Rsync via two ssh tunnels possible (standard method mentioned k times not possible?)

[Roman Fiedler]

Matt McCutchen wrote:
> On Fri, 2008-12-12 at 12:56 +0100, Roman Fiedler wrote:
>> The goal is to sync two directories using rsync without running the
>> rsync daemon on one of the two hosts...
 >
> I suggest you just replace your "nc" command with an rsync daemon
> listening on port 6666 on the SRC machine.  This isn't any less secure
> than what you're trying now.

In the end I hope to run the two rsyncs just using the stdin/out of the 
two ssh connections and the relay connection is done on the local host, 
the nc thing is just for testing because it's easier to strace.

> If you really don't want an rsync daemon anywhere in the picture (though
> I'm not clear on why),

I hoped that I do not have to create a .conf file for the daemon (or 
fiddle around with the cmd parameters) and that I do not need to care 
about the auth settings for the daemon. Apart from that my rsync via 
cmdline would run only once and terminate immediately after sync and 
closing the ssh connection would stop the server thread if not 
terminated before.

> you could use a single-colon source argument with
> "-e 'nc localhost 4444 #'" to have the client connect to the forwarded
> port and expect a bare server.

I tried that, but rsync tried to execute

[pid  8953] execve("/bin/nc", ["nc", "localhost", "1235", "#", 
"localhost", "rsync", "--server", "--sender", ".", "src"], [/* 23 vars 
*/]) = 0

The additional args caused nc to fail. Also \x00 instead of # did not 
work. But following worked:

* Server side:
   nc -vnlp 1235 -c 'rsync --server --sender -r . src'
* Client side
   rsync -e './direct.sh' localhost:src dst
with direct.sh:
   #!/bin/bash
   nc -v localhost 1235

The direct.sh just ignores any additional arguments.

   But then it's your responsibility to
> make sure the server arguments on SRC are correct; rsync may crash if
> they aren't.

Yes, seems that that's exactly what happens, e.g with

nc -v -c 'rsync -v --server -a src .' localhost 7777
nc -vnlp 7777 -c 'rsync -v --server --sender -r . src'

[pid  8958] write(1, ".\0\0\10ERROR: buffer overflow in recv_rules 
[sender]\n", 50) = 50
[pid  8958] write(1, "Z\0\0\10rsync error: error allocating core memory 
buffers (code 22) at util.c(121) [sender=2.6.9]\n", 94) = 94