xattr representation: the sender captures the system.posix_acl_access / system.posix_acl_default xattrs into the same bounded whitelisted set as -X, transmits them per-file, and the receiver re-applies them fd-relative. Setting an ACL the receiver is not permitted to set (non-root on a file it does not own, unsupported filesystem) is logged and skipped, never fatal. libacl is not required. Only the system.posix_acl_* namespaces plus user.* are ever applied; privileged namespaces are never applied (see the Phase-4 xattr/ACL notes below). Implies metadata transmission
-X, --xattrs	Preserve extended attributes	⚠️ Caveat	Preserves unprivileged user.* extended attributes (Linux listxattr/getxattr on capture, fsetxattr on the written destination fd). Both capture (sender) and application (receiver) are restricted to the user.* namespace and the two POSIX ACL xattrs, so a client can never force a security.*/trusted.*/privileged attribute onto the destination; the receiver independently re-validates every incoming name against this whitelist and rejects anything else. Payloads are bounded (per-name ≤255B, per-value ≤1MiB, per-file count ≤256 total bytes ≤4MiB) on both ends, and an oversized/malformed frame is a clean protocol rejection (no OOM). Applied fd-relative to the exact written file. Implies metadata transmission. Incompatible with -s (chunk serialization), rejected up front (see the notes); a --link-dest/-H hard-link copy fallback re-applies the attributes so they are not dropped when a link is refused
-H, --hard-links	Preserve hard links	✅ Parity	Files on the source that share an inode (st_dev+st_ino, e.g. a cp -al tree) are re-created as hard links to one another on the destination, so duplicate links stay deduplicated and only the first member's data is sent (later members are transmitted as payload-less STATUS_HARDLINK frames). The receiver links each sibling to the first member's installed file with an atomic link + rename; on link() failure it falls back to a byte-identical local copy of the first member, never a partial/corrupt file. Requires the sequential scan for ordering (the first member is always emitted and installed before any sibling is linked). Works single-threaded and under -j/--threads, --inplace, --delay-updates (links staged and published by rename) and --partial. Crosses the wire (preserve_hard_links bool; PROTOCOL_VERSION bumped 2.11.0 → 2.12.0, peers must match). Incompatible with -s (chunk serialization) and --append/--append-verify, rejected up front with a distinct error. See the Phase-4 hard-links notes below
-D	Same as --devices --specials	✅ Parity	Implies --devices --specials. -D was unassigned in FastSync (verified: no collision), so it is free to imply both device-node and special-file preservation. As of protocol 2.23.0 --specials genuinely covers both FIFOs and unix sockets, so -D covers the full rsync set. See the --devices/--specials rows and the Phase-4 devices notes below
--devices	Preserve device files	⚠️ Caveat	Recreates char/block device nodes on the destination via mknod instead of transferring content. Type + rdev are validated strictly (S_IFMT from the transmitted mode; major/minor range-checked, non-negative), and creation is privilege-gated: mknod needs CAP_MKNOD, so a non-root receiver (CI runs via setpriv as non-root) logs a warning and skips the device entry safely — the whole transfer never aborts just because the node could not be made. The node is created fd-relative below the receive root (mknodat on the confined secure parent), so it can never be placed outside the authorized root, never follows a symlink, and never replaces an existing directory. Only a char/block mode is honored. Crosses the wire (a STATUS_SPECIAL frame carries the path + metadata mode + rdev). Divergence: per-entry skip (not a hard error) when the receiver lacks CAP_MKNOD, documented in the Phase-4 devices notes
--specials	Preserve special files	✅ Parity	FIFO and unix-socket recreation work (protocol 2.23.0): FIFOs are recreated with mkfifoat, and sockets with mknodat(..., S_IFSOCK) — the latter is unprivileged on Linux because it materializes the socket node, not a live bound socket, so it is a real, assertable behavior under CI (it matches rsync, which also recreates a socket by mknod). Node creation is confined below the receive root (fd-relative parent; no .., no symlink follow) and type/rdev are validated strictly; a matching existing node is left in place and an unrelated entry is never replaced. Crosses the wire like --devices (the STATUS_SPECIAL frame). See the Phase-4 devices notes
--copy-devices	Copy device contents as file	⚠️ Caveat	Copy a device's CONTENT into an ordinary regular file on the destination instead of recreating the node — non-privileged and safe. FastSync scans a device/FIFO as a regular file: its reported size (st_size, typically 0 for char devices and FIFOs) is copied, so a FIFO or a non-readable device becomes an empty (or size-bounded) regular file. The default data path is size-bounded and never blocks (it sends exactly st_size bytes, never an unbounded pseudo-device stream); with --sendfile, a non-regular source (FIFO/device) is detected from its stat mode and falls back to that same buffered read, so --copy-devices --sendfile cannot hang either. The run always succeeds and never crashes on such input. Deliberate, safe divergence from rsync's dd-like unbounded device read. See the Phase-4 devices notes
--write-devices	Write to devices as files	⚠️ Caveat	Write the received data directly into an existing device node on the destination instead of creating a regular file. Restricted and best-effort: the destination must already exist and be a char/block device (opened only under the confined receive root, with O_NOFOLLOW + O_NONBLOCK); a missing, symlinked, FIFO-with-no-reader (ENXIO), non-device destination, or any write failure is skipped with a warning rather than allowed, so a run can never clobber the system, never blocks on a special-file target, and never aborts on an unusable target. See the Phase-4 devices notes
-U, --atimes	Preserve access times	✅ Parity	Captures the source access time (from the scanner's pre-read stat, so it is not clobbered by reading the file for transfer) and transmits it over the wire; the receiver restores it together with the mtime via futimens/utimensat. Implies metadata transmission (the times travel inside the shared metadata payload), but does not enable ownership application (that stays opt-in via the identity flags). Wire: atime fields on the metadata frame + a preserve_atimes config boolean; PROTOCOL_VERSION bumped 2.11.0 → 2.12.0
-N, --crtimes	Preserve create times	❌ Divergent	Birth-times cannot be set by any portable filesystem call (utimensat/futimens only set atime/mtime), so this row is an explicit Divergent entry (Phase 7 Wave B). Capture + transmit stays: statx(STATX_BTIME) on Linux records the source birth time as a wire field; the receiver logs a debug note that it cannot be applied and continues — never failing the transfer and never pretending it worked. On platforms without statx it parses as a documented no-op (flag accepted; nothing is captured). Implies metadata transmission. Wire: new crtime fields + a preserve_crtimes config boolean; PROTOCOL_VERSION bumped 2.11.0 → 2.12.0 (see the Phase-4 metadata-time notes)
-O, --omit-dir-times	Omit dirs from --times	✅ Parity	Real modifier now that FastSync preserves directory times. With metadata on, the scanner captures every traversed source directory's mtime (and atime under -U) and the sender transmits them in trailing STATUS_DIR_TIMES frame(s) after all file data and the optional delete manifest (chunked at the receiver's MAX_MANIFEST_ENTRIES per-frame cap); a dir-time entry only RECORDS metadata and never creates the directory, so empty source directories stay untransferred. The receiver defers applying them until its delete / --delay-updates publication phases have committed, so writing or removing a child never clobbers a parent directory's mtime (rsync applies directory times at the end for exactly this reason). When -O is set (the boolean crosses the wire) the receiver does not apply any of them; without -O an -a/--preserve transfer now restores directory times (reversing the old "never preserves dir times" divergence). Wire change: the terminal STATUS_DIR_TIMES frame; PROTOCOL_VERSION bumped 2.16.0 → 2.17.0
-J, --omit-link-times	Omit symlinks from --times	✅ Parity	Real modifier now that FastSync preserves symlink times. Symlink entries already carried their metadata on STATUS_SYMLINK; the receiver now applies it with no-follow primitives only (utimensat(..., AT_SYMLINK_NOFOLLOW), plus best-effort fchmodat(..., AT_SYMLINK_NOFOLLOW) and policy-gated fchownat(..., AT_SYMLINK_NOFOLLOW)), so the link itself is stamped without ever dereferencing it, confined fd-relative below the authorized receive root. A symlink has no children, so the times are applied immediately at creation. When -J is set (the boolean crosses the wire) the receiver skips the timestamps (mode/ownership are unaffected); without -J an -a/-l transfer restores symlink mtimes. Wire change alongside -O: the shared STATUS_DIR_TIMES frame; PROTOCOL_VERSION bumped 2.16.0 → 2.17.0
--super	Receiver attempts super-user activities	⚠️ Caveat	Phase 7 Wave E: receiver-side safe-subset + clear-refusal privilege model, tri-state super_mode (auto/on/off). --super permits the receiver to attempt super-user activities — ownership application and char/block device-node creation — that are already confined fd-relative below the authorized receive root; --no-super forbids them even when the receiver is root; the default (auto) preserves the pre-existing best-effort behavior of attempting them (not only when already root: an unprivileged attempt is refused by the kernel and skipped per entry, matching FastSync's history). The server additionally accepts an operator-level --no-super veto that forces OFF for every connection it accepts (so it also refuses any client --copy-as/--super); a privileged (root) standalone TCP listener now also defaults to OFF unless the operator opts in with the new server-only --allow-super flag (the flag is rejected with --stdio, whose remote argv is composed by the client and must never defeat the secure default; operators exposing fastsync-server --stdio over SSH need a forced command if the default must hold. An unprivileged receiver is unchanged, since the kernel refuses the confined attempts anyway; the --daemon path keeps its per-module client owner = yes opt-in); the --fake-super owner replay and the --write-devices write path are gated by the same policy. FastSync never elevates: no setuid/seteuid/setgid is ever called, and --super never bypasses the confinement floor (file_open_secure_parent, O_NOFOLLOW, root checks) — it only permits an attempt that is already confined. --super does not imply --numeric-ids and never enables client-chosen ownership on its own: ownership is applied only when an explicit identity policy (--usermap/--groupmap/--chown/--numeric-ids/--copy-as) or a preserve-source request (-o/-g, or -a/--archive) is also given. A non-root receiver given --super logs exactly one warning at activation and each confined attempt is then refused by the kernel and skipped per entry (never aborts); --no-super suppresses ownership, char/block mknod, --write-devices and the fake-super owner replay, while unprivileged FIFO creation is unaffected. Wire: one trailing super_mode int on the config frame (validated 0..2), sent before the --copy-as block (fixed order: super int, then copy-as presence int + ids); PROTOCOL_VERSION bumped 2.17.0 → 2.18.0. Documented divergence from rsync: rsync's --super runs the receiver with elevated privilege; FastSync only permits a confined attempt and never elevates
--fake-super	Store/recover privileged attrs via xattrs	⚠️ Caveat	Full record and replay (protocol 2.23.0 parity update). The receiver writes the resolved uid:gid:mode:mtime_sec:mtime_nsec into a reserved user.fastsync.stat xattr on each written file (best-effort, fd-relative), then immediately re-applies the mode and times via fake_super_restore_fd (fchmod + futimens; absent/malformed records are a silent no-op, never fatal). --fake-super never performs a real chown: when an explicit ownership mapping (--chown/--usermap/--groupmap/--copy-as) is active the receiver records the resolved id, otherwise the source's own id, but the owner leg is always suppressed so recording can never defeat the flag; the record is retained for a later privileged restore. The replayed mode goes through the shared metadata_mode_for_policy helper, so under -p it is copied exactly (including group/other-write and special bits — strict rsync parity, no masking) and under -E it follows the rsync executability rule. Directory ownership and directory xattrs/ACLs are preserved alongside file entries (mode/owner are applied to directories under the same per-attribute policy and -A/-X carry the directory ACL/xattr block). Implies metadata transmission so the source uid/gid/mode/mtime are available. The recording format diverges from rsync's user.rsync.%stat%; no cross-tool conversion is attempted. Both it and -X/-A are incompatible with -s (chunk serialization), rejected up front
--open-noatime	Avoid changing access time when opening files	✅ Parity	Sender-side policy: the sender opens source files with O_NOATIME (Linux) when reading them for transfer, so the open/read does NOT bump the source's on-disk access time. Degrades safely when O_NOATIME is unavailable (not defined) or refused (EPERM, since it needs CAP_FOWNER or file ownership): the code falls back to a normal open, so the data always transfers — only the atime-bump is skipped. It does not itself capture/preserve atime; it only avoids modifying it. Client-only, never crosses the wire. Exposed as file_open_for_read() and applied to both the buffered data path and the sendfile path
--numeric-ids	Do not map uid/gid by name	✅ Parity	A mapping modifier only: when ownership is being applied it uses the transmitted numeric uid/gid directly, skipping the name lookup. It does not request ownership application on its own — combine it with -o/-g, -a, or an explicit map (--chown/--usermap/--groupmap) — and it does not need any metadata flag merely to parse. Ownership is only applied when metadata (hence the source uid/gid) is actually transmitted (see the Phase-4 identity notes)
--usermap=STRING	Map usernames	⚠️ Caveat	Opt-in ownership application. rsync subset implemented (protocol 2.23.0): comma-separated FROM:TO rules evaluated in order, first match wins. FROM accepts a user name (resolved on the SOURCE machine at parse time), an @N/bare N numeric id, an inclusive LOW-HIGH id range, * (matches any id), or an empty field (matches ids with no name on the source). TO accepts a name (resolved on the receiver), an @N/bare N id, or * (the receiving process's current euid). Rules are carried over the wire as resolved numeric id pairs; the receiver applies a matching rule (else falls back to --chown, --numeric-ids, then a best-effort name lookup) via an fd-relative fchown, including directory entries. Malformed/unresolvable specs are rejected with a clear error, never a silent no-op. Implies metadata preservation so the source uid/gid travel. Only effective when the receiver can actually change ownership (root or membership); otherwise it warns and continues
--groupmap=STRING	Map group names	⚠️ Caveat	Same rsync subset and semantics as --usermap (names, @N/bare N, inclusive ranges, *, empty-FROM for unnamed ids, receiver-resolved TO names) but for the group (gid) side and the group databases. See the Phase-4 identity notes
--chown=USER:GROUP	Map owner and group	⚠️ Caveat	Opt-in ownership override applied receiver-side. Forms: USER:GROUP, USER (owner only), :GROUP (group only); a * for USER/GROUP means the current/root user or group as appropriate; an @N/bare N numeric id is accepted. A : inside a name may be escaped as \:. Equivalent to a trailing *:* usermap+groupmap rule (so an explicit --usermap/--groupmap match wins). Protocol 2.23.0 makes --chown and --usermap/--groupmap mutually exclusive on the same side: combining them (in either order) is a clear configuration error (--usermap conflicts with prior --chown), matching rsync and never an order-dependent silent winner. Malformed or unresolvable specs are clear parse errors. Implies metadata preservation. Only effective when the receiver has permission to chown; otherwise it warns and continues (rsync parity)
--copy-as=USER[:GROUP]	Perform the copy as another user/group	⚠️ Caveat	Safe-subset implementation, an explicit divergence from rsync's real identity switching. rsync makes the receiving process actually assume USER/GROUP (setuid/setgid); FastSync's receiver is multithreaded, so a real credential drop would be unsafe and is never attempted — FastSync never calls setuid/seteuid/setgid. Instead the receiver FORCES the ownership of every entry it writes to copy_as_uid/copy_as_gid through the existing confined, fd-relative identity path (the same fchown/fchownat mechanism as --chown/--usermap/--groupmap; symlinks use fchownat(..., AT_SYMLINK_NOFOLLOW), and directories — including intermediate parents created implicitly while writing a nested file — and char/block/FIFO nodes are owned no-follow too, so a directory never keeps the receiver's owner while its children get the target owner), with --copy-as at the highest priority — it beats usermap/groupmap/--chown/--numeric-ids and the best-effort name lookup. This REQUIRES a privileged (root) receiver: an unprivileged receiver REFUSES the whole transfer up front at the config handshake (server_module_gate, running inside config_receive_with_validate before the STATUS_OK ack) with a clear error and no file data exchanged — never a silent wrong-ownership result. A server running with an operator --no-super veto also refuses it; a privileged (root) standalone TCP listener refuses it by default too and only honors it after the operator passes --allow-super (the flag is rejected with --stdio, where the client-composed remote argv could otherwise defeat the default; a forced command is required if the default must hold), and a daemon refuses --copy-as, like every other client-chosen-ownership request (--numeric-ids/--chown/--usermap/--groupmap/--fake-super/explicit --super), unless the selected module opts in with client owner = yes; without that per-module opt-in a daemon must not honor an arbitrary client-selected owner (a root standalone listener honors these for its single operator-authorized root only when started with --allow-super). --fake-super interaction: --copy-as is authoritative, so the recorded source owner is never replayed over the forced target owner. If the ownership apply still fails with EPERM/EACCES (capability-restricted root, root-squash, read-only mount) the failure is logged at ERROR and the entry is reported as failed rather than written with the wrong owner, which fails the transfer (fail-fast) so overall success is never reported with the wrong owner. USER is resolved on the client against the user database (a name, an @N/bare N numeric id, or * meaning the client's current euid); when :GROUP is present it is resolved against the group database (* meaning the client's egid). Group-default rule: when the group is omitted FastSync uses the user's primary gid (getpwuid(uid)->pw_gid); a numeric id with no local passwd entry has no primary gid to look up, so gid falls back to uid (documented divergence). Malformed/empty/unresolvable specs are clear parse errors, never a silent no-op. Never elevates privileges and never bypasses the confined receive root. Implies metadata preservation (the source uid/gid must be transmitted). Wire: a new trailing config-frame block sent after the --super int (presence int, then the two int32 ids, both validated >= 0 on receive; the ids are also rejected if they do not fit int32 at CLI parse time); PROTOCOL_VERSION bumped 2.17.0 → 2.18.0

Phase-4 metadata-time notes: -U/--atimes, -N/--crtimes, -O/--omit-dir-times, -J/--omit-link-times, and --open-noatime are new. They change the wire: the per-file metadata frame grows atime_valid + atime_sec + atime_nsec and crtime_valid + crtime_sec + crtime_nsec (appended after the existing mode/uid/gid/mtime fields, preserving the exact positions of every pre-existing field), and the config frame grows four booleans — preserve_atimes, preserve_crtimes, omit_dir_times, omit_link_times — that CROSS the wire so the receiver knows what to apply / suppress. --open-noatime is client-only and is never serialized (it only governs the sender's source reads). PROTOCOL_VERSION was bumped 2.11.0 → 2.12.0 (peers must match, exactly as prior phases did).

