FastSync/README.md

#FastSync

FastSync is a high-performance file synchronization tool designed to become a drop-in replacement for common rsync workflows. It keeps the familiar source/destination model and rsync-style options while adding optional multithreading, streaming zstd compression, chunking, zero-copy TCP transfers, and native TCP/TLS transports.

The release version is FastSync's client/server protocol version (printed by fastsync --version); client and server must match. See CHANGELOG.md for the history.

The compatibility target is straightforward:

• Existing rsync commands should keep the same meaning.
• FastSync-only performance options should be additive and optional.
• A normal compatibility-mode transfer should prioritize rsync filesystem semantics over maximum throughput.

FastSync currently speaks its own protocol to fastsync-server. SSH mode starts that server remotely; it does not yet interoperate with an unmodified rsync client or rsync daemon. See Compatibility Status for the current boundary.

Why FastSync

FastSync uses a producer-consumer transfer pipeline and can combine several optimizations for large or high-latency transfers:

• Multithreaded scanning, loading, and sending.
• Streaming zstd compression with levels 1 through 22.
• Configurable file chunking and compact chunk serialization.
• sendfile() zero-copy transfers over TCP.
• Batched incremental checks to reduce round trips.
• Optional block-level delta transfer for FastSync peers.
• Bandwidth limiting, progress reporting, statistics, and backups.
• TCP, SSH, and TLS transports.
• Atomic temporary-file writes by default.

These optimizations are disabled or selected independently. Users can start with rsync-style commands and add FastSync options when they are useful.

Compatibility Status

FastSync is currently an rsync-compatible CLI in progress, not a complete replacement for every rsync feature or protocol mode.

Working today

• Recursive directory scanning.
• Rsync-style source and destination arguments.
• SSH transport using user@host:destination paths below the remote authorized root.
• TCP client/server transfers.
• Dry runs, excludes, includes, size filters, backups, statistics, and bandwidth limiting.
• Incremental size/mtime checks and optional xxHash64 content checks.
• FastSync-native delta transfer for changed files.
• Optional mode and timestamp preservation.
• Delete manifests with server-side delete authorization.
• Temporary-file writes with atomic rename by default.
• Path traversal checks and destination-root confinement.

Not yet equivalent to rsync

• The FastSync wire protocol is not the rsync wire protocol.
• SSH mode requires fastsync-server on the remote host.
• Archive mode does not yet provide all of rsync's -rlptgoD behavior.
• Symlink transfer is incomplete; link targets are not yet recreated in all modes.
• Owner/group, ACL, xattr, and hard-link handling is incomplete or unavailable.
• Device and special-file preservation is implemented with documented divergences: recreated device nodes require CAP_MKNOD on the receiver (a non-root receiver skips the entry), and sockets cannot be recreated (FIFOs are).
• Sparse-file hole preservation (-S, --sparse) is implemented receiver-side: long all-zero runs are written as holes (no wire change; the full file image is already in memory).
• --partial, --partial-dir, -P, --append, and --append-verify keep the write atomic (temp + rename). With --partial, a failed/interrupted write now retains the already-written temp at the destination path (best-effort) so a later --append/--append-verify run can resume it.
• --dirs is not implemented. Its compatibility aliases --old-dirs and --old-d are recognized but rejected explicitly rather than silently using FastSync's recursive directory behavior.
• Short-option names are now rsync-parity (Phase 7 Wave A): FastSync's former collisions were renamed (-j/--threads, --preserve, --sendfile, --chunk-serialization, --timeout, --ssh-port), so -m, -M, -f, -s, -T, -p, -c, -a, and -z follow rsync. See RSYNC_COMPAT.md.

The detailed flag matrix is maintained in RSYNC_COMPAT.md. It distinguishes implemented, partial, alternate, and planned behavior.

Quick Start

Build

compile_commands.json is a symlink to build/compile_commands.json and is used by clangd/editor tooling; its target is generated by the build, so it dangles until the first build.

