FastSync/CHANGELOG.md

Changelog

All notable changes to FastSync are documented here. Versions match PROTOCOL_VERSION (printed by fastsync --version); the client and server must run the same version because the handshake is strict.

[Unreleased]

Added

• Server-contacting --dry-run (protocol 2.21.0). --dry-run now performs a real handshake with a remote/daemon receiver and reports exactly what WOULD change based on receiver state (existing destination files, mtimes, checksums, basis dirs). The wire config carries the dry-run intent (Config.dry_run) and the receiver answers each per-file check with STATUS_DRY_RUN_TRANSFER (would transfer) or STATUS_OK (already up to date); the sender prints the would-transfer set and its trailer without sending any file data. The receiver performs the normal read-only incremental decision but mutates nothing: no temp files, writes, renames, deletes, metadata/xattr/chown, or directory creation. A plain local destination (no explicit --server-port/remote) keeps the original client-side dry-run. Would-delete reporting for --delete* is deferred to a follow-up; dry-run never deletes.

Security

• Enforce the daemon's per-module max connections cap and add a global max connections per host cap plus a cross-process auth lockout (auth lockout threshold / auth lockout duration). Because the listener forks one child per connection, the counters live in an anonymous shared mapping created before the accept loop and reclaimed by the parent's SIGCHLD handler, so the per-module, per-source and auth-failure state is shared across every child (including after SIGKILL). The per-source table now has a bounded lifetime (expired-lockout/idle entries are reclaimed, with a rate-limited warning when it is genuinely full), and the occupancy counters are re-derived from the shared slot table on every child exit so a child killed mid-registration cannot leak a count. Trusted loopback peers are exempt from the per-host cap and the auth lockout (they share one address); clients behind a shared NAT/proxy still share a single per-host budget and lockout, which is documented.

[2.21.0] - 2026-09-13

Added

• Optional server→client rejection detail (protocol 2.21.0). A rejected operation may now carry a bounded human-readable reason via STATUS_ERROR_DETAIL instead of a bare STATUS_ERROR, so the client can report why the server refused (daemon module gate, config validation, receiver-side path/node validation). receive_status() transparently maps the new status back to STATUS_ERROR for every existing call site and captures the reason into a thread-local buffer exposed by protocol_last_error(). The detail body is always consumed, so the stream cannot desynchronize, and messages are sliced to MAX_ERROR_DETAIL_BYTES (4096) on send.

Changed

• receive_incremental_check() (the per-file STATUS_CHECK fast path) is split into small static helpers with a short linear orchestrator. Pure refactor: the wire byte stream and all cleanup are unchanged.

[2.20.0] - 2026-09-13

Security

• Cap cumulative DirTimeList growth and bound pre-auth config-string memory (remote memory-exhaustion DoS).
• Daemon host access control (hosts allow/hosts deny, IPv4/IPv6/CIDR), configurable global max connections, connection audit logging, and a bounded auth failure delay throttle. IPv4-mapped peers are normalized and invalid patterns are rejected at parse time (no silent fail-open).
• Honor --timeout for protocol I/O and bound idle/session time to defeat keepalive slowloris; child-safe signal handling in the forked daemon.
• Compiler/linker hardening (_FORTIFY_SOURCE, stack protector, PIE, RELRO) and pinned build dependencies.

Fixed

• Use-after-free in the basis-dir oversize preflight.
• Placeholder Data leaks, missing_args leak, scanner chunk leak.
• Thread-safe logging; single fd owner and cleanup epilogue in the server handler.

Performance

• Metadata now crosses the wire as one packed frame (protocol 2.20.0).
• Delete keep-set and --files-from lookups indexed (O(n*m) → O(n)).
• Reused per-thread zstd contexts; TCP_NODELAY by default.
• Byte-bounded sender queues; removed a redundant scanner stat().

[2.19.0] - 2026-09-12

Security

• Daemon authentication rewritten as SCRAM-SHA-256 challenge/response (STATUS_AUTH_CHALLENGE → STATUS_AUTH_RESPONSE → STATUS_AUTH_OK/STATUS_AUTH_FAILED), replacing the old replayable static SHA-256(password) bearer credential. Each proof is bound to a fresh per-connection server nonce plus a client nonce, so a captured response can never be reused.
• Salted verifier store. --password-file/--early-input now hold user:$fastsync$1$pbkdf2-sha256$<iters>$<salt>$<stored_key>$<server_key> (PBKDF2-HMAC-SHA256, default 600000 iterations, range 100000–10000000). The legacy user:SHA256HEX form is hard-rejected; there is no auto-upgrade. Generate stores offline with fastsync-server --hash-credentials FILE [--iterations N].
• Username-enumeration hardening. Unknown/off-list users are answered with a dummy verifier whose salt is a deterministic per-username value (HMAC-SHA256(dummy_key, username)), using the store-wide uniform iteration count and a constant-time full-length membership scan. The dummy key is persisted in an owner-only <store>.dummykey sidecar (atomic publish, exact mode 0600) so challenges are stable across restarts.
• Verified transport for auth-required modules. A module with auth users accepts credentials only over verified TLS whose client certificate matches --client-cn, or — when --allow-unauthenticated is explicitly set — plaintext from a loopback peer. Remote plaintext is refused before any challenge. Clients must use --tls to send --password-file credentials to a non-loopback daemon; --client-cn is mandatory with --tls.
• Secret hygiene. The plaintext password, derived keys, nonces/proofs and the dummy key are wiped from memory on every path and never logged.
• Carried-over hardening: -K TOCTOU-safe directory walk (openat(O_NOFOLLOW) per component), always shell-quoted SSH remote path, TLS compression/renegotiation disabled, race-free (open-then-fstat) --password-file/--early-input checks, log-injection escaping, and lazy protocol debug escaping.

Added

• fastsync-server --hash-credentials FILE [--iterations N] offline tool.
• <store>.dummykey sidecar (auto-created, owner-only, 0600).
• Integration tests for auth replay rejection, malformed frames, legacy-store refusal, and the loopback/TLS transport policy; fuzz targets for config receive and daemon-auth parsing.

Changed

• Protocol version 2.18.0 → 2.19.0 (breaking). The config-frame auth block is now [present][username] (digest removed) and the auth challenge/response frames are interleaved between the config frame and its STATUS_OK. A 2.19.0 client and a 2.18.0 server (or vice versa) fail cleanly at the handshake.
• Daemon modules declaring auth users require a configured credential store at startup (fail closed); operators regenerate stores from plaintext with --hash-credentials.

Notes

• First tagged release. FastSync implements rsync-compatible file synchronization over TCP and SSH with TLS (OpenSSL), streaming zstd compression, multithreaded transfers, and incremental sync. See RSYNC_COMPAT.md for the flag-parity matrix.