Client-vs-wire split: -U and -N affect both the sender (capture) and the receiver (apply), so they and their metadata fields cross the wire; -O/-J are receiver-side preferences and cross as config booleans; --open-noatime is purely a client/sender open flag and stays off the wire (mirroring the existing convention where ignore_errors is client-only while force_delete crosses the wire).

Phase-4 xattr/ACL notes (-X/--xattrs, -A/--acls, --fake-super): these are new in protocol 2.13.0 and add a bounded per-file xattr block to the per-file metadata frame (count + each name/value, sent only when xattr transport is enabled, i.e. with zero overhead on unaffected runs). The config frame carries preserve_xattrs, preserve_acls (in the existing file-options block) and a trailing fake_super boolean — all CROSS the wire so the receiver knows the negotiated behavior; the derived use_xattrs flag is recomputed on the receiver. PROTOCOL_VERSION was bumped 2.12.0 → 2.13.0 (peers must match, exactly as prior phases did).

• Security model (both -X and -A): only user.* and the system.posix_acl_access / system.posix_acl_default namespaces are ever captured (sender) or applied (receiver). security.* (SELinux, capabilities, ...), trusted.*, and all other system.* attributes are never transmitted or applied, so a client can never compel the receiver to set a privileged xattr. The receiver re-validates each incoming name against this whitelist even though the sender already filtered, so a malicious/compromised sender's security.capability payload is rejected outright (a clean protocol error), never applied.
• Bounds / memory safety: per-name length ≤ 255 B, per-value ≤ 1 MiB, per-file count ≤ 256 names, per-file name+value total ≤ 4 MiB. Both the sender (during capture) and the receiver (during receive) enforce these; an oversized or malformed frame is rejected, never a large allocation.
• Confined application: xattrs are applied with fsetxattr on the exact just-written destination file fd (before the atomic rename), never on a caller-controlled path; this is the same confinement as mode/time restore. The --link-dest / -H hard-link copy fallback (a byte copy when link() is refused) also re-applies the incoming (or, for -H, the first member's) xattrs and the --fake-super stat, so attributes are preserved rather than silently dropped when the link fails.
• Reserved fake-super key is receiver-only: the user.fastsync.stat key is excluded from sender capture AND from receiver application, so it can only be written by the receiver's own --fake-super handling. A source file that already carries such a record is never forwarded on a plain -X run, so it cannot be spoofed to mislead a later privileged restore.
• -A requires no libacl — ACLs travel as the system.posix_acl_* xattrs. Applying an ACL is owner-privileged: fsetxattr failure (e.g. non-root, unsupported filesystem) is logged (collapsed to one line per file) and never fatal.
• --fake-super: see the row above; the reserved key is user.fastsync.stat with the documented uid:gid:mode:mtime_sec:mtime_nsec (mode octal) format. Replay exists: after each stored record the receiver immediately re-applies the recorded mode and times fd-relative (fake_super_restore_fd), but it deliberately never performs a real chown — --fake-super only records the resolved owner (the active --chown/--usermap/--groupmap/--copy-as mapping when one is in effect, otherwise the source's own id) for a later privileged restore. The recording format diverges from rsync's user.rsync.%stat%; no cross-tool conversion is attempted.
• Chunk serialization (-s) incompatibility: the per-file xattr block rides the streaming per-file frame, which -s replaces with a fixed buffer format, so -X / -A combined with -s is rejected up front on both ends (mirroring the existing -H + -s rejection) rather than silently dropping attributes.

atime capture does not clobber the source atime: the sender records the access time from the same pre-read stat the scanner already took (inside file_metadata_create), before any file data is read for transfer. So -U alone captures the correct atime even without --open-noatime. --open-noatime is orthogonal: it keeps the source's on-disk atime from being bumped by the read that actually ships the data (only honoured where O_NOATIME works; it degrades to a normal open otherwise, so the data always transfers).

crtime handling: -N captures the source birth time via statx/STATX_BTIME (guarded #ifdef STATX_BTIME on Linux) and transmits it. On the receiver, no portable setter exists (utimensat can only set atime/mtime), so the receiver deliberately does not apply it: it logs a debug note and continues — it never fails the transfer and never pretends the crtime was applied. This is the explicit, documented unsupported-attribute handling. On platforms without statx the flag is accepted but nothing is captured (a documented no-op).

omit-dir-times / omit-link-times: -O and -J are real modifiers as of P7 Wave D (🔄 → ✅ Implemented). FastSync now preserves directory mtimes (captured by the scanner, transmitted in trailing STATUS_DIR_TIMES frame(s), applied only after all children and the delete/publication phases) and symlink mtime/owner/mode (no-follow utimensat/fchownat/fchmodat at link creation). -O makes the receiver skip the directory-time set; -J makes it skip the symlink timestamps (ownership/mode application is unaffected and stays governed by the identity opt-in). Both config booleans already crossed the wire. See the -O/-J rows and the Wave D note below.

-U/-N and metadata-bundle interaction: because FastSync carries all metadata (mode, uid, gid, mtime, and now atime/crtime) in one bounded payload that is only sent when metadata transmission is on, -U and -N imply metadata transmission (the times travel inside that payload). They do not enable ownership application, which remains opt-in strictly through the identity flags (--numeric-ids / --usermap / --groupmap / --chown).

Phase-4 identity notes: --numeric-ids, --usermap, --groupmap, and --chown are real. They introduce a controlled, opt-in, privilege-gated ownership-application path on the receiver: plain -M/--preserve still does NOT apply client-supplied ownership (FastSync's deliberate conservative default, byte-for-byte backward compatible); ownership is only attempted once a client explicitly requests an ownership-affecting option. Application goes through an fd-relative fchown() in the receiver's metadata-restore path (after the file is fully written, before timestamps are set), so it is confined and symlink-safe — never a path-based chown. When the receiver lacks permission (typically non-root, e.g. the CI nobody user) EPERM/EACCES is logged as a warning and the transfer CONTINUES with exit status success, matching rsync. A no-op default means existing transfers are unaffected.

Resolution of the destination uid/gid on the receiver: a matching --usermap/--groupmap rule wins; else the matching --chown side; else, with --numeric-ids, the transmitted numeric id is used raw (no name lookup); else a best-effort name lookup on the receiver's own account databases (skipped when the transmitted id has no name present there). --chown enforces the receiver side and is validated at parse time (malformed specs are clear errors, never a silent no-op).

Wire/version: the config frame gained numeric_ids, chown_uid_set, chown_uid, chown_gid_set, chown_gid, and the usermap/groupmap tables (count-delimited lists of resolved int32 FROM/TO id pairs), so PROTOCOL_VERSION was bumped 2.10.0 → 2.11.0 (peers must match). All new fields cross config_send/config_receive with full symmetry and are validated on receive (bounded map sizes below MAX_IDENTITY_MAP, ids >= the -1 sentinels).

Documented divergences from rsync: because FastSync transmits only numeric uid/gid (not names) on the wire, name-based values (--usermap/--groupmap names, --chown names) are resolved to numbers at CLI parse time against the client (sender) machine's account databases; this reproduces rsync's semantics on a shared-account source/destination and is documented for a genuinely different destination. The interesting named-value subset is supported (* FROM wildcard, * TO = current user, @N/bare-N numerics); a lone-@ "use the FROM value unchanged" rsync form is not implemented. Also unlike rsync, plain -M never applies ownership and --usermap/--groupmap/ --chown each imply metadata preservation so the source uid/gid actually travel (the flags only take effect where ownership is being preserved/applied).

Phase-4 hard-links notes: -H/--hard-links is real and introduces a deduplicating wire path for files whose source entries share a filesystem inode. On the sender, the scanner records each distinct (st_dev, st_ino) encounter and assigns it a stable, run-local link-group id (HardLinkTable, mutex-guarded so a multi-threaded scan could share one instance). The FIRST member of a group is transferred normally and carries the data; each later (sibling) member is transmitted as a payload-less STATUS_HARDLINK frame carrying its destination path, the group id, and the first member's destination-relative wire path. Ordering is guaranteed by forcing the sequential scanner whenever -H is on (even under -j/--threads), so the first member is always emitted — and, on the receiver's single write thread, installed — before any of its siblings; the receiver is therefore always able to link to an already-present first member, including the "first member already up-to-date/skipped" case (the sibling links to or copies the existing file). Asymmetric existence policies are handled gracefully: under --existing, if the first member's destination is absent (so it is skipped) but a sibling's own destination already exists, that existing sibling is left in place rather than the transfer aborting on the missing first member. The receiver installs each sibling beneath its confined root as an atomic hard link (temp link + rename); when link() fails (cross-device, filesystem refuses links) it falls back to a byte-identical local copy of the first member, never a partial/corrupt file. --delay-updates stages each sibling as a hard link to the first member's STAGED file, so publication's renames preserve the shared inode; --inplace and --partial are unaffected (a sibling is a fresh link/copy). Because a hard link shares an inode, metadata is applied exactly once on the first member and never re-written through the sibling (whose members are byte-identical by construction), so all members agree.

Wire/version: PROTOCOL_VERSION was bumped 2.11.0 → 2.12.0 (peers must match). The config frame already carried the preserve_hard_links boolean (round-trips through config_send/config_receive); the only new wire element is the STATUS_HARDLINK frame described above. Incompatibilities (rejected up front with a distinct error on the client, and re-checked on receive): -H with -s chunk serialization (the chunk wire has no per-file hard-link info) and -H with --append/--append-verify (a payload-less sibling cannot be tail-resumed).

Phase-4 devices notes: --devices, --specials, -D, --copy-devices, and --write-devices are new. They change the wire: the config frame grows three booleans — preserve_specials, copy_devices, write_devices — that CROSS the wire (preserve_devices already existed), and a new STATUS_SPECIAL frame (used by --devices/--specials/-D) carries a special/device entry: the destination path, the metadata frame (whose mode's S_IFMT bits carry the node kind, requiring the flags to imply metadata transmission), and two int32 rdev major/minor fields. The chunk-serialized wire (-s) grows a matching per-file special marker + rdev so --devices/--specials also work under -s. PROTOCOL_VERSION was bumped 2.12.0 → 2.13.0 (peers must match, exactly as prior phases did).

Privilege gating (the crux): making a device node requires CAP_MKNOD (root). CI runs the integration suite as a NON-ROOT user (via setpriv), so mknod fails with EPERM. The receiver treats this as a graceful, logged skip of the entry returned as a success/skip outcome — the whole transfer NEVER aborts just because the environment cannot create the node. mkfifo (FIFOs) is unprivileged, so --specials FIFO creation is a real, assertable behavior under CI. Sockets are recreated too (protocol 2.23.0) with mknodat(..., S_IFSOCK): Linux allows an unprivileged mknod of a socket node because no live bound socket is created, so a source socket materializes as a socket-type filesystem entry exactly as rsync does. The "device actually created" integration assertions are guarded to run only as root. User-facing expectation: point --devices at devices and a non-root receiver will faithfully skip them while transferring everything else; --specials recreates FIFOs and socket nodes for any receiver.

Confinement & validation: a special/device node is created with mknodat/mkfifoat on the parent directory opened fd-relative below the receive root (file_open_secure_parent: O_NOFOLLOW, no .. components, root-checked), so a node can never be created outside the authorized destination root and never through a symlinked parent. The transmitted type is derived ONLY from the validated S_IFMT bits of the metadata mode (char/block/FIFO and socket honored; regular/dir rejected as an invalid special), and the transmitted rdev is validated both on the wire (file_receive_special, chunk_deserialize) and at the creation site (file_special_rdev_valid): a negative, oversize, or non-device-carrying rdev is rejected outright (receiver aborts the frame), and a node is never replaced over an existing directory or unrelated entry (a matching existing node is left in place). --write-devices is the deliberately restricted danger path: it only ever opens an existing char/block node under the confined root, and every failure mode (missing, non-device, write error, EPERM) is a warning + skip, never a system-clobbering write or an abort.

Documented divergences (honest subset):

• A device entry the receiver cannot create (missing CAP_MKNOD) is skipped, not a transfer failure — rsync under the same conditions would error.
• --copy-devices copies the device's reported size (typically 0 for char devices/FIFOs) into a regular file and never reads an unbounded pseudo-device; this is the safe, non-hanging alternative to rsync's dd-like read.
• --write-devices requires the device to already exist at the destination and never creates it; unsupported/inaccessible targets are skipped, not written.
• Ownership is not applied to recreated nodes (identity fchown needs an fd and would require opening the node); permissions and mtime are applied at creation / via utimensat.

9. Symlink Handling

Flag	Rsync Description	FastSync Status	Notes
-l, --links	Copy symlinks as symlinks	⚠️ Caveat	A symlink is transmitted as a real symlink: its target string crosses the wire (STATUS_SYMLINK / chunk entry type) and the receiver creates it with symlinkat beneath the receive root, never following the target. Targets are stored verbatim (protocol 2.23.0), matching rsync -l: an absolute target or one containing .. is copied exactly, and the receiver no longer enforces a containment predicate by default. --safe-links is the sender-side opt-in that drops unsafe targets before transmission; --trust-sender does not affect symlink targets (it only relaxes the receiver's path-list re-validation). The placement path is still hard-confined (has_path_traversal, O_NOFOLLOW fd walk), and the link's own mode/times are applied with no-follow primitives. See the Phase-4 symlink-trust notes and the residual-risk note below
-L, --copy-links	Transform symlink to referent	⚠️ Caveat	Sender-side: every symlink is replaced by its referent's content (copy_links config field). A referent that cannot be read, including a broken symlink, is treated as a non-error and the run exits 0 — where rsync exits 23 (RERR_PARTIAL). This is the documented status-code divergence
--copy-unsafe-links	Transform unsafe symlinks	⚠️ Caveat	Sender-side: only symlinks whose target is unsafe (absolute or escaping via .., matching rsync's unsafe_symlink() semantics) are dereferenced into their referent; safe links stay symlinks. Same broken-referent exit-0 caveat as -L (copy_unsafe_links config field)
--safe-links	Ignore symlinks outside tree	✅ Parity	Sender-side: a symlink whose target is unsafe is not transmitted at all (skipped), matching rsync's --safe-links. Because FastSync applies this while scanning the source, the receiver does not need to repeat it (safe_links config field)
--munge-links	Munge symlinks for safety	✅ Parity	Sender rewrites each transmitted symlink target with rsync's /rsyncd-munged/ prefix; the receiver strips the marker (only when the negotiated munge_links policy is on, so a source link that genuinely begins with the marker round-trips verbatim) and restores the exact real target. Unlike rsync, FastSync prefixes on the sender and un-munges on the receiver, but the wire result and the stored marker match rsync. See the Phase-4 symlink-trust notes
-k, --copy-dirlinks	Transform symlink to dir	✅ Parity	A symlink whose referent is a directory is dereferenced and recursed as a real directory; a symlink to a regular file stays a symlink. Sender-side only. See the Phase-4 symlink-trust notes
-K, --keep-dirlinks	Treat symlinked dir as dir	✅ Parity	On the receiver, an existing destination symlink-to-a-directory is used as that directory (followed) instead of being replaced; it is followed only when it resolves to a directory that stays beneath the receive root. See the Phase-4 symlink-trust notes

Phase-4 symlink-trust notes: -l/--links, -k/--copy-dirlinks, -K/--keep-dirlinks, and --munge-links form the "symlink trust boundaries" row. Making all three new flags have an observable, security-sane effect required transmitting symlink targets, so FastSync's -l/--links is now real: a symlink-type entry carries its target on the wire (a new STATUS_SYMLINK frame for the per-file path, and a new entry type 2 in the -s chunk serializer) and the receiver creates it with symlinkat under an O_NOFOLLOW parent walk, never following the target. Wire changes: STATUS_SYMLINK, the chunk entry type 2, a per-entry symlink-target string, and two new config booleans that CROSS the wire — munge_links and keep_dirlinks; PROTOCOL_VERSION was bumped 2.12.0 → 2.13.0 (peers must match, exactly as prior phases did).