Client

Argument	Description
Positional	<source> <dest> — automatic SSH detection if dest contains :
-c, --checksum	Verify content by checksum instead of size+mtime
-z, --compress [level]	Enable streaming zstd compression (level 1–22, default 5)
-a, --archive	rsync archive mode (-rlptgoD): links, metadata, devices and specials (not compression/multithreading)
-j, --threads[=N]	Multithreading mode; N (1–256) sets the parallel scanner worker count, bare -j/--threads uses the default
-m	rsync --prune-empty-dirs (short form now rsync-parity)
--chunk-serialization	Chunk serialization (batch all files per chunk; long form only)
-s	rsync --secluded-args compatibility no-op (remote SSH argv is already injection-safe)
--sendfile	Sendfile zero-copy. Incompatible with compression / chunk serialization. TCP only. Long form only.
--preserve	Preserve supported file metadata (mode and mtime; ownership and atime are unsupported)
-n, --dry-run	Scan and print what would be transferred
-p, --perms	Preserve permission bits (part of the metadata bundle)
--ssh-port <port>	SSH port (default: 22)
-v, --verbose	Enable debug logging
-q, --quiet	Suppress non-error output
--progress	Show real-time transfer speed
-P	Enables partial-transfer mode + progress output; interrupted writes retain the already-written temp for resumption
--delete	Delete files on receiver not present in source (default timing: delete-after, i.e. only after the whole transfer succeeded)
--delete-before	Delete extras before the transfer starts (implies --delete)
--delete-during, --del	Delete extras once the keep-set is known, before data is applied (implies --delete)
--delete-delay	Delete extras only after a successful transfer (implies --delete)
--delete-after	Explicit delete-after timing (implies --delete)
--exclude <pattern>	Exclude files matching glob pattern (repeatable)
--exclude-from <file>	Read exclude patterns from a file (one per line)
--include <pattern>	Only transfer files matching glob pattern (repeatable, whitelist)
--max-size <n>	Skip files larger than n bytes
--min-size <n>	Skip files smaller than n bytes
--max-alloc <SIZE>	Maximum single allocation (binary units: B, K, M, G, T, P, E; default 1G)
--incremental	Skip files unchanged since last transfer (size + mtime). Auto-enables --preserve. Incompatible with --chunk-serialization.
--existing	Skip files not already present at the destination; update existing files normally.
--bwlimit <KB/s>	Bandwidth limit in kilobytes per second
--chunk-size <n>	Chunk size in bytes (default: 10485760)
--timeout <sec>	Positive I/O timeout in seconds, applied to both the socket (SO_RCVTIMEO/SO_SNDTIMEO, built-in default 30 s) and the per-message protocol poll deadline (built-in default 60 s). Omit the option to keep both built-ins; 0 is rejected. The server side keeps the built-in 60 s protocol window (the value is not sent on the wire).
--contimeout <sec>	Connection timeout in seconds (default: 10)
--backup	Backup existing destination files before overwriting
--backup-dir <dir>	Target directory for backups (requires --backup)
--stats	Print transfer statistics at end (bytes, files, timing)
-h, --human-readable	Format transfer byte sizes with binary units
--max-depth <n>	Maximum directory depth to recurse (0 = unlimited, default: 0)
--log-file <path>	Write log messages to file instead of stderr
--source-dir <path>	Source directory (overrides FASTSYNC_SOURCE_DIR)
--dest-dir <path>	Server destination directory (overrides FASTSYNC_DEST_DIR)
--save-to-disk	Write received files to disk
--server-host <ip>	Server IP address (default: 127.0.0.1)
--server-port <n>	Server port (default: 8080)
--tls	Enable TLS encryption
--cert <path>	TLS certificate file (PEM)
--key <path>	TLS private key file (PEM)
--ca <path>	TLS CA certificate file for verification (PEM)
--client-cn <name>	TLS client certificate common name; mandatory with --tls (a TLS connection always verifies the client CN)

