This is kind of informal and may be wrong, but it helped me. It's basically a summary of clientserver.c and authenticate.c. -- Martin Pool <email@example.com> $Id: csprotocol.txt,v 1.4 2001/08/23 06:14:54 mbp Exp $ This is the protocol used for rsync --daemon; i.e. connections to port 873 rather than invocations over a remote shell. When the server accepts a connection, it prints a greeting @RSYNCD: <version> where <version> is the numeric version; currently 24. It follows this with a free text message-of-the-day. It expects to see a similar greeting back from the client. The server is now in the connected state. The client can either send the command #list to get a listing of modules, or the name of a module. After this, the connection is now bound to a particular module. Access per host for this module is now checked, as is per-module connection limits. If authentication is required to use this module, the server will say @RSYNCD: AUTHREQD <challenge> where <challenge> is a random string of base64 characters. The client must respond with <user> <response> where <user> is the username they claim to be, and <response> is the base64 form of the MD4 hash of challenge+password. At this point the server applies all remaining constraints before handing control to the client, including switching uid/gid, setting up include and exclude lists, moving to the root of the module, and doing chroot. If the login is acceptable, then the server will respond with @RSYNCD: OK The client now writes some rsync options, as if it were remotely executing the command. The server parses these arguments as if it had just been invoked with them, but they're added to the existing state. So if the client specifies a list of files to be included or excluded, they'll defer to existing limits specified in the server configuration. At this point the client and server both switch to using a multiplexing layer across the socket. The main point of this is to allow the server to asynchronously pass errors back, while still allowing streamed and pipelined data. Unfortunately, the multiplex protocol is not used at every stage. We start up in plain socket mode and then change over by calling io_start_buffering. Of course both the client and the server have to do this at the same point. The server then talks to the client as normal across the socket, passing checksums, file lists and so on. For documentation of that, stay tuned (or write it yourself!). ------------ Protocol version changes 25 (2001-08-20, 2.4.7pre2) Send an explicit "@RSYNC EXIT" command at the end of the module listing. We never intentionally end the transmission by just closing the socket anymore.