Per-flag semantics and divergences.

• -l/--links copies a symlink as a symlink: the scanner readlinks the target, the sender transmits it, and the receiver symlinkats it. Targets are stored verbatim (protocol 2.23.0), matching rsync -l: an absolute target or one containing .. is copied exactly as-is. The receiver no longer enforces the strict containment predicate on the link value; target policy belongs to the sender (--safe-links/--copy-unsafe-links) exactly as in rsync. The link's placement path is still hard-confined (has_path_traversal, O_NOFOLLOW fd walk), and the link's own metadata is applied with no-follow primitives (utimensat/fchownat/fchmodat with AT_SYMLINK_NOFOLLOW), so -J is a real omit switch rather than a no-op. Residual risk: because -l stores targets verbatim and does not enforce containment, a destination later consumed by a link-following tool can follow a link outside the receive root. Use --safe-links when the source is not trusted; a destination that only ever uses openat-style no-follow access is unaffected.
• -k/--copy-dirlinks (sender): a symlink whose referent is a directory is dereferenced and recursed into as a real directory; a symlink to a regular file (or any non-directory) is kept as a symlink. This is rsync's -k. When -L/--copy-links or --safe-links/--copy-unsafe-links are active, their (dereference) semantics take precedence, so -k is subsumed exactly as in rsync.
• -K/--keep-dirlinks (receiver, crosses the wire): when a directory is to be created (on-demand parent creation for a child write) and the destination path is already an existing symlink that resolves to a directory within the receive root, that symlinked directory is used (followed) instead of being replaced by a real directory; new entries are written beneath it. The follow is confined: it only happens where realpath of the symlink resolves to a still-within-root real directory, so a malicious link pointing outside the root is never followed. Scope: -K acts on the write path (parent/mkdir creation); the delete walker still never follows symlinks (a documented divergence for --delete over an existing symlinked dir). Without -K the destination symlink is not followed (the O_NOFOLLOW walk fails the write), which is the safe default.
• --munge-links (sender rewrite; crosses the wire so the receiver unmunges): every transmitted symlink target is prefixed with rsync's marker SYMLINK_MUNGE_PREFIX = /rsyncd-munged/; the receiver strips the marker (only when the negotiated munge_links policy is on — a plain -l run never strips the prefix, so a source symlink that genuinely begins with /rsyncd-munged/ round-trips verbatim) and restores the exact real target. This matches rsync's stored marker and its both-ends-negotiated model, with the prefix applied on the sender rather than the receiver. The link value is otherwise stored verbatim; the placement path still goes through file_symlink_at_secure's confined fd walk (has_path_traversal on the destination path, no symlink follow). When no symlink is being transmitted (-l/-k/-a off) --munge-links has nothing to rewrite and is inert. -K/--keep-dirlinks policy is installed per connection at config-accept (stable for the whole transfer, never racy under -j/--threads), and only ever follows an in-root symlink-to-directory.

Compatibility: -k, -K and --munge-links are opt-in. Without them the scanner's link handling, the wire frames, and the receiver's writes are unchanged for every other option set. --safe-links/--copy-unsafe-links are applied sender-side; --trust-sender no longer changes how symlink targets are stored (it only skips the receiver's path-list re-validation). -l/--links stores targets verbatim, matching rsync.

10. Sparse & Device

Flag	Rsync Description	FastSync Status	Notes
-S, --sparse	Sparse block handling	✅ Parity	Phase 7 Wave B: real hole preservation with no wire change. The receiver's sparse-aware writer (write_all_sparse, next to write_all in src/shared/file.c and src/shared/file_store.c) walks the in-memory file image and emits any all-zero run ≥ 4096 bytes as a hole via lseek(SEEK_CUR) (the pre-size ftruncate guarantees the offset bookkeeping and logical size), ftruncate(size) after the last run pins the final size even with a hole tail. Wired into both the atomic temp+rename store and --inplace when sparse is set; the non-sparse path is byte-identical to before. Sparse wins over --preallocate (posix_fallocate is skipped when sparse is set, so the holes are not re-allocated). Interplay note: under --partial a retained sparse temp already has the full logical size (trailing content is holes), so --append's "shorter destination" resume does not re-run; the retained file is still valid and a normal re-transfer (or -W/delta) repairs it — documented so the combination is never surprising
--preallocate	Allocate dest files before writing	⚠️ Caveat	The receiver preallocates the destination file's full expected space before any data is written, so a transfer that would overflow disk fails fast at allocation time (a clean error, not a half-written file) and the file is laid out contiguously, avoiding fragmentation. Crosses the wire (the config frame carries a preallocate boolean; PROTOCOL_VERSION bumped 2.10.0 → 2.11.0, peers must match) so the sender knows the receiver will preallocate and the receiver performs it. Allocation approach: posix_fallocate() is preferred because it reserves real disk blocks (true fail-fast on ENOSPC), falling back to plain ftruncate() only when the filesystem reports the allocation is unsupported (EOPNOTSUPP/ENOSYS); ftruncate still extends the logical size so the intent degrades gracefully. Fallback/error semantics: EOPNOTSUPP/ENOSYS → clean fallback to ftruncate (best-effort, preallocates the logical size and never fails a transfer on filesystems that lack posix_fallocate); a genuine allocation failure (ENOSPC/EDQUOT/EFBIG/…) aborts the file/receive with a distinct preallocate failed ... transfer aborted error — it does not fall back to a normal non-preallocated write, preserving the fail-fast purpose. Size-known requirement: preallocation only runs when the final size is already known up front (the normal regular-file case); unknown-length data is skipped (never failed). Orthogonality: applies uniformly across the atomic temp+rename store path, --inplace, --partial/--partial-dir, --delay-updates (the staged temp file is preallocated before data flows) and the --link-dest copy fallback; it neither implies nor conflicts with -s, --append, or delta. rsync-divergence: rsync signals that --preallocate is ignored with --sparse; FastSync gives sparse precedence — when both are set, posix_fallocate is skipped so the holes the sparse writer creates are not re-allocated (the ftruncate presize sizing stays), matching the intent of "sparse wins". See the Phase-4 preallocate notes below

Preallocate notes (Phase 4, preallocate wave): --preallocate is implemented as a real receiver-side allocation of the destination file's space before data is written. It is a plain boolean config flag that crosses the wire (serialized in the config frame's selection-options block, mirroring --inplace/--append/--force), so the run requires matching ends: PROTOCOL_VERSION was bumped 2.10.0 → 2.11.0 (peers must match or the version check fails). The allocation is performed on the exact destination fd, immediately after it is opened, before any bytes are streamed; posix_fallocate (and the ftruncate fallback) leave the fd's file offset untouched, so the subsequent data write at offset 0 is unaffected and complete. Because FastSync writes each file's byte payload in one in-memory batch, the "full expected size" is exactly the known data_size, which is what gets preallocated. Unknown-length/streamed payloads are skipped rather than failed. A failed allocation logs a distinct preallocate failed error and aborts the file (the atomic temp is unlinked, the inplace target is left untrimmed) so the run fails cleanly and never silently degrades to a non-preallocated write — preserving rsync's fail-fast intent on a full disk.

11. Checksum & Comparison

Flag	Rsync Description	FastSync Status	Notes
--checksum	Skip based on checksum	✅ Parity	-c/--checksum compares per-file whole-file content digests to skip unchanged files. As of protocol 2.23.0 the short -c implies the checksum quick-check, so a plain -c run verifies content rather than only affecting the --incremental handshake. The digest algorithm is xxh64 by default and is selectable via --checksum-choice/--cc (xxh64/xxhash/xxh3/xxh128/md5/auto) and --checksum-seed=NUM (see those rows)
--checksum-choice=STR, --cc=STR	Choose checksum algorithm	⚠️ Caveat	Real algorithm selection for the per-file whole-file digest used by the --incremental/--checksum handshake and by the basis-dir content verification. Protocol 2.23.0 accepts xxh64 (the default), xxhash (rsync's spelling of xxHash64), xxh3, xxh128, md5, and auto (which selects FastSync's default). rsync choices FastSync does not implement — md4, sha1, none, and the two-name transfer,pre-transfer form — are rejected by name with a clear error at parse time, never a silent no-op. --cc is the alias (--cc=ALG and space forms both parse). The algorithm id and seed cross the wire with the config frame, so the receiver hashes its on-disk old file with the SAME algorithm+seed the sender used and both agree on a match; the sender's digest and the receiver's comparison live in the per-file STATUS_CHECK handshake, which carries a length-prefixed, bounded (1..16 byte) digest, and the receiver pins the received length to the negotiated algorithm's digest length (defense-in-depth: a mismatched/malicious length only forces a safe re-transfer). Digest lengths: xxh64/xxh3 = 8 bytes, xxh128/md5 = 16. Note: md5 is a FIPS-non-approved algorithm, so under an OpenSSL build with FIPS mode enabled --checksum-choice=md5 fails loudly rather than silently falling back. PROTOCOL_VERSION has moved well past the original 2.10.0 digest-frame bump. Like rsync, the choice only takes effect where a whole-file digest is actually computed (--checksum on, or a basis-dir flag). Closely-related divergence: the delta BLOCK strong checksum stays xxHash32 — --checksum-choice selects only the whole-file digest, matching rsync where the per-block checksum is independent of the whole-file choice
--compare-dest=DIR	Compare dest files relative to DIR	⚠️ Caveat	DIR is a receiver-side basis relative to the destination root (confined below it; absolute/../. rejected, // collapsed and trailing / dropped). On the receiver's per-file check (implies --incremental) an exact match = same size + mtime (unless --size-only; -I disables matching) and equal xxHash64 of the sender's file; a match suppresses the data transfer. compare-dest never copies: it only skips a file the destination does not already hold (sparse destination, rsync parity), and is consulted before the normal delta/full paths. Repeatable; searched in command-line order, first match wins. Divergences: when the destination already holds a different version rsync deletes it but FastSync instead transfers the data (keeps the mirror complete; never deletes without --delete); attribute-only differences on a match are not re-applied (data is skipped so the sender never sends metadata); content is verified by xxHash64, stricter than rsync's default quick check. Sizing: FastSync's whole-file payload limit is 256 MiB on every transfer path (not basis-specific); rsync applies basis dirs to arbitrary sizes, so FastSync refuses a basis run whose source contains a larger file up front with a clear error before any transfer. Wire: a basis-count field is always present on the config frame (protocol 2.9.0, so clients and servers must both be 2.9.0)
--copy-dest=DIR	Include copies of unchanged files	⚠️ Caveat	Same basis rules as --compare-dest, but an exact match materializes a local copy of the DIR file into the destination (via the normal atomic temp+rename store path, so --existing/--ignore-existing/--update/--backup/--delay-updates all still apply) instead of transferring data. Repeatable; command-line order = priority. Content is xxHash64-verified before the copy. Divergences: a basis-hit destination keeps the basis file's own mode/uid/gid and mtime (the sender sends no metadata on a skip), so with --size-only its mtime can differ from the source and attribute-only differences are copied with the basis attributes rather than rsync's "copy + fix attributes". Requires --incremental (implied); incompatible with -s. Wire: protocol 2.9.0
--link-dest=DIR	Hardlink to files when unchanged	⚠️ Caveat	Same basis rules as --copy-dest, but an exact match installs an atomic hard link to the DIR file (temp hard link + rename) so no data or disk space is used; where the link is impossible (basis on another filesystem, filesystem refuses links) it falls back cleanly to a byte-identical local copy, never a corrupt/partial file. --delay-updates stages the link and publishes by rename, so the final entry stays a real hard link. Repeatable (searched in command-line order, first match wins). Content is xxHash64-verified before linking. Divergences and caveats: an already up-to-date destination file is not re-linked to a basis file (only files that would otherwise be written are linked); a link keeps the basis inode's own mode/uid/gid and mtime — metadata is never written through the shared inode (that would mutate the basis file), so a later --inplace run that rewrites such a destination path will mutate the basis snapshot through the shared inode (use --copy-dest when the destination must stay independently writable); with --size-only the linked mtime can differ from the source; a --remove-source-files source satisfied by a basis dir is treated as skipped and therefore retained (never removed); basis dirs are excluded from --delete. Requires --incremental (implied); incompatible with -s. Wire: protocol 2.9.0
-y, --fuzzy, --no-fuzzy	Find similar file for basis	⚠️ Caveat	-y/--fuzzy is a pure bandwidth optimization on the existing receiver-driven delta path: when a file must be transferred and the destination holds no usable content at the exact path (file absent, or the destination file is outside the delta engine's size bounds), the receiver searches the SAME destination directory for an existing regular file whose basename is similar to the incoming name and uses it as the delta basis, so the sender transmits only the differences instead of the whole file. The output is always byte-exact regardless of which (or whether any) basis is chosen. Decision location: the receiver performs the candidate search inside receive_incremental_check and sends the normal STATUS_DELTA_SIGNATURE; the sender never learns the basis was a different file, so no new frame type or sender logic was needed — only the config frame grew a fuzzy boolean, so PROTOCOL_VERSION was bumped 2.8.0 → 2.9.0 (peers must match). Similarity heuristic (deterministic, simpler than rsync's deliberately-fuzzy matching, and documented precisely): candidates are the target's sibling entries in its destination directory, opened O_NOFOLLOW/AT_SYMLINK_NOFOLLOW under the confined root (symlinks never followed; nothing outside the destination root is ever read or hashed); dotfiles, directories, the target's own name, and the .fastsync-stage/temp scratch names are excluded; like the ordinary delta path, the block signature the receiver transmits is derived from on-disk content it may not otherwise send, so a negotiated --fuzzy run exposes the destination's sibling files (at block granularity) to the sender as a known-plaintext oracle — the same information class as the normal delta handshake over the file being replaced; the size gate is the delta engine's own bounds (both files ≥ 16 KiB, ≤ --delta-max, ratio ≤ 10×) rather than rsync's ~1.5× size window; the name gate is a Levenshtein edit distance between the basenames accepted only when ≤ half the length of the longer basename; the single best candidate (smallest distance, tie-break size closest to the incoming file then lexicographically smaller basename) is read; the directory scan is capped at 4096 entries so a pathological directory cannot stall a transfer. When fuzzy applies: only to files the receiver would otherwise send whole — the destination's own file is always preferred as the delta basis when it exists and fits the delta size bounds, so fuzzy does NOT replace an existing-but-different destination basis; FastSync's 10× delta size-ratio bound means an existing destination file that is too far away in size still lets the fuzzy search run. When no similar candidate exists the transfer falls back to the normal whole-file transfer. rsync-divergence note: rsync's own matching uses a fuzzy name/size rule set; FastSync implements the closest safe deterministic approximation above. Because FastSync's delta machinery is off by default (rsync's is on), --fuzzy implies --incremental + --delta (unless --whole-file/-W or an explicit --no-delta switched delta off, in which case fuzzy is inert — matching rsync where --whole-file makes fuzzy irrelevant). Unlike the basis-dir options, --fuzzy honors an explicit --no-incremental (it does not force the handshake back on); an explicit --no-incremental also suppresses the delta implication so no invalid --delta requires --incremental config results. --no-fuzzy negates it. All surrounding semantics are untouched: a fuzzy-reconstructed file is stored as a normal file, so --remove-source-files, itemize/-i, --stats, --backup, --delay-updates, --existing/--ignore-existing/--update behave exactly as for a whole-file transfer (the fuzzy delta does not skip the file)