Per-message vs. connection timeouts. --timeout bounds each individual protocol send/receive (the poll() deadline), so a peer that stops mid-frame is dropped. It does not, by itself, stop a peer that keeps sending well-formed frames forever. The receiver therefore also enforces two wall-clock (CLOCK_MONOTONIC) bounds on a connection: a 1 hour idle limit and a 24 hour overall session cap. Only frames that move real work (not STATUS_KEEPALIVE/STATUS_ABORT and not an empty STATUS_CHECK_BATCH/STATUS_DIR_TIMES) refresh the idle timestamp, so a peer cannot hold a connection slot by emitting cheap empty frames; a peer that fabricates minimal non-empty frames can still occupy a slot until the 24 hour cap, since no bound can require actual payload without risking a legitimate long operation. Both are deliberately generous so a legitimate long-running transfer is never aborted.

Server

Argument	Description
--stdio	Run in stdio mode (for SSH transport; single connection then exits)
-p <port>	TCP listen port (default: 8080, range: 1–65535)
--tls	Enable TLS encryption
--cert <path>	TLS certificate file (PEM)
--key <path>	TLS private key file (PEM)
--ca <path>	TLS CA certificate file for verification (PEM)
--destination-root <path>	Authorized destination root (default: .)
--allow-delete	Permit manifest deletion
--allow-super	Standalone TCP listener only: keep super-user activities enabled for a root receiver. Without it a root standalone server forces SUPER_MODE_OFF, so client --devices/--write-devices/--super and client-chosen ownership requests are skipped/refused. Rejected with --stdio (the SSH remote argv is client-composed, so a client could otherwise pass it and defeat the secure default; operators exposing fastsync-server --stdio over SSH must use a forced command if the default must hold). No effect when not root.
--allow-unauthenticated	Permit plaintext TCP clients. For an auth users module this opts in loopback plaintext only; remote auth still requires verified TLS, so the flag never permits remote plaintext auth.
-v, --verbose	Enable debug logging
--help	Show help

Environment Variables

Variable	Default	Description
FASTSYNC_SOURCE_DIR	—	Source directory fallback
FASTSYNC_DEST_DIR	—	Destination directory fallback
FASTSYNC_SAVE_TO_DISK	false	Disk persistence fallback
FASTSYNC_SSH_PORT	22	Default SSH port
FASTSYNC_SERVER_HOST	127.0.0.1	Default server host
FASTSYNC_SERVER_PORT	8080	Default server port
FASTSYNC_TLS_CERT	—	Default TLS certificate path
FASTSYNC_TLS_KEY	—	Default TLS private key path
FASTSYNC_TLS_CA	—	Default TLS CA certificate path

Implementation Details

Data Structures

1. Chunk — collection of files (~10 MB total by default)
2. File — path, content (Data), optional FileMetadata pointer
3. FileMetadata — mode, uid, gid, mtime_sec, mtime_nsec; uid / gid are advisory wire fields and are never applied by the receiver; atime is unsupported
4. Config — runtime parameters (transported over wire, TLS settings excluded). Includes timeout, contimeout, quiet, backup, backup_dir, stats, max_depth, log_file.
5. Queue — thread-safe bounded queue with condition variables
6. DirectoryScanner — recursive BFS traversal with exclude and include pattern support, max-depth enforcement

