OpenBSD's openrsync merges into base, ships ISC-licensed rsync alternative

OpenBSD has merged openrsync into its base system, providing an ISC-licensed reimplementation of rsync that speaks protocol 27 and interoperates with mainline rsync 3.1.3. The project supports a subset of upstream’s command-line flags and was built primarily to serve rpki-client, OpenBSD’s RPKI validator, with funding from NetNod, IIS.SE, SUNET, and 6connect. While OpenBSD is the supported target, the code compiles on other UNIX systems and can coexist with an existing rsync install.

Architecturally, openrsync mirrors the classic Tridgell design: a sender and receiver exchange a sorted file list, then negotiate block-level deltas using a fast Adler-32 rolling hash backed by an MD4 verification hash, with block size set to roughly the square root of file size (minimum 700 bytes, rounded up to a multiple of eight). Symlinks and directories are handled from metadata alone, while regular files go through the full block-exchange protocol and a final MD4 integrity check.

The significance is licensing and provenance rather than features. Replacing GPL-licensed rsync with a permissively licensed, audited, OpenBSD-style implementation removes a license-incompatible dependency from the BSD ecosystem and gives projects like rpki-client a synchronization tool that fits OpenBSD’s security and code-quality standards.