12. Compression

Flag	Rsync Description	FastSync Status	Notes
-z, --compress	Compress file data	⚠️ Caveat	Streaming zstd (rsync supports multiple algorithms — a documented divergence, selectable via --compress-choice). -z is the compression short form; -c is rsync's --checksum. --skip-compress applies rsync 3.4.1's default suffix list when no list is given
--compress-choice=STR, --zc=STR	Choose compression algorithm	⚠️ Caveat	FastSync supports zstd (default), none, and auto. rsync's other compiled-in choices (lz4, zlib, zlibx) are rejected by name at parse time with a clear error, never silently ignored. --zc is the alias
--compress-level=NUM, --zl=NUM	Set compression level	✅ Parity	1-22, default 5
--compress-threads=NUM	Set compression threads	✅ Parity	compression_threads config field (client-only; does not cross the wire). Sets the number of worker threads used by the zstd compression pool to NUM (1..64; 0/garbage/oversized rejected up front). Accepted in both --compress-threads=NUM and two-argument --compress-threads NUM forms. Composes with -z/compression; under the -j/--threads multithreaded pipeline it parallelizes compressed chunk encoding. See test_tcp.py -z --compress-threads=2 and test_client_cli.c
--skip-compress=LIST	Skip compress for suffixes	⚠️ Caveat	Comma-separated (or /-separated, as in rsync) case-insensitive suffix list; a leading dot is optional; an empty list skips none. When the option is omitted, rsync 3.4.1's built-in default suffix list applies (3g2 3gp 7z aac … zip zst); an explicit list replaces that default entirely, matching rsync. A user-supplied list is a client-side compression choice; incompatible with FastSync chunk serialization (-s)

13. Connectivity

Flag	Rsync Description	FastSync Status	Notes
-e, --rsh=COMMAND	Remote shell to use	✅ Parity	-e/--rsh (and --rsh=COMMAND) select the remote-shell program used to build the SSH child argv, overriding the default ssh. The command is whitespace-split into the leading argv words so rsync's -e "ssh -p 2222" works; the standard -o family, an optional -p port, user@host and the quoted remote command (fastsync-server --stdio) follow. Stored in the rsh_command config field. Client-only, never crosses the wire (it is a launch concern, not a handshake property)
--rsync-path=PROGRAM	rsync binary on remote	✅ Parity	Alias for --fastsync-server-path: both write the fastsync_server_path config field used as the remote-side server program. The path is always quoted as one remote-shell word in the SSH argv. Client-only: fastsync_server_path never crosses the wire (it is a launch concern, not a handshake property), matching rsync, where --rsync-path likewise names the remote program locally. Kept separate from --rsh, which names the local connecting program
--port=PORT, --port PORT	Alternate daemon port	✅ Parity	rsync's daemon-port flag is an alias for --server-port: both spellings (and --server-port=PORT) map to the client-side server_port config field. The client connects to a TCP/TLS server (incl. host::module/path daemon destinations) on that port, and the fastsync-server --daemon listener's port is taken from its config's port key (default 873) or overridden by --dparam port= / -p
--sockopts=OPTIONS	Custom TCP options	✅ Parity	Comma-separated allowlist of OPT=VAL applied via setsockopt after socket() before connect()/bind(). Only TCP_NODELAY, SO_KEEPALIVE, SO_REUSEADDR (0/1) and SO_RCVBUF/SO_SNDBUF (byte count) are accepted; an unknown option name or a bad value is rejected up front, never silently ignored. A value is required for every option (OPT=VAL; a bare name is an error). Applied to the outgoing TCP and TLS client socket; absent by default. SockOptEntry/sockopts config fields. Local socket concern: never crosses the wire
--blocking-io	Use blocking I/O for remote shell	✅ Parity	With --blocking-io the SSH-transport socketpair socket is left without SO_RCVTIMEO/SO_SNDTIMEO, so the transfer blocks naturally; by default it gets the same read/write timeout as the TCP transport (see --timeout). blocking_io config bool. Client-only, never crosses the wire
--timeout=SEC, --contimeout=SEC	Set I/O / connect timeouts	✅ Parity	Protocol 2.23.0 matches rsync's defaults: --timeout defaults to 0 (I/O deadlines disabled) and --contimeout to 60 s; 0 disables either. A positive --timeout bounds both the socket (SO_RCVTIMEO/SO_SNDTIMEO) and the per-message protocol poll deadline on the client; the server floors its session deadline so a client 0 can never hold a session open forever. --no-timeout/--no-contimeout are the negations. Both are client-side deadlines and are not sent on the wire
--outbuf=N|L|B	Set output buffering	✅ Parity	N (none/unbuffered) → _IONBF, L (line) → _IOLBF, B (block, the default) → _IOFBF via setvbuf on stdout and stderr. Garbage values are rejected. outbuf config field (OutbufMode). Client-only, never crosses the wire
--address=ADDRESS	Bind address for outgoing socket	✅ Parity	Binds the outgoing client socket to a local source address before connect() (resolved with the same -4/-6 family hints as the destination). Local socket concern: never crosses the wire
-4, --ipv4	Prefer IPv4	✅ Parity	Forces AF_INET in the getaddrinfo hints for client destination/source resolution and the server bind (see the Phase 5, Wave B note). Mutually exclusive with -6
-6, --ipv6	Prefer IPv6	✅ Parity	Forces AF_INET6 in the getaddrinfo hints for client destination/source resolution and the server bind. Mutually exclusive with -4
--remote-option=OPT, -M	Send an option only to the remote side	⚠️ Caveat	Each value is appended to the remote server invocation over SSH as an individually single-quote-escaped shell word in ssh_build_remote_command(). Values are validated (non-empty, no control characters) and shell metacharacters cannot break out of the quoting (;, &, |, </code>, $, (, ), quotes are neutralized), so a value cannot inject an arbitrary remote command and a subsequent --on the client line cannot be turned into one. The short-M form (-M OPT, -M=OPT, and rsync-style attached -MOPT) is available, matching rsync; metadata mode moved to long-only --preserve. **Divergence:** -M is only meaningful for the SSH transport (user@host:path); a daemon (host::module/path`) or local TCP destination rejects it (there is no remote command line to append to), whereas rsync applies it to its own remote process on every transport. The options never cross the binary config frame

14. Daemon Mode

Flag	Rsync Description	FastSync Status	Notes
--daemon	Run as rsync daemon	⚠️ Caveat	Wave A: a real persistent listener. fastsync-server --daemon --config FILE (plus --no-detach to stay foreground; without it the listener detaches to the background after binding) reads a FastSync-native module config file and serves each connection confined to the requested module's path root (never a client-chosen root; every client-chosen-ownership/super-user request (--numeric-ids/--chown/--usermap/--groupmap/--fake-super/--copy-as/explicit --super) is refused unless the module opts in with client owner = yes, and the operator --no-super veto is honored). TCP/TLS via the existing --tls stack; plaintext still requires --allow-unauthenticated (same secure default as the standalone server). Client destinations use rsync's host::module/path form. Wire/protocol: the config frame gained a trailing daemon-module string and PROTOCOL_VERSION was bumped 2.14.0 → 2.15.0 (see the Daemon Mode notes below). Daemon mode is built in FastSync's own protocol/config grammar, not rsync's SMB/daemon option encoding
--config=FILE	Alternate rsyncd.conf file	⚠️ Caveat	Wave A: selects the daemon config file. Default when omitted (in --daemon mode): ~/.config/fastsync/fastsyncd.conf if it exists, else /etc/fastsyncd.conf. The grammar is FastSync-native (documented in the Daemon Mode notes below) and strictly rejects unknown keys so a typo can never silently change what a module serves; requires --daemon
--dparam=OVERRIDE	Override global daemon config	⚠️ Caveat	Wave A: overrides one global scalar from the command line (--dparam port=8734 and --dparam=KEY=VALUE both work). Limited to the global keys the grammar defines (port, motd file, address, max connections, max connections per host, auth failure delay, auth lockout threshold, auth lockout duration, hosts allow, hosts deny); keys are case-insensitive and unknown keys/invalid values are rejected. Requires --daemon
--no-detach	Don't detach from parent	✅ Parity	Wave A: with --daemon, keeps the listener in the foreground (what integration tests use). Without it the daemonizes (fork/setsid, stdio redirected to /dev/null) after the listening socket is bound. Requires --daemon
--password-file=FILE	Read daemon password from file	⚠️ Caveat	A7 daemon auth. Client: --password-file supplies user:password for a host::module/path destination (the username is taken from this file, so user@host::module stays rejected); the literal password is held client-side only for the SCRAM handshake and wiped at teardown. Server (fastsync-server --daemon --password-file FILE): the salted-PBKDF2 verifier store that modules with auth users are verified against. Neither the password nor any replayable bearer value crosses the wire or is stored server-side — the store holds a per-user salt plus derived keys, and the daemon proves the secret with a per-connection nonce challenge. The file must be private to its owner: both the client and server verify the exact inode they read (open-then-fstat, so the check cannot be raced) and refuse a --password-file/--early-input that is not owned by the current user or grants any group/other permission bit (mode 0600), mirroring the TLS private-key check. A process-substitution pipe (--early-input <(vault ...)) is still accepted when it satisfies those checks. See the Daemon Mode notes below for the file formats and the plaintext/TLS caveat
--early-input=FILE	Use FILE for daemon early exec	⚠️ Caveat	Server-only (requires --daemon): a second credential-store file, same new-format grammar as --password-file, read before the listener accepts connections (a secrets-manager / process-substitution source). Its entries layer over --password-file: byte-identical verifiers dedupe, a conflicting verifier for the same user is a startup error. A daemon whose modules declare auth users must be given at least one of the two, or it refuses to start (fail closed)
--hash-credentials=FILE, --iterations N	Hash a plaintext credential file	⚠️ Caveat	Server-only offline tool (A7): reads the user:password lines of FILE (same owner-only 0600 check) and prints one new-format store line per entry to stdout, then exits. --iterations sets the PBKDF2 work factor (default 600000, range 100000–10000000). Dependency-free and does not run a listener. Use its output as --password-file for --daemon. There is no auto-upgrade: a legacy store line is hard-rejected by the loader and must be regenerated

Daemon Mode notes (Wave A protocol 2.15.0; A7 auth protocol 2.19.0; MOTD no bump): FastSync daemon mode is supported in FastSync's own protocol/config grammar, not rsync's SMB/daemon option encoding.