Key Algorithms

1. File scanning — BFS directory traversal; entries matched against exclude and include patterns, max - depth enforced 2. * *Chunking ** — files accumulated until chunk_size threshold, then flushed 3. * *Compression ** — streaming zstd via ZSTD_compressStream2 / ZSTD_decompressStream 4. * *Network protocol ** — status - code - driven exchange with metadata packing, keep - alive, and abort support 5. * *Incremental check ** — client sends STATUS_CHECK + path + size + mtime and, with --checksum, XXH64 content checksum; server compares against destination. Can be batched via STATUS_CHECK_BATCH for reduced round-trips.
2. Bandwidth limiting — token-bucket algorithm with nanosleep throttling on 64 KB write chunks
3. Metadata restoration — chmod(), chown(), utimensat() on the receiving side
4. --delete — sender tracks all sent paths; receiver walks destination tree and removes unlisted files / directories 9. * *SSH transport * * — socketpair() + fork() + execvp("ssh", ...) with ControlMaster and port support 10. * *TLS transport ** — OpenSSL SSL_CTX with TLS 1.2 minimum, mutual CA verification, transparent SSL_read/SSL_write via io_set_ssl() 11. * *Path traversal protection ** — has_path_traversal() rejects any file path containing .. components, preventing directory escape attacks 12. * *Connection limiting ** — server tracks active connections and rejects new ones beyond max_connections (default 100)13. * *Keep - alive ** — idle connections receive periodic STATUS_KEEPALIVE to detect half - open TCP connections 14. * *Abort handling ** — SIGINT sets an abort flag; the next protocol operation sends STATUS_ABORT for clean server cleanup
5. Atomic writes — files are written to a .tmp suffix then atomically renamed via rename(), preventing partial files
6. Backup — before overwriting, existing files are moved to --backup-dir (or same directory with ~ suffix) preserving the original

Security Features

Path Traversal Protection

All received file paths are validated by has_path_traversal() before any disk operation. Any path containing .. components is rejected with STATUS_ERROR, preventing directory escape attacks.

TLS Certificate Verification

TLS requires --ca and performs mutual TLS verification (SSL_VERIFY_PEER with depth 4). Connections without certificate verification are rejected.

Connection Limits

The server enforces a maximum of 100 concurrent connections (configurable via max_connections in Server). When the limit is reached, new connections are immediately rejected and closed.

Abort Handling

If the client receives SIGINT (Ctrl+C) during a transfer, it sends STATUS_ABORT to the server. The server then cleans up temporary files and exits the child process, preventing incomplete files from remaining on disk.

Atomic Writes

Received files are written to a temporary path (suffixed with .tmp) and then atomically renamed to the final filename via rename(). This prevents partial or corrupted files from appearing at the destination if the transfer is interrupted.

Build Requirements

• C11 compiler
• CMake >= 3.22
• zstd library
• OpenSSL (development headers and libraries)
• pthreads
• SSH client (for SSH transport mode only)

Installing Dependencies

Ubuntu/Debian:

sudo apt install cmake build-essential libzstd-dev libssl-dev openssh-client

Nix:

nix-shell  # provides zstd, openssl, cmake, gcc

Building

cmake -B build -S .
cmake --build build -j$(nproc)

With Nix:

nix-shell
cmake -B build -S .
cmake --build build -j$(nproc)

SSH transfer

The remote host must have fastsync-server available in PATH, or use --fastsync-server-path. SSH starts fastsync-server --stdio in its remote working directory, so use a destination below that directory unless the remote server is otherwise configured with a matching authorized root.

The remote --stdio server argv is composed by the client, so it must never be trusted to opt a root receiver into super-user activities: --allow-super is rejected with --stdio and super stays off on that path. Operators exposing fastsync-server --stdio over SSH must use a forced command (e.g. an authorized_keys command= entry) if the default must hold.

ssh user@host 'mkdir -p destination'
./build/client /path/to/source user@host:destination

FastSync is push-only: the source (first argument) is always a local directory and only the destination may be remote. A remote source such as client user@host:src ./local (a "pull") is intentionally not supported; see RSYNC_COMPAT.md.

TCP transfer

Start the FastSync server:

./build/server --destination-root /path/to -p 8080 --allow-unauthenticated

Then run the client:

./build/client --server-host 127.0.0.1 --server-port 8080 \
  --source-dir /path/to/source --dest-dir /path/to/destination \
  --save-to-disk

Plain TCP requires the explicit --allow-unauthenticated server option. Use TLS for authenticated network connections.

