When ssh is run to create an ssh tunnel and to do nothing else, it does not exit
if the local port is already in use. I tried a few different command lines to
make sure that ssh was aware that I was only trying to create a tunnel. I hoped
that if the tunnel could not be established completely, ssh would exit with an
error status. This is important to programs and scripts that use ssh to only
create tunnels.

I tried the following:

$ ./ssh -n -a -N -L 80:localhost:17 remote
bind: Address already in use
channel_setup_fwd_listener: cannot listen to port: 80
Could not request local forwarding.

$ ./ssh -f -a -N -L 80:localhost:17 remote
bind: Address already in use
channel_setup_fwd_listener: cannot listen to port: 80
Could not request local forwarding.
$ ps
jeremyp  25826  0.0  0.2  3112 1628 ?        S    15:08   0:00 ./ssh -f -a -N -L
80:localhost:17 remote

In the first example, I had to C-c out... in the second, the process still
appears in ps output.

I'm not sure whether this is a minor bug or an enhancement. It seems that ssh
should always exit if what it was able to setup cannot be used and it has
nothing else to do (I can't use the tunnel without a local port to connect to).