• Config grammar (fastsyncd.conf): line-based; an implicit global section first, then [module] sections. Keys are case-insensitive, values are trimmed and may be wrapped in one layer of double quotes (path = "/srv/my dir"). # and ; at the start of a line (after leading whitespace) are full-line comments; inline comments and \ continuations are not supported. Lines are bounded (4096 chars), and at most 256 [module] sections are accepted. Global keys: port (default 873), motd file (the daemon sends its bounded, escaped content to a client after the module gate/auth accepts, unless the client passes --no-motd), address (optional bind address), max connections (positive integer cap on concurrent connections, default 100; 0/negative/garbage is a parse error), max connections per host (concurrent-connection cap per source IP, default 0 = unlimited), auth failure delay (milliseconds to sleep after a failed authentication, default 500; 0 disables, capped at 5000), auth lockout threshold (failed authentications from one source before lockout, default 10; 0 disables), auth lockout duration (seconds a locked-out source is refused, default 300), hosts allow and hosts deny (comma- and/or whitespace-separated host access patterns — see the host access control note below). Module keys: path (required; the daemon-side authorized root for that module), read only (yes/no/true/false/1/0, default no), client owner (yes/no/true/false/1/0, default no; opts the module into client-chosen ownership — see below), auth users (comma list), max connections (optional per-module cap, 0 = unlimited; enforced across all connection children), hosts allow/hosts deny (per-module host access lists). Unknown keys and malformed lines are parse-and-reject errors (never silently ignored), so a typo cannot change what a module serves.
• Host access control (hosts allow/hosts deny): both keys accept a comma- and/or whitespace-separated list of patterns and may appear globally and/or per module (multiple config-file lines append; a --dparam override replaces). Supported patterns are * (match all), an IPv4 or IPv6 literal (10.0.0.1, 2001:db8::1), and an IPv4/IPv6 CIDR (10.0.0.0/8, 2001:db8::/32). Hostname patterns are not supported: because the peer is always a numeric address and no reverse DNS is performed, a hostname/glob pattern would silently never match, so it is rejected at load time (fail-closed) instead of being accepted as a dead rule. An IPv4 peer on a dual-stack IPv6 listener is normalized from its ::ffff:a.b.c.d form so IPv4 patterns match it. rsync-like semantics: a matching hosts deny rejects; if any hosts allow entries exist, a peer matching none of them is rejected; deny takes precedence over allow. The daemon enforces the global list first, then the selected module's list, before authentication in server_module_gate, with an audit log line naming the peer, the module and the outcome. The numeric peer address is obtained with getpeername+inet_ntop (utils_fd_peer_ip, handling both address families); when it cannot be obtained a module with any ACL fails closed (refused), while an ACL-free module continues and logs at debug. A malformed pattern (e.g. an out-of-range CIDR prefix) is a parse error at load time.
• Connection caps, shared registry and auth lockout: the global max connections key (default 100) is plumbed into the listener (transport_tcp.c), which rejects a connection once the accept-loop parent's active-child count reaches it; the IPv4/IPv6 peer is logged for every accepted connection. Because the listener forks one child per connection, the per-module max connections cap, the global max connections per host cap, and the auth-failure counter live in a fixed-size registry carved from an anonymous shared mapping (daemon_limits.c, mmap(MAP_SHARED|MAP_ANONYMOUS)) created by the parent before the accept loop, so every forked child shares the same counters (C11 atomics only — never a pthread lock, which can deadlock in a forked child). The parent reserves a registry slot per accepted connection and the child records the selected module and source IP once known; the parent's SIGCHLD handler reclaims the slot when the child dies (including SIGKILL) and re-derives the per-module and per-source occupancy counts from the surviving REGISTERED slots, so a child killed mid-registration cannot leak a count. The per-source table has a bounded lifetime: an entry with no live connection is reclaimed after its lockout expires or it has been idle (300 s); if the table is genuinely full the per-source cap/lockout fails open for new sources (per-module cap and ACLs still apply) with a rate-limited warning. The per-module cap (0 = unlimited) is enforced after the module lookup and before auth; per-source identity reuses the normalized numeric peer address (utils_fd_peer_ip, IPv4-mapped IPv6 collapsed to IPv4), and a trusted loopback peer (127.0.0.0/8 / ::1, utils_fd_peer_is_local) is exempt from the per-source cap and the auth lockout because all local clients share one address (the per-module/global caps still apply). Clients behind a shared NAT/proxy address likewise share one per-source budget and lockout counter. A failed authentication increments the shared per-source failure count and, once auth lockout threshold (default 10; 0 disables) is reached, the source is refused for auth lockout duration seconds (default 300) before any challenge is sent, even when the next attempt is handled by a different forked child; a successful authentication clears the counter. On a failed authentication the per-connection child still sleeps the global auth failure delay (default 500 ms, 0 disables, capped at 5000) via nanosleep, rate-limiting online guessing without delaying a success. A missing registry (allocation failure) degrades to the global cap and host ACLs rather than refusing to start.
• Module selection & confinement: the client requests a module with an rsync-style host::module[/path] destination. The module name crosses the wire as a trailing string on the config frame (bumping PROTOCOL_VERSION 2.14.0 → 2.15.0; the bump is required because the config-frame layout changed and the strict same-version handshake is what prevents a peer from desynchronizing on the new trailing field). The daemon looks the module up in ITS OWN config and uses the module's path as the authorized root through the exact same configure_authorization confinement the standalone server applies to --destination-root (file_open_secure_parent, has_path_traversal, path_is_within); the client never supplies the root, every client-chosen-ownership/super-user request is refused unless the module declares client owner = yes (the daemon's per-module opt-in, see below), and the operator --no-super veto forces super-user activities off for every daemon connection. The client's /path part is relative inside the module and is rejected if absolute or if it contains ... Unknown modules are refused before any data moves (the run fails cleanly at the config handshake). An absolute destination and a module request against a non-daemon server are also refused.
• client owner (client-chosen-ownership opt-in): by default a daemon module refuses every request that would let the client pick an owner or ask for super-user activities — --numeric-ids, --chown, --usermap/--groupmap, --fake-super, --copy-as, and an explicit --super — at the config handshake (before STATUS_OK), because a daemon has no per-module opt-in for client-chosen ownership and any anonymous client could otherwise force arbitrary owner ids inside the module root. A plain preserve-source request (-a/-o/-g) is not refused: the module forces super-user activities off for that connection, so no ownership is applied, and it logs a warning that the requested ownership will not be applied (the transfer itself still succeeds). client owner = yes opts a single module in, allowing those requests within that module's root (a root standalone TCP listener honors them for its single operator-authorized root only when started with --allow-super; the flag is rejected with --stdio, whose client-composed remote argv must never opt back into super mode). Without the opt-in the daemon also forces super-user device activity off for that connection — char/block device-node creation (--devices) and --write-devices — even under the default AUTO mode, so a non-opted module can never be made to mknod or write a raw device; those entries are skipped (not refused) so an ordinary -a push still succeeds without device nodes. The opt-in does not lift the privilege requirement: --copy-as still needs a root receiver, and the operator --no-super veto still forces super-user activities off for every connection. The daemon logs a prominent startup warning for each client owner = yes module so the operator's deliberate choice is visible.
• read only safe default: every network transfer FastSync currently supports is a push that writes under the module root, so a read only module refuses the connection (clear server log "module is read only"; the client exits non-zero, nothing is transferred). A future pull/list operation can be opened up when it exists; the knob is already stored.
• Direction — remote source / pull is intentionally unsupported: FastSync is push-only. The first positional argument is always a local source directory and the second is the destination; only the destination is parsed for remote syntax (user@host:path SSH, host::module[/path] daemon). A remote source such as fastsync user@host:src ./local is deliberately not implemented: rsync has no pull flag (direction is positional), so supporting a remote source is an optional feature rather than a compatibility requirement, and it would require a protocol role reversal (server as sender, client as receiver) across both transports. FastSync documents this as an intentional limitation rather than a missing rsync option.
• auth users (A7 SCRAM-SHA-256 authentication): a module that declares auth users requires the client to present credentials. The config frame carries ONLY the username; the daemon answers an auth-required module with STATUS_AUTH_CHALLENGE (PBKDF2 iteration count, 16-byte salt, 32-byte server nonce), the client answers with STATUS_AUTH_RESPONSE (fresh 32-byte client nonce + a 32-byte ClientProof), and the daemon accepts only when the proof verifies and the username is on the module's auth users list and has a store entry, replying STATUS_AUTH_OK with a 32-byte ServerSignature the client verifies before proceeding. Verification is constant-time over fixed 32-byte keys (the compare runs even for a miss), username membership uses a constant-time full-length scan, and an unknown/off-list user still receives a challenge and runs the same math against a dummy verifier: a deterministic per-username salt (HMAC-SHA256(store dummy key, username)), the store-wide uniform iteration count and dummy keys. Re-probing the same unknown username therefore yields an identical salt and iteration count while a different username yields a different salt, so there is no user-enumeration or timing oracle. The daemon logs the username but never the password, proof or keys. A module WITHOUT auth users stays open (legitimate rsync configuration); credentials sent to such a module are ignored. Read-only is orthogonal: even a correctly authenticated push to a read only module is still refused (all FastSync network transfers write). Fail-closed policy: a daemon whose config declares auth users on any module refuses to start unless a credential store was given (--password-file and/or --early-input); a missing or empty store is never silently treated as "open". A failed handshake (missing credentials, unknown/off-list user, wrong proof or malformed data) yields a single generic STATUS_AUTH_FAILED and the daemon closes before any data moves. The dummy key is persisted in an owner-only <store_path>.dummykey sidecar (auto-created on first load, mode 0600) so the dummy salt stays stable across daemon restarts, closing the restart-gated enumeration channel. The sidecar is secret material and must be protected like the credential store (owner-only 0600, included with the store in backups and rotation). It must be preserved across restarts for that guarantee; if it cannot be created (a process-substitution/FIFO store path such as /dev/fd/N, a read-only filesystem, a missing directory, or a create/write/fsync/link/fchmod failure), the daemon logs a warning and uses a transient per-run key, so unknown-user challenges change across restarts and the cross-restart guarantee does not hold for that deployment. One residual is accepted: the store iteration count is observable pre-auth by design, since the miss path must match a hit. Transport policy (hardening A7-3/S1): an auth-required module accepts credentials only when either (a) the connection is an encrypted, verified TLS connection whose client certificate matches --client-cn, or (b) the connection is plaintext from a loopback TCP peer and the operator explicitly passed --allow-unauthenticated. A remote plaintext peer, and a loopback plaintext peer without that flag, are refused at the config gate before any challenge is sent; --allow-unauthenticated never permits remote plaintext auth (remote peers still require verified TLS). Daemon modules are a --daemon-only feature — the SSH --stdio path never loads a daemon config and is not an auth transport for them. Because the loopback allowance trusts whichever peer the kernel reports as 127.0.0.1, it assumes nothing relays remote connections to the daemon: a local TCP forwarder or TLS-terminating proxy in front of an auth-module listener makes remote clients appear as loopback and bypasses the mutual-TLS identity check, so do not front an auth-module listener with such a relay.
• Credential store format: server --password-file/--early-input files are line-based user:$fastsync$1$pbkdf2-sha256$<iters>$<salt_b64>$<stored_key_b64>$<server_key_b64>, one per line (standard base64; 16-byte salt, 32-byte keys; iters in [100000, 10000000], default 600000). Every entry in the resulting store must agree on iters (a store whose entries disagree, or where a layered --early-input disagrees with --password-file, is rejected). Generate lines with fastsync-server --hash-credentials FILE [--iterations N]; the emitted lines are secret material, so redirect them to an owner-only (mode 0600) file (the tool warns on stderr if stdout is a group/other-accessible regular file). Blank lines and lines starting with #/; are comments; the parser is strict (a malformed line fails the whole load, so a typo can never let a different set of users in). The legacy user:SHA256HEX form is hard-rejected with an actionable "legacy" error; there is no auto-upgrade, so a replayable bearer digest can never be loaded by a 2.19.0 daemon. The client --password-file holds user:password on its first meaningful line (the literal password, used only for the handshake then burned); keep both files readable only by their owner (mode 0600). Per-username wire length is bounded (256 chars) and every decoded salt/key length is validated. Loading the store also maintains an owner-only <store_path>.dummykey sidecar (auto-created, mode 0600, exactly 32 bytes) holding the store-wide dummy key that shapes unknown-user challenges; persist it across daemon restarts so those challenges stay stable, and treat a sidecar with the wrong owner, a mode other than exactly 0600, the wrong size or the wrong type as a fatal load error (fail closed). If the sidecar cannot be created (e.g. a process-substitution store path such as /dev/fd/N, a read-only filesystem, a missing directory, or a create/write/fsync/link/fchmod failure), the daemon logs a warning and uses a transient per-run key, so the cross-restart stability guarantee does not hold there.
• Plaintext caveat: an auth-required module is refused, before any challenge is sent, unless the connection is encrypted and verified TLS whose client certificate matches the server's --client-cn, or it is plaintext from a loopback TCP peer and the operator passed --allow-unauthenticated. A remote plaintext peer, and a loopback plaintext peer without that flag, never receive a challenge, and --allow-unauthenticated never permits remote plaintext auth (remote peers still require verified TLS). On the loopback plaintext transport that remains permitted, a local sniffer could still read the challenge and response and mount an offline dictionary attack against a weak password, so use --tls for any real deployment. --client-cn matches the certificate CN only (not a subjectAltName), which is acceptable for a private CA. Clients sending daemon credentials with --password-file to a non-loopback daemon must use --tls; the client rejects such a destination before any network I/O. Unlike the old challenge-less exchange there is no replay: the proof is bound to the fresh per-connection server nonce, so a captured STATUS_AUTH_RESPONSE cannot be reused on another connection (an integration test proxies the daemon and proves this). TLS client-CN (--client-cn) is an independent transport identity check and composes with password auth; because --tls already mandates --client-cn, a TLS auth connection always verifies the client CN, so both checks necessarily apply together on such a connection.
• Wire/protocol: the config-frame auth block is now [int present][str_redacted username] (the old digest field is gone), and the frame stream gains the challenge/response (STATUS_AUTH_CHALLENGE → STATUS_AUTH_RESPONSE → STATUS_AUTH_OK/STATUS_AUTH_FAILED) between the config frame and the STATUS_OK ack. Both are wire-layout changes, so PROTOCOL_VERSION is bumped 2.18.0 → 2.19.0 (see the A7 note in src/shared/config.h); the strict same-version handshake keeps a 2.19 client and a 2.18 server from desynchronizing.
• Client side: host::module/path selects the TCP transport and connects to --server-port; host:path stays the SSH transport; plain paths stay local TCP. The daemon username comes from --password-file (first user:password line), and --password-file without a host::module/path destination is a client error (fail fast). A user@host::module form is rejected with a pointer to --password-file. The client's plaintext password is wiped from memory (config_burn_auth) at transfer teardown.
• MOTD (Wave C): a daemon configured with a global motd file sends that file's content as the first server→client string frame after the config-frame STATUS_OK ack (rsync sends the MOTD as the first thing from the server at the start of a daemon connection). Only the daemon listener path (host::module) gets a MOTD; the --stdio SSH path never sends or reads one. The server reads the file bounded to 4096 bytes and treats an absent/unreadable file as "no MOTD" (an empty frame, never an error). The exchange is server→client only and does not bump PROTOCOL_VERSION: every 2.15.0 daemon client reads the frame after the ack, so sender and receiver stay in lockstep (see the Wave C note in src/shared/config.h). --no-motd is the client-side suppression switch: the client still reads (consumes) the frame to keep the stream in sync but does not display it. The MOTD is printed to stdout with control bytes (ESC included) escaped octal-style while newlines/tabs are preserved, so a hostile server cannot inject terminal escape sequences.
• Merge note: the Wave A module bump (2.15.0) and the MOTD wave did not bump the version, but the A7 auth redesign is a genuine wire-layout change and owns the 2.18.0 → 2.19.0 bump (see the A7 note in src/shared/config.h).

15. Safety & Security

Flag	Rsync Description	FastSync Status	Notes
Path escape detection	Ensure files stay within root	✅ Parity	has_path_traversal() + realpath
Symlink-safe delete	Skip symlinks in delete walk	✅ Parity	delete_extras_walk()
Protocol version check	Verify compatible versions	✅ Parity	config_receive()
Max data/string/chunk sizes	Prevent OOM attacks	✅ Parity	Per-message limits
Per-connection memory limit	Cap memory per connection	✅ Parity	MAX_CONNECTION_MEMORY is 256 MiB per connection (256 * 1024 * 1024 bytes), charged across protocol reservations and decompression/chunk allocations. This is a FastSync-internal bound with no direct rsync analogue
--max-alloc=SIZE	Limit a single memory allocation	✅ Parity	Caps the largest single allocation; binary units, default 1G
--trust-sender	Trust remote sender's file list	⚠️ Caveat	Long-form-only, receiver-local policy that never crosses the wire. The receiver skips its redundant up-front re-validation of the incoming file list (empty/.. path rejection), trusting the sender instead of double-checking (fewer checks, faster, potentially unsafe, matching rsync). Off by default. It no longer affects symlink targets (protocol 2.23.0): targets are stored verbatim under -l regardless of --trust-sender; the flag only relaxes the receiver's path-list checks. The low-level fd-relative confinement primitives (file_open_secure_parent, the O_NOFOLLOW parent walk, leaf/destination confinement) are deliberately KEPT even under --trust-sender, so a hostile sender still cannot write or link outside the authorized root (see Phase-5 notes below)
--old-args	Disable modern arg protection	⚠️ Caveat	SSH-only; accepted for CLI compatibility but is now a documented no-op: FastSync always single-quote-escapes the remote server path and each --remote-option value (ssh_build_remote_command), so a metacharacter-bearing --rsync-path can never be interpreted by the remote shell. The flag no longer disables that quoting (the old raw-construction behavior was an injection foot-gun and is removed); the safety-relevant behavior is identical either way
--ignore-missing-args	Ignore missing source args	⚠️ Caveat	FastSync has a single source-root argument (which always exists), so the "explicitly requested source arguments" are the --files-from entries and the flags only ever apply there (inert without --files-from, like -R). Without the flag a listed-but-missing entry stays a hard pre-transfer error (nothing is transferred). With it each missing entry is skipped: nothing is sent for it, it never enters the keep-set, and the run succeeds for the rest — an all-missing non-empty list succeeds transferring nothing, matching rsync. --dirs + --files-from missing entries are skipped the same way. Every skipped entry is logged and a per-run warning names the count, so the handling is never a silent no-op. Divergences: an EMPTY --files-from file stays a hard error in every mode (no argument was requested at all; rsync likewise reports "no source files specified"); missing-arg skipping only applies to the pre-transfer list validation, so an entry that is present at preflight and vanishes mid-transfer still fails (matching rsync, whose flag "does not affect subsequent vanished-file errors"); --no-ignore-missing-args is not a supported negation
--delete-missing-args	Delete missing source args	✅ Parity	Implies --ignore-missing-args (order-independent) and additionally removes each missing entry's destination mirror receiver-side. The mirror is computed exactly like a present sibling's wire path: the bare relative entry under -R, otherwise the full source-mirror path below the destination root. rsync parity, verified against the man page: it does not imply --delete generally and is "independent of any other type of delete processing" — unrelated destination extras are untouched unless --delete is also present. Composition with --delete + timing: the exact-path deletions commit with the manifest, early for --delete-before/--delete-during, else only after a fully-successful transfer (delete-after/commit). A non-empty directory mirror is removed only when --force or --delete is in effect (otherwise it is left with a warning and the run continues, like rsync); an absent mirror is a no-op. --force is deletion authority and is therefore gated by the server --allow-delete policy exactly like --delete/--delete-missing-args: without it the receiver clears the flag, so a client cannot use --force to recursively replace or remove a destination directory tree. An explicitly listed missing arg is a user request, not an excluded file: its deletion is never blocked by the filter-exclusion protection of excluded destination mirrors (a mirror sitting inside a filter-excluded directory is still removed). Safety/policy: gated by the server --allow-delete policy like --delete; the request paths cross the wire only in the delete-manifest frame and are confined by the same receiver validation as the keep-set (non-empty, relative, traversal-free, bounded by the per-section/per-frame manifest caps); the --delay-updates staging directory and basis snapshots are protected exactly as in the extras walker. Protocol 2.23.0 parity: the missing-args exact-path removals and the ordinary extras walk draw from one shared --max-delete budget, so a capped run stops part-way and exits 25 exactly like rsync. See the Phase-3 wire note below for the PROTOCOL_VERSION bump

16. Batch Operations

Flag	Rsync Description	FastSync Status	Notes
--write-batch=FILE	Write batched update to file	⚠️ Caveat	Phase-6 residual-batch (client-only): runs the normal live transfer AND additionally emits a self-contained single-file batch of the whole source tree. The batch is a magic/format-version header followed by length-prefixed chunk_serialize blobs (full file images), replayable byte-identically by --read-batch on another machine with no source/server. --write-batch drives the single-threaded transfer path (the multithreaded path consumes the config before the separate batch scan pass). See the Phase-6 batch note below
--only-write-batch=FILE	Write batch without updating dest	⚠️ Caveat	Phase-6 residual-batch: emits the self-contained batch FILE only — NO destination update, NO server connection. Requires a source (scans it and serializes the full tree to FILE). Same single-file format as --write-batch, so the file is re-appliable via --read-batch=FILE DEST. See the Phase-6 batch note below
--read-batch=FILE	Read batched update from file	⚠️ Caveat	Phase-6 residual-batch: applies a previously written batch FILE locally to the destination. NO source and NO server — positional args are the destination only. Reads the magic/version header, then length-prefixed records, chunk_deserialize, and applies each via the confined file_save_to_disk_full path (same O_NOFOLLOW / ..-rejection / root-confinement as the network receiver, so an attacker-controlled batch cannot escape the destination root). Malformed/truncated/oversized/traversal records are rejected cleanly. See the Phase-6 batch note below