TLS transfer

./build/server --destination-root /path/to --tls --cert server.pem --key server-key.pem -p 8443
./build/client --tls --cert client.pem --key client-key.pem --ca ca.pem \
  --server-host example.com --server-port 8443 \
  --source-dir /path/to/source --dest-dir /path/to/destination \
  --save-to-disk

Common Workflows

These examples show the intended rsync-style workflow. Options marked as FastSync-native are optional performance or transport extensions.

#Basic synchronization
./build/client /source/ /destination/

#Archive - style synchronization(current FastSync archive behavior)
./build/client -a /source/ user@host:destination/

#Preview a transfer without changing the destination
./build/client -n /source/ /destination/

#Exclude temporary and object files
./build/client --exclude '*.tmp' --exclude '*.o' \
  /source/ user@host:destination/

#Remove destination entries not present in the source
./build/client --delete /source/ user@host:destination/

#Skip unchanged files using size and modification time
./build/client --incremental /source/ user@host:destination/

#Verify content when size and time are not sufficient
./build/client --incremental --checksum /source/ user@host:destination/

#Preserve supported mode and timestamp metadata
./build/client --preserve /source/ user@host:destination/

#Keep backups of overwritten destination files
./build/client --backup --backup-dir backups \
  /source/ user@host:destination/

FastSync Extensions

FastSync-native options are intended to add performance or operational features without changing the meaning of ordinary compatibility options.

Option	Purpose
-j, --threads[=N]	Enable the multithreaded scanner/loader/sender pipeline. N (1–256) sets the parallel scanner worker count; bare -j/--threads uses the default.
-z [level], --compress [level]	Enable streaming zstd compression, levels 1-22.
--compress-level <n>	Set the zstd compression level.
--zc <alg>	Alias for --compress-choice. FastSync supports zstd and none.
--zl <n>	Alias for --compress-level.
--skip-compress <list>	Skip compression for comma-separated suffixes; incompatible with --chunk-serialization.
--compress-threads <n>	Use n zstd compression workers. Requires compression and a zstd build with threaded support; the setting affects sender CPU work only.
--chunk-size <bytes>	Set the transfer chunk size.
--chunk-serialization	Enable FastSync chunk serialization (long form only; -s is rsync's --secluded-args).
--sendfile	Use TCP sendfile() zero-copy transfer. Incompatible with compression and chunk serialization. Long form only.
--delta	Use FastSync-native block delta transfer. Requires --incremental.
--delta-block <bytes>	Set the FastSync delta block size (--block-size is an alias).
--delta-max <bytes>	Limit files eligible for FastSync delta transfer.
--server-host <host>	Select the TCP server host.
--server-port <port>	Select the TCP server port (--port <port> and --port=<port> are rsync-friendly aliases).
--tls	Enable TLS for TCP transport.
--bwlimit <KB/s>	Apply token-bucket bandwidth limiting.
--progress	Show transfer progress and throughput.
--stats	Print transfer statistics.
--timeout <seconds>	Set the socket and per-message protocol I/O timeout (positive seconds). Omit to keep the built-in 30 s socket / 60 s protocol defaults.
--contimeout <seconds>	Set connection timeout.

Short-option conflicts with rsync have been resolved for the CLI namespace (Phase 7): -c is now rsync's --checksum, -m is --prune-empty-dirs, -M is --remote-option, -f is --filter, -s is --secluded-args, -p is --perms, and -T is --temp-dir. FastSync's own flags were renamed to long-form-only or new shorts: multithreading is -j/--threads, metadata is --preserve, sendfile is --sendfile, chunk serialization is --chunk-serialization, timeout is --timeout, and SSH port is --ssh-port. -a/--archive is now real rsync archive (-rlptgoD).

--secluded-args (and its short form -s) is accepted as a compatibility no-op. It does not change FastSync's transport or protocol behavior, because remote SSH argv is already built injection-safe.

