I have a script which connects to a remote host to replicate some files via SSH. I must connect to an rsync daemon listening on port 873. To accomplish this I have a ssh configuration like this.
Host blah LocalForward 10873 127.0.0.1:873 Hostname 10.2.4.1
Unfortunately because of the timing of this script, there is a small chance that on occasion when it attempts to establish the SSH session another similar script will have the port open to a different host.
If I connect while another ssh session is open I get the obvious message Address already in use.
me@local:~$ ssh blah bind: Address already in use channel_setup_fwd_listener: cannot listen to port: 10873 Could not request local forwarding. Linux remote 2.6.26-2-amd64 #1 SMP Tue Jan 25 05:59:43 UTC 2011 x86_64 motd... remote:~$
What I want to have happen is to have the SSH client immediately exit with a exit code so I can put some logic into my script to report a failure instead of trying to run any commands against the port forwarded to the incorrect host.