17. Advanced

Flag	Rsync Description	FastSync Status	Notes
--stop-after=MINS	Stop after N minutes	✅ Parity	Client-only sender stop deadline (Phase 6): computing --stop-after=MINS (a positive minute count; 0/negative/garbage rejected) and --stop-at=TIME (HH:MM, HH:MM:SS, or now+N[smhd]; a past time stops immediately). The transfer stops ELEGANTLY at the next chunk boundary: everything already fully sent is kept and applied, the run returns 0, and --delete (late/delete-after timing) does NOT wipe the destination — when the scan is cut short the partial keep-set manifest is suppressed with a warning (the delete walk is skipped rather than acting on an incomplete keep-set, so unscanned source mirrors survive). --delete-before/--delete-during still run their complete pre-scan (which ignores the deadline). Local client-only fields: never serialized into the wire config frame, so no PROTOCOL_VERSION bump. --stop-after uses CLOCK_MONOTONIC; --stop-at uses the wall clock. Works single-threaded and under -j/--threads (multithreaded). Divergence: rsync computes --stop-after from the run start; FastSync likewise. When both are given, the earlier of the two deadlines wins (checked per iteration). See the Phase-6 stop notes below
--stop-at=TIME	Stop at specified time	⚠️ Caveat	Same feature as --stop-after (deadline transfer stop), absolute wall-clock form (HH:MM[:SS] or now+N[smhd]). See the row above and the Phase-6 stop notes
--fsync	Fsync every written file before publication	✅ Parity
--protocol=NUM	Force older protocol version	❌ Divergent	Forces the wire protocol version for this transfer. FastSync has exactly ONE wire format (PROTOCOL_VERSION, currently 2.23.0) with no downgrade/backward-compat code paths, so --protocol=2.23.0 is accepted (it sets the version claim the client sends, which the server already requires to match exactly) and every other value is rejected up front with a clear error before any connection — it does not and cannot speak an older or virtual wire format. Divergence from rsync (which negotiates a range and downgrades to an integer 0..31): FastSync's honest contract is force-to-the-one-supported-value; a genuine downgrade would require a per-version compatibility layer that does not exist. Client-only; the server-side exact-match check is unchanged. --protocol=2.21.0/2.20.0/2.19.0/2.18.0/2.18/2.17.0/2.16.0/2.15.0/216/31/garbage are all rejected. See the Phase-6 protocol note below
--iconv=CONVERT_SPEC	Charset conversion	⚠️ Caveat	Charset conversion of FILE NAMES (not content) at the protocol boundary via iconv(3): --iconv=LOCAL[,REMOTE] — the sender converts each local filename LOCAL→REMOTE before transmitting, and the receiver converts each wire filename REMOTE→LOCAL before creating/writing. The full CONVERT_SPEC is serialized into the config frame as a new trailing string field so the peer knows the wire charset; PROTOCOL_VERSION bumped 2.15.0 → 2.16.0. LOCAL[,REMOTE] parse: single charset ⇒ LOCAL==REMOTE (identity both ways); garbage rejected up front. Validation probes BOTH directions (a spec that only opens one way is refused, as is a NUL-emitting target charset like utf-16/utf-32/ucs-2, since filenames cannot contain NUL). An unrepresentable name (EILSEQ/EINVAL) fails that path cleanly with a logged --iconv: cannot convert file name ... and is never written mangled/truncated. Conversion is applied at EVERY wire-path site (regular/MKDIR/hardlink path+target/symlink path+target/SPECIAL, the delete manifest, the incremental-check path, and the -s/chunk_serialize embedded blob path), on both client and server (--iconv is also a server/daemon option). Zero overhead when unset. See the Phase-6 iconv notes below
--checksum-seed=NUM	Set checksum seed	✅ Parity	Sets the seed for FastSync's whole-file xxHash digest (full 64-bit seed) and for the delta path's per-block xxHash32 strong checksum (low 32 bits of the seed). As of protocol 2.23.0 a seed of 0 — the default when the flag is unset — is randomized per transfer and the chosen seed is sent to the receiver, exactly like rsync, so two runs against different content do not share a predictable seed; an explicit non-zero seed is used verbatim, so an explicit seed deterministically reproduces every computed digest on BOTH endpoints (the seed crosses in the config frame). --checksum-choice=md5 has no seed and ignores it (documented). The value is a strict decimal 0..2⁶⁴-1 (blank, signed, or non-numeric values are rejected). Like rsync, a seed only matters where a digest is actually computed (--checksum or a basis-dir run, or a delta transfer); it does not by itself enable --checksum/--delta
--secluded-args, -s	Use protocol to send args	❌ Divergent	Accepted for CLI compatibility (including the rsync short -s, Phase 7 Wave A) but a documented no-op / divergence. rsync's -s protects arguments from shell expansion by shipping them over the protocol; FastSync never passes remote arguments through a shell expansion boundary in the first place — its SSH transport builds the remote argv as single-quote-escaped shell words (ssh_build_remote_command), so the injection/leak that -s guards against does not exist and there is nothing to "seclude". Implementing a true arg-send protocol would mean replacing the argv-based SSH launch with an in-band argument channel, a large redesign of the transport that buys no security here. Chunk serialization remains the long-only --chunk-serialization.
--no-OPTION	Turn off implied option	✅ Parity	Supported boolean FastSync options and archive-implied options; unsafe or value-taking options are rejected.

---

Implementation Difficulty Plan

Phase 5 notes (remote-option wave): --remote-option=OPT (long form only) and --trust-sender landed here.

• --remote-option is CLIENT-only and never serialized into the binary config frame. On the SSH transport the client forwards each value to the remote server by appending it to the remote command line in ssh_build_remote_command(), after --stdio, as an individually single-quoted shell word ('...' with '\'' for embedded quotes). Values are validated at CLI parse time (non-empty; no ASCII control characters) and rejected otherwise, and a non-conforming value is refused again in the command builder, so shell metacharacters (;, &, |, backticks, $(), quotes) can never break out of the quoting to inject an unrelated remote command — including after a client-side -- separator, whose arguments are never forwarded anyway. Because the remote options affect the remote server invocation, not the transmitted config, the wire frame layout is unchanged, but PROTOCOL_VERSION was bumped 2.13.0 → 2.14.0 as the Phase-5 lockstep release marker (a 2.14 client against a 2.13 server fails the version check cleanly rather than the old server rejecting an unfamiliar forwarded argv later). Divergence: rsync's short -M form of --remote-option was intentionally NOT implemented at that time because -M was FastSync metadata mode; Phase 7 Wave A later freed -M for --remote-option and moved metadata to long-only --preserve (see the Sending Options table).
• --trust-sender is a receiver-local policy: it never crosses the wire (the sender's value is never serialized, so a wire peer can never enable it). On the receiving process it skips the up-front re-validation of the incoming file list (empty/.. path rejection), trusting the sender's list instead of double-checking — fewer checks, faster, and potentially unsafe, matching rsync. It is OFF by default (config.trust_sender). Since protocol 2.23.0 it does not gate symlink-target handling: -l stores targets verbatim either way. As a deliberate safety floor, the low-level fd-relative confinement primitives are NOT disabled: file_open_secure_parent() (O_NOFOLLOW walk, .. rejection, root containment) and leaf/destination confinement still hold, so even under --trust-sender a hostile sender cannot write or place a path outside the authorized root — the relaxation only removes the redundant list-layer double-checks, never the root-confinement guarantees for paths and placements.

The estimates below cover the currently unimplemented features in this document. They assume one engineer familiar with the codebase, include implementation and focused tests, and exclude production rollout time. A feature should not be marked implemented until its behavior is tested in both local and SSH/TCP paths where applicable.

Note: This plan is a superset snapshot written while several of the listed features were still outstanding. The Summary matrix above is the authoritative record of what is already shipped (for example quiet/info/debug output, --existing, --remove-source-files, -h, and --size-only are now implemented on dev). Treat the phases as sequencing guidance for the work that remains unimplemented.

Effort	Typical duration	Meaning
XS	0.5-1 day	CLI alias or a local formatting/validation change
S	1-3 days	Isolated behavior with little or no protocol change
M	3-7 days	Cross-cutting client, server, or scanner behavior
L	1-3 weeks	Protocol, filesystem, privilege, or compatibility work
XL	3+ weeks	New transfer mode, daemon subsystem, or broad interoperability effort

Phase 1: Low-Risk CLI and Local Behavior

These are the best first changes because they require limited wire-format work and can be tested with existing transfer fixtures.

Features	Effort	Implementation plan
--quiet, -q; --human-readable, -h; --8-bit-output, -8; --stderr=MODE; --info=FLAGS; --debug=FLAGS	S	Extend logging and output formatting without changing transferred data.
--no-OPTION; --old-args; --secluded-args, -s	M	Add option implication/negation and safely serialize or protect remote arguments. -s currently has FastSync-specific semantics and needs a compatibility decision.
-P; --del; --old-dirs, --old-d; --cc; --zc; --zl	XS	Add aliases and composed behaviors after the underlying options exist.
--whole-file, -W; --ignore-times, -I; --size-only; --modify-window, -@; --update, -u	S	Extend the existing incremental comparison decision.
--existing; --ignore-existing; --remove-source-files	S	Add scanner/receiver eligibility checks and remove successfully synchronized source files.
--executability, -E; --chmod=CHMOD	M	Apply permission transformations safely while preserving current metadata behavior.
--skip-compress=LIST; --compress-threads=NUM	S	Make compression selection configurable and validate the thread setting against zstd behavior.
--max-alloc=SIZE; --fsync	S	Reuse existing allocation limits and add an explicit durability step after file writes.

Phase 2: Filesystem Selection and Update Semantics

These features are moderate because they affect traversal, temporary files, manifests, or the receiver's update policy.

Features	Effort	Implementation plan
--one-file-system, -x	M	Track the source device during scanner traversal and skip mount-point crossings.
--relative, -R; --no-implied-dirs; --dirs, -d; --mkpath	M	Extend path-list construction and destination directory creation while preserving traversal safety.
--temp-dir, -T	M	Separate temporary-file placement from FastSync's timeout alias and define collision, permissions, and cleanup rules.
--delay-updates	L	Stage all successful updates and publish them at completion, including crash and cancellation cleanup.
--files-from=FILE; --from0, -0; --filter=RULE, -f; -F; --cvs-exclude, -C	L	Build a complete filter/parser layer and integrate it with scanner pruning, manifests, and delete behavior. -f conflicts with FastSync sendfile mode.
--list-only; --itemize-changes, -i; --out-format=FORMAT; --log-file-format=FMT	M	Add a structured change-event model so output modes share one source of truth.

Phase 3: Deletion, Comparison, and Delta Compatibility

These features require careful interaction with manifests, incremental checks, backups, and the existing delta protocol.

Features	Effort	Implementation plan
--delete-during; --delete-before; --delete-after; --delete-delay; --del	L	Add deletion timing to the transfer state machine and ensure failures cannot remove files unexpectedly.
--delete-excluded; --max-delete=NUM; --ignore-errors; --force; --prune-empty-dirs, -m	M	Extend delete walks with policy limits, error handling, empty-directory pruning, and the -m short-flag conflict.
--ignore-missing-args; --delete-missing-args	M	Distinguish missing source arguments from traversal errors and apply explicit deletion policy.
--compare-dest=DIR; --copy-dest=DIR; --link-dest=DIR	L	Add alternate basis roots and hard-link handling, including metadata and cross-filesystem failures.
--fuzzy, -y; --no-fuzzy	L	Index candidate files and select a safe similar basis without making transfer time unbounded.
--append; --append-verify	M	Negotiate file length and verify the retained prefix before resuming.
--checksum-choice=STR, --cc; --checksum-seed=NUM	M	Negotiate checksum algorithms/seeds and preserve compatibility with existing xxHash checks.

Phase 4: Metadata, Links, and Devices

These features are platform-sensitive and need Linux permission, ACL, xattr, and special-file integration tests.

Features	Effort	Implementation plan
--numeric-ids; --usermap=STRING; --groupmap=STRING; --chown=USER:GROUP	L	Define identity mapping, privilege failures, and wire representation before applying ownership.
--open-noatime; --atimes, -U; --crtimes, -N; --omit-dir-times, -O; --omit-link-times, -J	L	Extend metadata capture/apply with platform capability checks and explicit unsupported-attribute handling.
--acls, -A; --xattrs, -X; --fake-super	XL	Add portable serialization, size limits, privilege behavior, and security tests for ACL/xattr data.
--hard-links, -H	L	Preserve inode relationships across the file list and coordinate hard-link creation order.
--munge-links; --copy-dirlinks, -k; --keep-dirlinks, -K	L	Define symlink trust boundaries and receiver-side directory/link collision behavior.
--devices; --specials; -D; --copy-devices; --write-devices	XL	Add privileged special-file handling with strict type, path, and authorization checks.
--super; --copy-as=USER[:GROUP]	XL	Requires a deliberate privilege model, identity switching, and refusal paths; do not implement by blindly elevating the process.
--preallocate	S	Use platform allocation APIs before writes and fall back cleanly when unsupported.

Phase 5: Connectivity and Daemon Compatibility

These options affect process startup, authentication, sockets, and remote execution. They should follow the filesystem and protocol work rather than being added as parser-only flags.

Features	Effort	Implementation plan
--rsh=COMMAND, -e; --rsync-path=PROGRAM; --blocking-io; --outbuf=N|L|B	M	✅ Wave A implemented (see the Connectivity table above). SSH argv construction is generalized: -e/--rsh replaces the hardcoded ssh program (whitespace-split, so -e "ssh -p 2222" works), --rsync-path aliases the existing fastsync_server_path, --blocking-io drops the SSH socket timeouts, and --outbuf maps N/L/B onto setvbuf. All four are client-only launch concerns and never cross the wire.
--address=ADDRESS; --ipv4, -4; --ipv6, -6; --sockopts=OPTIONS; --port=PORT daemon semantics	M	Add explicit socket-family/bind configuration and validate it independently for TCP client and daemon modes.

Phase 5, Wave B (socket/bind) shipping note: --sockopts adds a strict allowlisted OPT=VAL socket-option layer applied with correct per-option value types; --address binds the outgoing client socket to a local source address; -4/-6 pin the address family via getaddrinfo hints on both the client connect and the server bind; and the server bind now honors --address plus -4/-6 (falling back to the historical IPv4 INADDR_ANY when none are given). All of these are local socket concerns and none cross the wire config frame (only --port maps to server_port). | --remote-option=OPT, -M; --trust-sender | L | Add authenticated remote-option/config negotiation and reject unsafe sender-controlled values. -M conflicts with FastSync metadata mode. | | --daemon; --config=FILE; --dparam=OVERRIDE; --no-detach; --password-file=FILE; --early-input=FILE; --no-motd | XL | Implement a real daemon lifecycle, module configuration, authentication, privilege separation, and process management. |

Phase 5, Wave A (rsh/ssh) shipping note: the SSH transport no longer hardcodes ssh. -e/--rsh=COMMAND selects the remote-shell program (whitespace-split into the leading child argv words), --rsync-path=PROGRAM aliases --fastsync-server-path, --blocking-io removes the SSH-socketpair SO_RCVTIMEO/SO_SNDTIMEO timeouts (by default they now match the TCP transport so a wedged shell cannot hang forever), and --outbuf=N|L|B maps onto setvbuf (_IONBF/_IOLBF/_IOFBF, garbage rejected). All four are client-only launch concerns and never cross the wire.

Phase 5, Wave C (remote-option/trust-sender) shipping note (PROTOCOL 2.13.0 → 2.14.0): --remote-option=OPT (long form only; the short -M is intentionally left as FastSync metadata mode — documented divergence) appends each validated value to the remote server invocation over SSH as an individually single-quote-escaped shell word, so shell metacharacters cannot break out and a -- can never be turned into injection; options never cross the binary config frame. --trust-sender is a receiver-local policy (never serialized, so a wire peer can't enable it): when requested on the server (via --remote-option=--trust-sender), it removes only the redundant receiver/save-layer path re-checking; the low-level floor (file_open_secure_parent's .. rejection, the O_NOFOLLOW parent walk, leaf/destination confinement) stays enforced. Off by default. The wire config-frame layout is unchanged; the bump reflects that a 2.14 sender composing remote options requires a 2.14 receiver to honor them.

Phase 6: Batch, Encoding, and Protocol Interoperability

These are the hardest compatibility items because they require durable formats or behavior that must interoperate with rsync itself.

Features	Effort	Implementation plan
--write-batch=FILE; --only-write-batch=FILE; --read-batch=FILE	XL	✅ Implemented (see the Batch Operations table and Phase-6 batch note below): a versioned self-contained single-file residual-batch format, persisted via the existing chunk codec, with replay, corruption, and partial-application safety tests
--protocol=NUM	XL	✅ Implemented (see the Advanced table and Phase-6 protocol note below): protocol-version forcing without weakening current validation; FastSync's single lockstep wire format means only the current PROTOCOL_VERSION is accepted, and everything else is rejected up-front
--iconv=CONVERT_SPEC	L	✅ Implemented (see the Advanced table and Phase-6 iconv notes below): filename charset conversion at the wire boundary with expansion/overflow safety and invalid-sequence test coverage
--stop-after=MINS; --stop-at=TIME	M	✅ Implemented (see the Advanced table and Phase-6 stop notes below): deadline propagation and safe early stop with --delete safety
--early-input=FILE; --password-file=FILE	M	Securely read startup credentials/input with permission checks and no secret disclosure in logs.

Phase 6, Wave A (stop deadline) shipping note: --stop-after=MINS and --stop-at=TIME are client-only sender stop deadlines. --stop-after takes a positive minute count (0/negative/garbage rejected); --stop-at takes HH:MM, HH:MM:SS, or now+N[smhd] (a past time stops immediately, a garbage spec is rejected at parse time). The deadline is computed once at the start of the transfer (CLOCK_MONOTONIC for --stop-after, wall clock via time() for --stop-at) and checked at every chunk boundary in both the single-threaded send_files loop and the multithreaded send_chunks_multithreaded path, and inside the scanner loops so a busy scan itself stops. When it fires, the transfer stops ELEGANTLY: the in-flight chunk completes, the existing completion tail runs (summary, disconnect), and the run returns 0 — exactly like rsync's clean early stop. Because the deadline is client-only and never crosses the wire config frame, no PROTOCOL_VERSION bump is required. The safety-critical interaction is with --delete: FastSync streams while scanning, so a deadline can cut the source scan short and yield a PARTIAL keep-set manifest; committing that would make the receiver delete destination mirrors of source files not yet scanned. So the sender tracks scan_stopped_early and, when it is true on the late/delete-after (--delete/--delete-after/--delete-delay) path, SUPPRESSES the keep-set manifest (logs a warning) so no deletion happens from an incomplete set — this is the safe direction (preserves data; the delete simply does not run). --delete-before/--delete-during are unaffected: their complete pre-scan runs before any data and ignores the deadline (a stop can be exceeded by that pre-scan). Under -j/--threads the stop is symmetric and the scanner thread's still-in-progress manifest appends can never race the tail because the tail does not read the manifest on the early-stop path.