Client Options

Selection and transfer

Option	Description
-a, --archive	rsync archive mode (-rlptgoD): links, metadata, devices and specials.
-n, --dry-run	Scan and report without writing files.
--delete	Request removal of destination entries absent from the source. The server must allow deletion. Default timing is delete-after: extras are removed only after the whole transfer succeeded.
--delete-before	Delete extras before the transfer starts (implies --delete).
--delete-during, --del	Delete extras once the keep-set manifest is known, before data is applied (implies --delete; early mode, same engine behaviour as --delete-before).
--delete-delay	Delete extras only after a successful transfer (implies --delete; commit mode, same behaviour as --delete-after).
--delete-after	Explicit delete-after timing: delete only after the transfer succeeded (implies --delete).
--exclude <pattern>	Exclude matching paths. Repeatable.
--include <pattern>	Include matching paths. Repeatable.
--exclude-from <file>	Read exclude patterns from a file.
--include-from <file>	Read include patterns from a file.
--max-size <bytes>	Skip files larger than the limit.
--min-size <bytes>	Skip files smaller than the limit.
--max-depth <n>	Limit recursive scanning depth;
zero means unlimited.
--checksum	Include xxHash64 content checks in incremental comparisons.
Back up overwritten files.
--suffix<suffix>	Set the backup filename suffix.
Select partial - transfer handling. On failed/interrupted writes the
already-written temp file is retained (best-effort) for resumption.
With --partial --partial-dir <dir>, completed files are written under the
partial directory and installed atomically.
Set a relative partial - transfer directory below the server destination root.
Use with --partial.
--inplace	Write directly to the destination instead of using a temporary file.

Metadata and links

Option	Description
--preserve	Preserve supported file metadata, currently mode and modification time (long form only).
-l, --links	Request symlink preservation;
link-target transfer remains incomplete.
--copy-links	Copy symlink referents.
--safe-links	Skip symlinks that point outside the transfer tree.
--copy-unsafe-links	Copy unsafe symlink referents.
-S, --sparse	Sparse-file handling: receiver preserves holes (zero runs are written as holes; no wire change).

Output and logging

Option	Description
-v, --verbose	Enable debug logging.
--progress	Show live transfer progress.
--stats	Print transfer statistics.
--log-file <path>	Write log output to a file.
-V, --version	Print the FastSync protocol version.
--help	Print command usage.

Paths and transport

Option	Description
--ssh-port <port>	SSH port for the SSH transport (default: 22). Note the short -p is now rsync's --perms.
--fastsync-server-path <path>	Remote FastSync server path for SSH mode.
--source-dir <path>	Set the source directory explicitly.
--dest-dir <path>	Set the destination directory explicitly.
--save-to-disk	Enable server-side disk persistence.
--server-host <host>	TCP server address.
--server-port <port>	TCP server port. --port <port> / --port=<port> is an alias.
--tls	Enable TLS. Requires --cert and --key.
--cert <path>	TLS certificate file.
--key <path>	TLS private key file.
--ca <path>	CA file for peer verification.

Server Options

Option	Description
--stdio	Serve one SSH connection over standard input/output.
-p <port>	TCP listen port.
--tls	Enable TLS.
--cert <path>	TLS certificate file.
--key <path>	TLS private key file.
--ca <path>	CA file for peer verification.
--destination-root <path>	Confine received files to this server-side root;
defaults to the current directory.
--allow-delete	Permit client delete manifests. Deletion is refused by default. This also gates --force (which can recursively replace/remove a destination directory tree).
--allow-super	Standalone TCP listener only: keep super-user activities enabled for a root receiver. Without it a root standalone server forces SUPER_MODE_OFF, so client --devices/--write-devices/--super and client-chosen ownership requests are skipped/refused. Rejected with --stdio (the SSH remote argv is client-composed; use a forced command if the default must hold). No effect when not root. Daemon modules opt in per module with client owner = yes.
-v, --verbose	Enable debug logging.
--help	Print server usage.

