Unable to use autossh in background even with absolute path

Question

I would love to set autossh to run at boot adding it to /etc/rc.local.

This command works:

autossh -i /root/.ssh/id_rsa -R 2522:localhost:22 user@address

But, if I add the -f option

autossh -f -i /root/.ssh/id_rsa -R 2522:localhost:22 user@address

The ssh session is not started.

As you can see, I'm using an absolute path for my identity file, so this seems to be a different problem from the one stated here: autossh in background does not work

From /var/log/syslog:

Oct 18 11:08:39 raspberrypi autossh[2417]: starting ssh (count 1)
Oct 18 11:08:39 raspberrypi autossh[2417]: ssh child pid is 2418
Oct 18 11:08:39 raspberrypi autossh[2417]: ssh exited with status 0; autossh exiting

I'm using it with debian wheezy on a raspberry pi, autossh version 1.4c.

Could it be that it's passing the -f option to ssh instead?

Answer 1

When you start autossh without -f, you get a shell. While the shell is working, you get port forwarding. After you logout, ssh terminates with exit code 0 and autossh knows it does not need to launch ssh session again.

When you start autossh with -f, it passes -f to ssh too. ssh is then running in the background and does not give you a shell. As you did not specify any other flags or remote command, ssh exits immediately with status 0 (nothing to do), and autossh does not start it over.

Just add -N option to avoid it:

-N      Do not execute a remote command.  This is useful for just forwarding
        ports (protocol version 2 only)

Like this:

autossh -f -N -i /root/.ssh/id_rsa -R 2522:localhost:22 user@address