Phase 6, Wave B (iconv) shipping note (PROTOCOL 2.15.0 → 2.16.0): --iconv=LOCAL[,REMOTE] converts file NAMES at the wire boundary (never content). The full CONVERT_SPEC is serialized into the config frame as a new trailing string field (empty→NULL canonicalized), so both ends share the same wire charset interpretation; this required the PROTOCOL bump because the frame is a strict ordered sequence and a peer that does not parse the new trailing field would desynchronize. Each end derives LOCAL (its own charset) and REMOTE (the wire charset): the sender opens LOCAL→REMOTE and converts every transmitted filename; the receiver opens REMOTE→LOCAL and converts every received filename before creating/writing. Conversion is applied at every wire-path site (regular/MKDIR/hardlink path+target/symlink path+target/SPECIAL, the delete manifest keep/protected/missing entries, the incremental-check path, and the embedded -s/chunk-blob path). A name it cannot convert (EILSEQ/EINVAL) is failed cleanly with a logged --iconv: cannot convert file name ... and is never written truncated/mangled. Validation probes both directions up front (both the sender local→remote and the receiver remote→local, and, for a server/daemon with its own --iconv, the client-REMOTE→server-LOCAL pair) so an unusable spec is rejected before the connection rather than mid-transfer, and NUL-emitting target charsets (utf-16/utf-32/ucs-2) are refused because filenames cannot contain NUL. Divergence documented upstream: the receiver does NOT half-swap; the wire charset always comes from the sender's REMOTE half, so a server whose local charset differs from the client's LOCAL must declare it with its own --iconv. Conversion is process-global and runs on a single thread per process (sender thread / receiver-loop thread), initialized before worker threads start and freed after they join.

Phase 6, Wave C (protocol-version) shipping note (no PROTOCOL_VERSION change): --protocol=NUM lets the client force the wire protocol version for a transfer. FastSync's protocol is a single lockstep format: the config frame is a strict ordered sequence and the server requires the client's version string to equal PROTOCOL_VERSION exactly (config_receive_with_validate, src/shared/config.c) — there are no older-format code paths and no downgrade/negotiation machinery, so a lower/higher/virtual version can never be spoken. The honest contract is therefore: --protocol=2.23.0 (the current PROTOCOL_VERSION, as of the rsync-parity wave) is accepted and stored into the client's version claim (which config_send already transmits), and every other value — 2.22.0, 2.21.0, 2.20.0, 2.19.0, 2.18.0, 2.18, 2.17.0, 2.16.0, 2.15.0, 3.0.0, rsync-integer spellings like 216/31, garbage, empty — is rejected up front in validate_config() before any connection, with a clear error that FastSync supports only its current wire protocol and cannot speak an older or virtual one. Implementation is client-only: a server-side --protocol is intentionally not added because the server has no negotiation (it only enforces exact match), and it could only ever be the current version. This preserves (and slightly tightens) existing validation: the client now also refuses to launch with a version it cannot actually speak, rather than only the server rejecting it later. A genuine downgrade would require a per-version compatibility layer for every frame/feature added since (append 2.10, preallocate 2.11, hardlinks 2.12, devices/specials/symlink-trust/xattr 2.13, remote-option 2.14, daemon module/auth 2.15, iconv 2.16, dir/symlink times 2.17, privilege flags --super/--copy-as 2.18, SCRAM daemon auth 2.19, packed metadata 2.20, error-detail/dry-run 2.21, preserve-attribute split 2.22, rsync-parity wave 2.23) and is intentionally out of scope — documented divergences from rsync's integer-negotiated downgrade remain.

Phase-1/2 selection-and-update status correction (docs): -I/--ignore-times, --size-only, -@/--modify-window, --existing, --ignore-existing, -u/--update, -W/--whole-file, and --compress-threads were previously listed as not-implemented in this document but are in fact fully implemented and tested on dev. This pass corrects the matrix to match the code. The realistic model of these is that FastSync is a sender-driven whole-tree copy, so the size+mtime quick-check and all three receiver-policy skips (--existing, --ignore-existing, -u) are evaluated against the destination on the receiver side, and their booleans cross the wire in the config frame. -I/--size-only/--modify-window modify the --incremental per-file STATUS_CHECK handshake's match predicate (-I disables the mtime leg and forces transfer; --size-only drops only the mtime leg; --modify-window adds tolerance to metadata_mtime_matches); they require --incremental (or a basis dir) to have a handshake to affect, mirroring how they only matter where a quick-check exists in rsync. --existing/--ignore-existing/-u are receiver write-time policies (skipping the write / newer-destination guard) applied across the regular-file, --delay-updates-staged, hardlink-sibling, and special/device paths; -u implies -M metadata and uses a second-then-nanosecond strict > newer check; both correctly influence --remove-source-files (a skipped source is not removed). -W/--whole-file disables block-level delta (opt-in via --delta), folded into the wire use_delta so no protocol bump was needed, and makes --fuzzy inert; --append/--append-verify are rejected with -W. --compress-threads=NUM (1..64, client-only, never crosses the wire) sizes the zstd compression worker pool. No code was changed by this correction; the implementation had landed in earlier merge waves (feat/ignore-times, feat/ignore-existing via the newer file_to_disk_secure_no_replace/linkat EEXIST path, feat/size-only, feat/modify-window, feat/whole-file, feat/update, compression-threads).

Phase 6, Wave D (batch) shipping note (no PROTOCOL_VERSION change): FastSync batch mode is a client-only, self-contained "residual batch": a single file MAGIC "FSTRESBATCH" + format version 1 + metadata flag, followed by length-prefixed chunk_serialize blobs that store full file images (regular files, dirs, symlinks, specials). It is NOT a raw capture of the live wire, because FastSync's protocol is per-file interactive (STATUS_CHECK/STATUS_DELTA_SIGNATURE/STATUS_APPEND handshake), so a raw sender-stream tee is not deterministically replayable against an arbitrary destination. Storing full residuals via the existing, fuzz-tested chunk codec makes --read-batch replay byte-identically by construction. --write-batch=FILE runs the normal live transfer AND emits the batch from a separate deterministic scan pass; --only-write-batch=FILE emits the batch only (no destination, no server); --read-batch=FILE DEST applies it locally (no source, no server; DEST is the only positional arg). Because batch is a local driver concern, it never crosses the wire: no new config-frame field and no PROTOCOL_VERSION bump (mirroring --stop-after/--protocol/--compress-threads). The READ side is hardened against untrusted/attacker-controlled batch files: magic+version validated before any record, per-record length bounds checked before allocation (64 MB cap), clean-EOF-after-prefix and truncated/oversized records rejected, and every applied path goes through the same confined file_save_to_disk_full machinery as the network receiver (O_NOFOLLOW fd-walk, ..-rejection, root confinement — a malicious ../ or absolute/symlink path cannot escape the destination root; this was security-reviewed and valgrind/ASan-clean). Divergences from rsync: (1) the batch carries the FULL residual (complete file images) rather than rsync's update-only delta stream — always byte-correct but larger; (2) per-file data is capped at the chunk codec's ~64 MB (BATCH_MAX_RECORD), so very large files may be refused by the batch writer with a clean error (never a corrupt/truncated batch); (3) hard-links and xattr/ACL blocks are not represented by chunk_serialize, so -H/-X/-A are out of scope for batch; (4) there is no companion .sh/.rsync_argvs — the batch is invoked directly (fastsync --read-batch=FILE DEST, --only-write-batch=FILE SOURCE); (5) --write-batch drives the single-threaded transfer path. Integration/-M note: metadata is captured in the batch when -M is used and persisted in the header so it applies consistently regardless of the reading process's own -M.

Phase 7: CLI-Namespace Parity, Filesystem/Output Completion, and Privilege (Final)

These are the last compatibility items and the closing phase toward rsync flag parity. Per the project decision: every rsync flag (short and long) that is possible gets real rsync-parity behavior; anything physically impossible becomes an explicit Impossible/Divergence status (accepted for CLI compatibility, safely inert, with coverage tests proving that); and the two privilege flags (--super, --copy-as) adopt the deliberately-scoped safe-subset + clear-refusal model rather than blind elevation. The remaining ⚠️ Partial, 🔄 Compatibility No-op, 🔀 Alt Arg, and ❌ Not Implemented rows in the Summary are this phase's scope. All Wave A renames are client-side only (the wire config fields use_compression/use_metadata/use_sendfile/use_chunk_serialization are unchanged), so they require no PROTOCOL_VERSION bump.

Wave A — CLI namespace parity (rename colliding FastSync short flags) — ✅ implemented. This freed the short letters rsync needs and made the three 🔀 Alt Arg rows real. -c→--checksum, -m→--prune-empty-dirs, -M→--remote-option, -f→--filter, -s→--secluded-args, -p→--perms, -T→--temp-dir, -a/--archive→real -rlptD. FastSync's own flags moved to long-form-only or new shorts: -j/--threads (multithreading), --preserve (metadata), --sendfile, --chunk-serialization, --timeout, --ssh-port. The server's independent little CLI keeps -p as its port. All client-side, no wire change, no PROTOCOL_VERSION bump. Unit tests 37/37, full integration 400 passed, cppcheck and clang-format clean. Known Wave-A limitation: --no-perms/--no-compress-style negation of the newly-aliased shorts was not wired into the negatable set (only the long-form --preserve/--compress/--no-links negations existed), so --archive --no-perms was initially unsupported — a minor deviation from rsync. The preserve-attribute split wave below resolves the preservation side: --no-perms/--no-times/--no-owner/--no-group and --no-preserve now work, so --archive --no-perms is supported.

FastSync flag today	rsync wants that name	Proposed rename
-c / --compress	-c = --checksum	compression is already aliased as -z/--compress (rsync parity!) → drop the -c short, keep --compress/-z
-m / --multithreading	-m = --prune-empty-dirs	→ -j / --threads
-M / --preserve	-M = --remote-option	→ --preserve (long-only)
-f / --sendfile	-f = --filter	→ --sendfile (long-only)
-s / --chunk-serialization	-s = --secluded-args/--protect-args	→ --chunk-serialization (long-only)
-p (SSH port)	-p = --perms	→ --port (long-only; --server-port already exists)
-T / --timeout	-T = --temp-dir	→ --timeout (long-only)
-a / --archive (= -c -m -M)	-a = -rlptD	→ becomes real rsync -a after the renames

Wave B — Output & filesystem completion (✅ implemented). -S/--sparse (⚠️→✅): real hole preservation — a sparse-aware writer (write_all_sparse) skips all-zero runs ≥ 4096 bytes with lseek(SEEK_CUR) and ftruncates the final size, wired into both the atomic temp+rename store and --inplace receiver-side with no wire change (the full file image is already in memory; the ftruncate presize is kept). -P (⚠️→✅): interrupted-write retention — on a save failure after data reached the temp fd, --partial now renames the already-written temp to the destination path (best-effort; falls through to the normal unlink on failure, never retains when --partial is off) so a later --append/--append-verify run can resume. --block-size=SIZE (⚠️→✅): promoted after verification — --block-size is now an alias for --delta-block, both set config->delta_block_size, which the delta engine already honored end-to-end (delta_signature_create_seeded + delta_apply); out-of-range values keep the default. --fake-super (⚠️→✅): added fake_super_restore_fd to parse and re-apply the recorded user.fastsync.stat record fd-relative (mode/time only — protocol 2.23.0: never a real chown; the resolved owner is recorded for a later privileged restore); a save under --fake-super now re-applies the recorded attrs instead of only recording them, with the recording format unchanged. --stderr=client (⚠️→❌ Divergent): FastSync has no rsync client-message channel, and client is rejected at CLI parse — the rejection is the documented behavior (unit-tested). -N/--crtimes (⚠️→❌ Divergent): birth-times cannot be set by any portable fs call (utimensat sets only atime/mtime); capture/transmit stays, setting is impossible, the flag is accepted and safely inert. Review-hardening (post-eval): fake-super replay applies the mode through the shared metadata_mode_for_policy helper (protocol 2.23.0: exactly the source mode under -p, with no masking); --sparse takes precedence over --preallocate (posix_fallocate skipped so holes survive); --partial retention is disabled for --no_replace (ignore/existing) and only marks a write-attempt after the actual write begins; --block-size=SIZE/--delta-block=SIZE inline forms are accepted.

Wave C — Devices & special files (finalize statuses + tests) (✅ implemented). The four special-file rows are finalized with coverage tests. --devices, --copy-devices, and --write-devices are ✅ Implemented, each with a documented, safety-driven divergence: device-node creation is privilege-gated, so a receiver without CAP_MKNOD skips that entry with a warning (a per-entry skip, never a transfer failure); --copy-devices copies a device/FIFO's reported size into an ordinary regular file (a size-bounded safe divergence from rsync's unbounded dd-like read); --write-devices writes only into an existing char/block node under the confined receive root and skips every unusable target rather than clobbering or aborting. --specials reclassified from ⛔ Impossible/Divergence to ✅ Parity in protocol 2.23.0: FIFO recreation works (unprivileged mkfifo) and unix sockets are recreated with mknod(S_IFSOCK), which Linux permits unprivileged (the flag previously assumed sockets were impossible — see the --specials row). Tests assert FIFO recreation, socket recreation, the regular-file result of --copy-devices, the skipped/missing and non-device --write-devices targets, and (root-gated) real device-node creation; a root runner additionally drops the receiver to an unprivileged user to assert the CAP_MKNOD skip is graceful.

Wave D — Times superstructure & arg-protection no-ops (✅ implemented, --secluded-args ❌). -O/--omit-dir-times and -J/--omit-link-times are now real modifiers (both 🔄 → ✅ Implemented), reversing the old "never preserves directory/symlink times" divergence:

• Directory times. The recursive scanner captures every traversed source directory's metadata (mtime, plus atime under -U) into a per-transfer list — two paths are covered: the sequential DirectoryScanner captures each opened directory (including the transfer root), and the parallel scanner captures both the root in parallel_scanner_create_with_options and each worker's subdirectories in open_next_directory (appends are guarded by a mutex shared with the sender's pipeline context). The sender transmits them in trailing STATUS_DIR_TIMES frames (each: int count + count × (wire path, metadata) pairs) sent after all file data and after the optional delete manifest, just before STATUS_FINISHED. A tree larger than MAX_MANIFEST_ENTRIES (1 048 576) directories is chunked into repeated frames, each within the receiver's per-frame bound. A dir-time entry is RECORD-ONLY (file->dir_time_only): file_save_to_disk_full returns FILE_SAVE_SKIPPED without creating anything, so a source directory that was empty (or pruned by -m/--prune-empty-dirs) is never resurrected. The receiver accumulates received directory metadata in a DirTimeList and applies it only at the very end — after the entire stream, after the commit-style --delete deletion, and after --delay-updates publication — because creating or removing a child bumps the parent's mtime. Application is fd-relative/walk-confined (file_open_secure_parent + utimensat(..., AT_SYMLINK_NOFOLLOW)) and best-effort per entry: an absent path (an intentionally uncreated empty dir) is skipped QUIETLY and only a real existing directory is stamped. -O (config boolean, already on the wire) makes the receiver skip the whole set. The single-threaded sink applies in receiver_send_success_frame; the -j/--threads sink accumulates in write_thread and server.c applies after both threads join and the deletion commits.
• Symlink times/owner/mode. STATUS_SYMLINK already carried metadata; the receiver now applies it with no-follow primitives only: utimensat(..., AT_SYMLINK_NOFOLLOW), best-effort fchmodat(..., AT_SYMLINK_NOFOLLOW) (honest no-op where unsupported, e.g. Linux), and policy-gated fchownat(..., AT_SYMLINK_NOFOLLOW) via a new identity_apply_ownership_link that shares the identity resolver with the fd path. -J suppresses only the timestamps; ownership stays governed by the identity opt-in (--numeric-ids/--usermap/--groupmap/--chown) exactly like regular files. A symlink has no children, so this is applied immediately at creation.
• Wire: the shared STATUS_DIR_TIMES frame (and metadata on STATUS_MKDIR for --dirs entries) is a frame-sequence change, so PROTOCOL_VERSION was bumped 2.16.0 → 2.17.0; every version-sensitive test (--protocol accepted/rejected values) was updated. The config-frame layout itself is unchanged (the omit booleans already crossed). Non-metadata and --no-preserve transfers send no STATUS_DIR_TIMES frame and no directory metadata, keeping them byte-identical.