Daemon configuration

fastsync-server --daemon --config FILE reads a line-based module config (an implicit global section, then [module] sections). Besides port, motd file, and address, the global section accepts:

• max connections = N — global cap on concurrent connections, default 100. The listener enforces it; 0, negative, and non-numeric values are parse errors.
• max connections per host = N — cap on concurrent connections from a single source IP, default 0 (unlimited). Enforced across all forked connection children through a shared registry.
• auth failure delay = MS — milliseconds to sleep after a failed authentication, default 500. 0 disables it and the value is capped at 5000, so online password guessing is rate-limited per connection. Successful auths are never delayed.
• auth lockout threshold = N — number of failed authentications from one source IP before that source is locked out, default 10; 0 disables the lockout. The failure counter is shared across every connection child, so the lockout holds even when the next attempt is handled by a different forked child.
• auth lockout duration = SECONDS — how long a locked-out source is refused (default 300). A locked-out client is refused before any SCRAM challenge is sent; a successful authentication clears the counter.
• hosts allow / hosts deny — comma- and/or whitespace-separated host access patterns.

A [module] may also set max connections (0 = unlimited; enforced per module across all connection children) and its own hosts allow/hosts deny.

The per-host cap and the shared auth lockout identify a source by its numeric peer IP. Loopback peers (127.0.0.0/8, IPv6 ::1) are exempt: every local client shares that one address, so counting or locking them out would let one local process deny service to all the others. The per-module and global max connections caps still apply to loopback. Because the key is the peer IP, max connections per host and auth lockout also cannot distinguish clients behind the same NAT, proxy, or reverse-proxy address — they share one budget and one lockout counter, so an over-aggressive lockout can affect unrelated users behind that address. Prefer TLS client certificates (--client-cn) plus hosts allow/hosts deny for per-client policy when clients share an address, and size auth lockout threshold accordingly.

The shared per-source table has a bounded lifetime: an entry with no live connection is reclaimed once its lockout has expired, or after it has been idle (300 s). If every entry is still live or locked, a new source is admitted without per-host accounting (fail open) and a rate-limited warning is logged; the per-module cap and host ACLs still apply. The occupancy counters are re-derived from the shared slot table after every child exit, so a child killed mid-transfer (or mid-registration) cannot leak a slot or an occupancy count.

Host patterns are * (match all), IPv4/IPv6 literals, or IPv4/IPv6 CIDR (10.0.0.0/8, 2001:db8::/32). Hostnames are not resolved, so hostname globs are rejected at parse time rather than silently never matching. 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 global list is checked before the module list, before authentication, and the connecting peer address (IPv4 or IPv6) appears in the connection and authentication audit log lines.

Architecture

Client

• Recursively scans the source tree with include, exclude, size, and depth filters.
• Sends individual files or serialized chunks.
• Performs incremental checks and optional content checksums.
• Uses a multithreaded producer-consumer pipeline when requested.
• Sends over TCP, TLS-wrapped TCP, or an SSH subprocess.
• Supports progress, statistics, backups, timeouts, and bandwidth limiting.

Server

• Runs as a TCP listener or one-shot SSH --stdio server.
• Receives and reassembles files and decompresses streaming zstd data.
• Applies supported metadata and writes files through a confined destination root.
• Uses temporary files and atomic rename by default.
• Handles delete manifests only when explicitly authorized.
• Enforces connection, message-size, and path-safety limits.

Protocol and Security

FastSync protocol version 2.21.0 is shared by the client and server. The current protocol is sender-driven and includes configuration negotiation, including the maximum allocation limit, incremental checks, checksums, manifests, keep-alives, abort handling, per-file remove-source results, and FastSync-native delta messages. Client and server versions must currently match exactly.

