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+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 <mbp@samba.org>
+
+
+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 newline-terminated
+greeting line:
+
+  @RSYNCD: <version>.<subprotocol> <digest1> <digestN>
+
+The <version> is the numeric version (see PROTOCOL_VERSION in rsync.h)
+The <subprotocol> is the numeric subprotocol version (which is 0 for a
+final protocol version, as the SUBPROTOCOL_VERSION define discusses).
+The <digestN> names are the authentication digest algorithms that the
+daemon supports, listed in order of preference.
+
+An rsync prior to 3.2.7 omits the digest names.  An rsync prior to 3.0.0
+also omits the period and the <subprotocol> value.  Since a final
+protocol has a subprotocol value of 0, a missing subprotocol value is
+assumed to be 0 for any protocol prior to 30.  It is considered a fatal
+error for protocol 30 and above to omit it.  It is considered a fatal
+error for protocol 32 and above to omit the digest name list (currently
+31 is the newest protocol).
+
+The daemon expects to see a similar greeting line back from the client.
+Once received, the daemon follows the opening line with a free-format
+text message-of-the-day (if any is defined).
+
+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>
+
+The <user> is the username they claim to be. The <response> is the
+base64 form of the digest hash of the challenge+password string. The
+chosen digest method is the most preferred client method that is also in
+the server's list.  If no digest list was explicitly provided, the side
+expecting a list assumes the other side provided either the single name
+"md5" (for a negotiated protocol 30 or 31), or the single name "md4"
+(for an older protocol).
+
+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
+
+31	(2013-09-28, 3.1.0)
+
+	Initial release of protocol 31 had no changes.  Rsync 3.2.7
+	introduced the suffixed list of digest names on the greeting
+	line.  The presence of the list is allowed even if the greeting
+	indicates an older protocol version number.
+
+30	(2007-10-04, 3.0.0pre1)
+
+	The use of a ".<subprotocol>" number was added to
+	@RSYNCD: <version>.<subprotocol>
+
+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.