Ssh – way to send custom arguments to an rsync server

rsyncssh

I have a backup server that clients use with rsync over SSH. The server is configured to run a custom shell script that wraps the invocation of the rsync server side. The client side is unaware of this and can issue any valid rsync command-line.

I would like to pass some arguments from the client into that shell script. Something like

$ rsync -av -customarg1 value1 -customarg2 value2 file1 file2 user@server

The client-side rsync rejects any arguments that it doesn't know, as does ssh (via rsync -e). The best I have come up with is to pass arguments as fake server-side file specs that the script extracts from the server-side command-line prior to executing it to launch the rsync server. i.e a command like

$ rsync -av file1 file2 user@server:some/path\;customarg1=value1\;customarg2=value2

would provide two arguments and then behave as if the command was

$ rsync -av file1 file2 user@server:some/path

(This uses the fact that SSH sets a number of environment variables including $SSH_ORIGINAL_COMMAND which contains the command issued by the ssh client (i.e the rsync server command launched by the rsync client) regardless of what the ssh server is configured to execute (i.e. the custom shell script))

Best Answer

Perhaps you could use the -M option (long version --remote-option), which does not seem to be checked by the client rsync. -M-xxxx is passed to the remote with the -M stripped off as -xxxx. You can easily play with this using -M-M-xxxx, which is ignored by client and remote:

rsync -av -M-M-myvar=val /tmp/test/ remote:test/

If your server front-end recognises and removes the resulting flags before calling rsync, you get what you needed.

You can play further with --rsync-path which allows you to run any script. It will get the remote args concatenated. Eg

rsync -a -M-myvar=val --rsync-path='echo hello >/tmp/out; ./mysync' /tmp/test/ remote:test/

will run on the remote something like

echo hello >/tmp/out
./mysync --server -logDtpre.iLsfx -myvar=val . test/

Related Solutions

Shell – RYSNC over SSH not working for restricted shell

You should use rrsync (probably available gzipped on your system e.g. in /usr/share/doc/rsync/scripts/) and associate a line in your authorized_keys file with the public key installed there for the rsync:

command="$HOME/bin/rrsync -ro ~/rsyncdir/",no-agent-forwarding,no-port-forwarding,no-pty,no-user-rc,no-X11-forwarding

If necessary more detail can be found here and here

Does rsync require both source host and destination host to run rsync as client, server, or daemon

When it comes to rsync, think of client and server as roles, that the same rsync binary can fulfill. When it's running as a daemon, you build an rsyncd.conf file, and run rsync with the --daemon argument.

One rsync binary can connect to another (remote) rsync binary either directly - if rsync is running as a daemon listening for it's own connections on a remote host, or via remote shell execution - where rsync uses ssh to connect to a remote host and execute the rsync binary at that host, with both rsync processes passing data back and forth to each other.

To hopefully answer your questions:

When transferring files between local and remote by accessing via rsync daemon, is the remote host required to run rsync as a daemon?
- Yes. If you're going to access an rsync daemon, that daemon has to be running.
When transferring files between local and remote by accessing via a remote shell, does the remote host need to run rsync? If yes, does the remote host run rsync as a server but not as a daemon, or just as the local host runs rsync as a client?
- Yes. When rsync has to be available on both hosts, either in the default $PATH, or with the full path to the binary specified in the command options.
When transferring files within the local host, does the local host need to run a rsync client and a rsync server?
- No. The rsync client binary can function as both roles at the same time if both the SRC and DEST paths are locally available.
When listing the source files instead of copying, if the source is in a remote host, does the remote host need to run rsync as a server or daemon?
- No, running an rsync daemon is not necessary, a remote client can be triggered if you use rsync -e ssh to access rsync (the client) on the remote host. (The remote-shell-spawned rsync in Q 6.)
Which use cases does "Note that rsync must be installed on both the source and destination machines" apply to, and not apply to, respectively?
- The rsync binary must be installed on both machines. In the case that the SRC and DEST are the same machine, the lone rsync binary will essentially perform both roles.
In the GENERAL part, "a server can be either a daemon or a remote-shell spawned process". Does a remote host running rsync as a server as "a remote-shell spawned process" mean that the remote host runs rsync in the same way as the local host runs rsync?
- Yes. Effectively so. The only item of note is that the remotely spawned rsync session is fulfilling the server role.

Best Answer

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