Cross-version behaviour checks - confirming whether a behaviour a user reported on an old release is version-specific or option-driven.
The version-mixing test suite: testsuite/runtests.py --rsync-bin2=... runs the current code against one of these as the daemon or remote-shell peer. CI (.github/workflows/ubuntu-version-mix.yml) exercises every binary against its peer profile in testsuite/profiles/.

Binaries are statically linked so they run regardless of the host's shared libraries and named rsync_<version>:

Binary	Version	Protocol	Notes
rsync_2.6.0	2.6.0	27	2004; needs autoconf regeneration
rsync_3.0.0	3.0.0	30	2008
rsync_3.1.0	3.1.0	31	2013
rsync_3.1.3	3.1.3	31	Ubuntu 18.04 and Debian Buster era, 2018
rsync_3.2.0	3.2.0	31	2020; zstd, lz4 and xxhash negotiation added
rsync_3.2.7	3.2.7	31	2022
rsync_3.3.0	3.3.0	31	2024
rsync_3.4.0	3.4.0	32	2025
rsync_3.4.1	3.4.1	32	2025

These are every x.y.0 release from 2.6.0 (2004) onward plus a few point releases. 2.6.0 is the practical floor: older tags need progressively more porting to build on a current toolchain.

All built --disable-openssl and with _FORTIFY_SOURCE disabled (see below); xxhash/zstd/lz4 are compiled in where the version supports them.

Adding a version

./build_static.sh 3.2.7            # uses git tag v3.2.7
./build_static.sh 3.0.9 v3.0.9     # explicit tag if naming differs

The script checks out the tag into a throwaway git worktree, applies the minimal patches needed to compile old sources on a modern toolchain, links statically and verifies the result. It then installs rsync_<version> here and removes the worktree.

Override the source repo with RSYNC_REPO=/path/to/rsync ./build_static.sh ... (defaults to ../rsync.4).

Why the patches?

Modern GCC (>= 14, C23 default) and glibc reject things old rsync relied on. build_static.sh handles these, each guarded so it's a no-op when not needed:

K&R lseek64() redeclaration in syscall.c clashes with glibc's real prototype - removed.
gettimeofday() - glibc only has the 2-arg form; configure misdetects the 1-arg form, so HAVE_GETTIMEOFDAY_TZ is forced on in config.h.
C23 () == (void) breaks K&R prototypes called with arguments such as the qsort comparator and pool->bomb - built with -std=gnu11.
Assorted modern -Werror promotions (incompatible pointer types, implicit declarations) downgraded to warnings; bundled zlib/popt used to keep the static link self-contained.
OpenSSL (3.2+) is disabled with --disable-openssl: linking libcrypto.a statically drags in jitterentropy (jent_*) and zlib's uncompress (OpenSSL's COMP module), which don't resolve here. OpenSSL only provided optional MD4/MD5, which rsync implements natively, so checksum behaviour is unaffected.
_FORTIFY_SOURCE disabled (-U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=0): modern Ubuntu defaults it to =3, whose stricter object-size checks turn latent (historically benign) over-reads in OLD rsync into hard *** buffer overflow detected *** aborts when the binary runs as a server or daemon - which made 3.1.3 and 3.2.7 unusable as peers. Disabling it makes the archival binaries behave as the released versions did.
Pre-3.0 tags such as 2.6.0 ship configure.in, not a generated configure. The script runs autoheader/autoconf to generate it, after neutralizing the AC_CHECK_FUNCS(fn,,AC_LIBOBJ(lib/...)) fallbacks for inet_ntop/inet_pton/getaddrinfo/getnameinfo - modern autoconf emits broken shell for those never-taken branches (the funcs exist in glibc). It also generates proto.h (no make rule in that era) and stubs the vendored lib/addrinfo.h the tag dropped (modern glibc supplies struct addrinfo). All guarded so they no-op on 3.x.

Newer versions may need fewer or different tweaks; if a build fails, the script prints the first compiler errors from its log.