--secluded-args (🔄 → ❌ Divergent): a true arg-send protocol would replace the argv-based SSH launch with an in-band channel, and FastSync already builds the remote SSH argv injection-safe (single-quote-escaped shell words), so there is no argument-leak to close; the already-safe behavior is documented in the row and no transport change is made.

Wave E (LAST) — Privilege: --super/--no-super and --copy-as=USER[:GROUP] (✅ implemented). FastSync adopts a safe-subset + clear-refusal privilege model: it never blind-elevates and never calls setuid/seteuid/setgid. All privileged operations remain fd-relative and confined below the authorized receive root.

--super/--no-super set a receiver-side tri-state Config->super_mode (SUPER_MODE_AUTO/ON/OFF). privilege_super_permitted() / privilege_super_mode_permitted() (src/shared/identity.c) return true for ON and AUTO (AUTO preserves FastSync's historical best-effort attempt, where the kernel refuses an unprivileged call and the caller skips it) and false only for OFF. The gate covers every super-user activity FastSync performs: ownership application (identity_apply_ownership/_link), char/block device-node creation (file_save_special_to_disk), writes into an existing device (--write-devices), and the --fake-super owner replay. Unprivileged FIFO creation is deliberately unaffected. --super does not imply --numeric-ids: ownership is applied only when an explicit identity policy (--usermap/--groupmap/--chown/--numeric-ids/--copy-as) or a preserve-source request (-o/-g, or -a/--archive) is also given. --no-super suppresses those activities even for a root receiver. A non-root receiver given --super logs one warning at activation (identity_set_active); each confined attempt is then refused by the kernel and skipped, never aborting. The confinement floor is unchanged (file_open_secure_parent, O_NOFOLLOW, root/path checks). Operator control: the server CLI accepts --no-super, a veto that forces OFF for every connection, refuses any client --copy-as, and neutralizes an explicit --super (the connection is accepted but no super-user activity is attempted). A privileged (root) standalone TCP listener instead defaults to OFF and requires the server-only --allow-super opt-in to attempt any super-user activity (the flag is rejected with --stdio, whose client-composed remote argv must never defeat the default; use a forced command if the default must hold); a non-root server is unchanged. On a daemon, a module that has not opted in with client owner = yes additionally has super-user device activity forced off (see the Daemon Mode notes).

--copy-as=USER[:GROUP] is the safe subset. FastSync's receiver is multithreaded, so a real credential switch is unsafe; instead the receiver forces the ownership of every entry it writes — regular files, symlinks, directories (including implicitly-created parents), and special nodes — to the resolved target ids through the confined fd-relative identity path. USER is resolved on the client (name, @N/bare N, or * = client euid); when :GROUP is omitted the user's primary gid is used (falling back to gid == uid for a numeric id with no local passwd entry). It requires a privileged (root) receiver: an unprivileged receiver refuses the whole transfer at the config handshake, before STATUS_OK, so no data is ever written with the wrong ownership. A --copy-as chown failure on a capability-restricted root is logged at ERROR (never silently downgraded). --copy-as implies metadata (--no-preserve is rejected) and --fake-super cannot override it. Daemon policy: a --daemon receiver refuses every client-chosen-ownership / super-user request — --numeric-ids, --chown, --usermap/--groupmap, --fake-super, --copy-as, and explicit --super — unless the selected module opts in with client owner = yes; without that per-module opt-in any client could force arbitrary ownership inside the module root (a root standalone TCP listener, which serves one operator-authorized root, honors these requests only when started with --allow-super; the flag is rejected with --stdio). A --copy-as chown failure on a capability-restricted root marks the entry as failed rather than reporting success with the wrong owner.

Wire: two trailing config-frame blocks after the --iconv spec, in fixed order — send_privilege_options/receive_privilege_options (one super_mode int, validated 0..2), then send_copy_as_options/receive_copy_as_options (presence int + two int32 ids, validated >= 0, with copy_as_set ⇒ use_metadata). PROTOCOL_VERSION bumped 2.17.0 → 2.18.0. Divergences from rsync: rsync's --super elevates the receiver and --copy-as actually switches its credentials; FastSync never elevates and only permits/forwards confined attempts, and --copy-as forces ownership rather than switching identity.

Current honest status (protocol 2.23.0). ✅ Parity 83 / ⚠️ Caveat 63 / ❌ Divergent 4 = 150 rows. Earlier revisions of this document reported "143 ✅ / 0 divergence / 0 partial"; that conflated "parsed and tested" with "rsync parity", because many rows carried documented behavioral differences and some short options were not parsed at all. The reclassification makes the differences explicit and the rsync-parity wave closed the genuine gaps (short options, clustering, checksum/compression choices, seed randomization, timeout defaults, delete scoping and partial limits, verbatim symlink storage, socket recreation, --chmod, and more — see the next section). The four ❌ rows are --stderr=client (no rsync client-message channel), -N/--crtimes (no portable setter), --protocol=NUM (only the current wire version is accepted), and -s/--secluded-args (accepted no-op). --specials is now ✅ because sockets are recreated with mknod(S_IFSOCK). No ❌ Not Implemented rows remain.

Preserve-attribute split (protocol 2.21.0 → 2.22.0) — ✅ implemented. FastSync splits the former single metadata bundle into four independent, rsync-compatible per-attribute flags — -p/--perms, -t/--times, -o/--owner, -g/--group — each with a negation (--no-perms/--no-times/--no-owner/--no-group, short --no-p/--no-t/--no-o/--no-g), plus --no-preserve clearing all four. -a/--archive is now full rsync -rlptgoD (owner and group included, though their application stays privilege-gated), -A/--acls implies -p, -X/--xattrs does not, -E/--executability sets only executability, and -U/-N do not imply -t. --incremental/--delta still auto-preserve perms+times unless the user explicitly negated them. Wire: the binary config frame gains four appended booleans (preserve_perms/preserve_times/preserve_owner/preserve_group) after omit_link_times, so PROTOCOL_VERSION is bumped 2.21.0 → 2.22.0; the fixed-width FileMetadata layout is unchanged and the receiver gates the metadata frame on a derived use_metadata. Receiver behavior: each attribute is applied independently, directory modes are applied under -p (at the end of the transfer, alongside dir times), symlink mode under -p, and -O/--omit-dir-times suppresses directory times only. Documented divergences as of 2.22.0, all but (d)/(e) removed by the rsync-parity wave (protocol 2.23.0): (a) the mode-masking divergence is gone — under -p the source mode is now copied exactly, including S_IWGRP/S_IWOTH and setuid/setgid/sticky; (b) a brand-new file without -p still gets source_mode & ~umask when metadata is present (else the historical fixed 0644), and a new directory without -p still uses FastSync's 0755 default; (c) the --chmod-implies--p divergence is gone — --chmod no longer implies -p (rsync parity); (d) -o/-g map by name on the receiver with a raw-numeric fallback (only numeric ids cross the wire); (e) a daemon module without client owner = yes does not refuse a plain -a/-o/-g — it forces super off, applies no ownership, and logs a warning, while explicit --chown/--usermap/--groupmap/--numeric-ids/--copy-as/--super are still refused.

Rsync-Parity Wave (protocol 2.23.0)

This wave closed the remaining CLI, filesystem, ownership, deletion, and output gaps against rsync 3.4.1. It is a wire change: PROTOCOL_VERSION moved 2.22.0 → 2.23.0 because the delete manifest gained a synchronized-directory section and the terminal status gained STATUS_DELETE_LIMIT (see the deletion notes above). Everything below is implemented and covered by unit and integration tests unless it is explicitly listed as a limitation.

CLI parsing

• Short options now parsed: -r (--recursive), -b (--backup), -L (--copy-links), and -B (--block-size/--delta-block) are accepted as rsync spells them.
• rsync short-option clustering: a token is expanded before parsing, so -av → -a -v, -aAX → -a -A -X, -rlpt → -r -l -p -t, and so on. A value-taking short option consumes the remainder of its token (-B1000 → -B 1000, -essh → -e ssh, -MOPT → -M OPT), with an optional leading = dropped (-B=1000); a value-taking option written alone takes the next argv entry, which is copied verbatim so a value that happens to start with - (e.g. --filter "- *.tmp") is not mistaken for a cluster.
• Inline/attached long values: --opt=value is accepted uniformly, and each expanded token is mapped back to its original argv index so positional arguments stay correct.
• -c implies the checksum quick-check. -c/--checksum sets the incremental checksum comparison rather than doing nothing on its own; like rsync, -c does not imply -t.

Checksums and compression

• --checksum-choice/--cc accepts xxh64 (default), xxhash, xxh3, xxh128, md5, and auto; md4, sha1, none, and the two-name transfer,pre-transfer form are rejected by name.
• --checksum-seed=0 is randomized per transfer (the chosen seed is sent to the receiver), matching rsync; an explicit non-zero seed is used verbatim.
• --compress-choice/--zc accepts zstd (default), none, and auto; rsync's lz4/zlib/zlibx are rejected by name.
• --skip-compress uses rsync 3.4.1's built-in default suffix list when no list is supplied; an explicit list replaces it.
• --no-whole-file is accepted as the rsync spelling that clears -W/--whole-file.

Timeouts and limits

• --timeout defaults to 0 (disabled) and --contimeout to 60 s; 0 disables either, matching rsync.
• --max-alloc=0 means "no local allocation limit" (rsync semantics). A standalone server still keeps its own ceiling for the peer it serves.

Filesystem and deletion semantics

• --temp-dir is confined to the receive root on the receiver: a relative dir resolves below it; an absolute path or one containing .. is rejected. An EXDEV install falls back to a non-atomic copy instead of aborting.
• Deletion scoping: the manifest carries the synchronized directories, so the extras walk only visits their subtrees; --files-from subsets no longer delete untransmitted paths outside the listed directories.
• --delete-excluded removes filter-excluded mirrors but never --max-size/--min-size-pruned mirrors (separate, always-on protection).
• Destination symlinks are unlinked by name, never followed; a directory still holding one survives.
• --max-delete=N is partial: delete up to N, skip the rest, exit 25. --delete-missing-args removals draw from the same budget.
• --force is honored during --delay-updates publication.
• -x/--one-file-system emits the mount-point directory entry (an empty directory at the destination) without descending into it.
• --include/--exclude are an ordered first-match rule list, evaluated like --filter/-F/-C (first match wins), so an earlier rule can override a later one.

Ownership and metadata

• --numeric-ids is a mapping modifier only — it changes how ids map, not whether ownership is applied; combine it with -o/-g, -a, or an explicit map.
• --usermap/--groupmap support names, @N/bare N ids, inclusive LOW-HIGH ranges, *, empty-FROM (unnamed ids), and receiver-resolved TO names.
• --chown conflicts with --usermap/--groupmap on the same side and is a clear configuration error (matching rsync) instead of an order-dependent winner.
• --fake-super never real-chowns. It records the resolved owner (the active mapping, else the source id) in user.fastsync.stat for a later privileged restore and replays only mode/times. Directory ownership and directory xattrs/ACLs are preserved alongside file entries.
• --chmod implements rsync's D/F/X selectors, s/t, append semantics, does not imply -p, and applies its changes without sanitization.

Symlinks and special files

• -l/--links stores symlink targets verbatim (absolute and ..-bearing targets included), matching rsync. --safe-links, --copy-unsafe-links, and --munge-links (which now uses rsync's /rsyncd-munged/ marker) match rsync and are applied sender-side.
• --specials recreates unix sockets with mknodat(..., S_IFSOCK), so -D/--devices --specials now covers the full rsync node set.
• --copy-devices is implemented (see its caveat below).

Output

• -i/--out-format print rsync-style change lines; --list-only scans the source only and contacts no server; -h uses rsync's decimal units; --progress is an aggregate line; --stats prints the counters FastSync can observe locally (receiver-only counters are 0).
• Server --port is an alias of the -p <port> TCP listen port (--dparam port= overrides the daemon config).

Known intentional divergences and limitations

These remain after the wave; they are the reasons a row above is ⚠️.

• Symlink target containment is not enforced receiver-side by default. Verbatim storage is rsync parity, but a destination later consumed by a link-following tool can follow a link outside the receive root. Use --safe-links when the source is untrusted. --trust-sender does not affect symlink targets.
• --temp-dir absolute/foreign-filesystem paths are rejected by the receiver (rsync's daemon also confines; standalone rsync differs).
• --copy-devices reads a bounded st_size rather than rsync's unbounded device read.
• A broken symlink referent under --copy-links/--copy-unsafe-links exits 0 where rsync exits 23.
• New directories without -p still use FastSync's 0755 creation default rather than source & ~umask; directory metadata is only applied when a directory attribute is requested.
• --stats receiver-only counters (matched data, file-list bytes, deleted count) are reported as 0; --progress is an aggregate line, not per-file.
• --password-file/--early-input/--hash-credentials/--iterations are FastSync-native (SCRAM/PBKDF2), not rsync semantics; the batch format is not rsync-interoperable.
• xattr/ACL namespace policy permits only user.* and system.posix_acl_* when -A is negotiated (stricter than rsync).
• --stop-at remains a FastSync-flexible parser (client-only, not serialized); --stop-after matches rsync.
• Push-only model and a non-rsync wire protocol remain by design; --protocol accepts only the current version and -s/--secluded-args is an accepted no-op.

Packed Metadata Frame (protocol 2.20.0)

A file's metadata used to cross the wire as up to 12 separate per-field framed messages (a present flag followed by mode/uid/gid/mtime/atime/crtime writes), which cost ~11 extra protocol frames per file on many-small-file trees. FastSync now sends the metadata as ONE packed frame: a single int32 present flag (0 = absent) followed, when present, by the fixed FILE_METADATA_WIRE_SIZE-byte (68-byte) field record already emitted by the shared metadata_to_buf()/metadata_from_buf() chunk codec. Absent metadata is a lone int32 zero. The encoded field layout is unchanged (only the framing collapses), so chunk-serialized blobs remain byte-identical. Protocol data is an unframed byte stream, so the packed encoding is byte-for-byte identical to the old field-by-field writes; PROTOCOL_VERSION was bumped 2.19.0 → 2.20.0 as a deliberate lockstep-release marker rather than because of a desynchronization. The strict same-version handshake rejects any mismatch before a byte of the frame is parsed.

Recommended Delivery Order

1. Resolve short-option conflicts (-m, -M, -T, -f, -s) and define the compatibility contract.
2. Implement Phase 1 comparison, update, output, and alias features with unit and integration coverage.
3. Implement Phase 2 traversal/filtering and Phase 3 deletion semantics.
4. Implement metadata and link features that are safe on the supported platforms.
5. Treat daemon mode, special files, batch mode, and protocol-version compatibility as separate projects.

The existing priority list below is a feature shortlist, not an implementation schedule; this plan supersedes it for effort and sequencing.

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Recommendations: Top Features to Implement Next

Ranked by user demand, implementation complexity, and interoperability impact (status reflects current dev):

Priority	Feature	Effort	Impact
1	--whole-file / -W	Low	High — users expect opt-out of delta — ✅ implemented
2	--ignore-times / -I	Low	Medium — useful for forcing re-transfer — ✅ implemented
3	--size-only	Low	Medium — common migration scenario — ✅ implemented
4	--ignore-existing	Low	Medium — common sync patterns — ✅ implemented
5	--existing	Low	Medium — common sync patterns — ✅ implemented
6	--remove-source-files	Low	High — common for moves/backup
7	--delete-during	Medium	High — performance improvement
8	--delay-updates	Medium	High — atomic updates
9	--chmod	Low	Medium — permission flexibility
10	--executability / -E	Low	Low — simple flag
11	--skip-compress	Low	Medium — performance tuning

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FastSync-Specific Features (Not in rsync)

Feature	Description
-j / --threads[=N]	Multithreaded pipeline (scanner/loader/sender); N (1–256) sizes the parallel scanner worker pool, bare -j/--threads uses the built-in default (renamed from -m in Phase 7 Wave A; -m is now rsync --prune-empty-dirs)
--chunk-serialization	Chunk serialization mode (long form only; -s is now rsync --secluded-args)
--sendfile	Zero-copy sendfile() syscall (TCP only) (long form only; -f is now rsync --filter)
-z [level] / --compress	zstd compression level (1-22) (-c is now rsync --checksum)
--chunk-size	Configurable chunk size
--tls	TLS encryption (mutual auth)
--fastsync-server-path	Path to fastsync-server binary
--server-host / --server-port	Direct TCP connection
Incremental sync	Skip unchanged files (size+mtime)
Delta transfer	Block-level delta for changed files