Daemon modules that declare auth users authenticate with a SCRAM-SHA-256-style challenge/response against a salted PBKDF2 verifier store: no password and no replayable bearer credential crosses the wire or is stored on the daemon. All store entries share one iteration count, and an unknown user is answered with a deterministic per-username dummy challenge, so probing the daemon cannot enumerate users. Store lines are generated with fastsync-server --hash-credentials <plaintext-file> (see RSYNC_COMPAT.md); redirect that output to an owner-only (mode 0600) file, and note that legacy user:SHA256HEX stores are rejected. FastSync also maintains an owner-only (mode 0600) <store>.dummykey sidecar next to the store: it holds the store-wide dummy key, is auto-created on first load, and must be preserved across daemon restarts so the dummy challenge for an unknown user stays stable (the key is never regenerated while the sidecar exists). The sidecar is secret material and must be protected like the credential store: keep it owner-only (mode 0600) and include it with the store in backups and credential rotation. If the sidecar 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, or fchmod failure), the daemon logs a warning and uses a transient key, so the cross-restart guarantee does not hold for those deployments. One residual is accepted: the store iteration count is observable pre-auth by design, since the miss path must match a hit.

An auth users module accepts credentials only when one of two conditions holds: (a) the connection is an encrypted, verified TLS connection whose client certificate matches the server's --client-cn, or (b) the connection is plaintext from a loopback peer and the operator explicitly passed --allow-unauthenticated. A remote plaintext peer is refused before any challenge is sent, and --allow-unauthenticated never permits remote plaintext auth: remote peers still require verified TLS regardless of the flag. Clients sending daemon credentials with --password-file to a non-loopback daemon must therefore use --tls; the client rejects a non-local plaintext credential destination before any network I/O. 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 a 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. --tls always mandates --client-cn, so a TLS connection to an auth-required module always has its client CN verified (--client-cn matches the certificate's CN only, not a subjectAltName, which is acceptable for a private CA).

TLS provides encrypted TCP transport. Supplying --ca enables certificate verification; without it, traffic is encrypted but peer identity is not verified. Use certificate verification for deployments where authentication matters. The default TCP transport is not encrypted.

The receiver protects its destination root with path validation, openat() directory traversal, O_NOFOLLOW, temporary files, and atomic renames. Delete operations require the server's explicit --allow-delete policy.

Compatibility Roadmap

The project will reach the drop-in replacement goal in stages:

1. Correct rsync option meanings, including short options, combined options, and --option=value syntax.
2. Add differential tests that compare FastSync and rsync contents, metadata, links, deletes, filters, dry runs, and exit codes.
3. Make -a implement the expected recursive, links, permissions, times, owner/group, and supported special-file behavior.
4. Complete symlink, sparse-file, metadata, delete-policy, and resumable-write semantics.
5. Add rsync remote-shell and daemon protocol interoperability.
6. Keep FastSync performance options as negotiated, optional extensions.

The exhaustive implementation matrix and compatibility notes are in RSYNC_COMPAT.md.

Testing

Run the unit test binary:

./build/tests

Run the Python integration suite:

python3 -m pytest tests/integration/ -n 4 --dist=load -m "not setpriv"

For stricter local validation:

cmake -B build-strict -S . -DSTRICT_WARNINGS=ON
cmake --build build-strict -j$(nproc)
cmake -B build-asan -S . -DSANITIZER=address
cmake --build build-asan -j$(nproc)

The benchmark tool compares FastSync configurations with rsync under controlled local and network conditions:

python3 benchmark/bench.py --help

Benchmark results measure transfer performance only. They do not establish rsync protocol or filesystem-semantic compatibility.

Performance Guidance

• Use -m for workloads with many files or enough CPU parallelism.
• Use -c or -z when network bandwidth is more constrained than CPU.
• Tune --chunk-size for file sizes, memory limits, and network latency.
• Use -f for large uncompressed TCP transfers where zero-copy I/O helps.
• Use --incremental to avoid retransmitting unchanged files.
• Use --delta for changed files when both endpoints are FastSync peers.
• Use --bwlimit when sharing a link with other traffic.

Always validate the compatibility behavior required by a deployment before replacing an existing rsync job.