RSYNCD(5) File Formats Manual RSYNCD(5)

rsyncd — rsyncd wire protocol

The rsyncd protocol described in this relates to the BSD-licensed openrsync(1), a re-implementation of the GPL-licensed reference utility rsync(1). It is compatible with version 27 of the reference.

The rsyncd protocol is an envelope protocol for rsync(5) between a client and an rsync://-capable server. It provides a means to exchange capabilities information prior to file transfer.

In this document, "client" refers to the openrsync(1) utility making the request. It follows that "server" refers to the daemon servicing the request.

A connection between a client and server consists of host, a module, and zero or more paths.

openrsync rsync://host/module/path1 rsync://host/path2... dest

At this time, operating in sender mode (with the rsync:// host receiving information) is not described in this document.

These are the same as in rsync(5). A newline is always a standalone \n.

After initialising a connection, the client and server exchange the following information, in order. This portion of the process is multiplexed.

  1. client sends requested module followed by newline
  2. server responds with preamble followed by standalone newline

The requested module must have non-zero length. The preamble consists in a sequence of lines. Each line either contains free-form text sent by the server as a "motd" (message of the day) or a command:

@RSYNCD: command\n

The only supported command is the server protocol specification:

@RSYNCD: xx[.yy]\n

The optional component is the submodule, which may be discarded. The version may only be specified once. Both the motd and commands end in the special termination command:

@RSYNCD: OK\n

Following that, the client must send the command-line arguments that would otherwise be used to start a openrsync(1) server. Each argument must be specified on its own line, e.g.,

--server\n--sender\n-r\n-t\n.\npath1\npath2

This must be followed by a standalone newline.

If the server does not understand or accept any of the command-line arguments, it will exit at this point.

Following this, the client must read the integer-length session checksum seed. Multiplexing is subsequently enabled.

The sequence that follows is stipulated in rsync(5) following the handshake.

openrsync(1), rsync(5)

February 21, 2025 Mac OS X 14