autossh pid is not equal to the one in pidfile when using start-stop-daemon

Question

I am trying to create a service on my Debian Wheezy system.

When trying to use start-stop-daemon to run autossh, the pid contained into the pidfile does not match with the autossh process.

$ AUTOSSH_PIDFILE=/var/run/padstunnel.pid
$ sudo start-stop-daemon --make-pidfile --background --name mytunnel --start --pidfile /var/run/mytunnel.pid --exec /usr/lib/autossh/autossh -- -M 0 -p 22 user@server -f -T -N -R 31022:localhost:31222
$ ps -elf |grep autossh
1 S root       447     1  0  80   0 -   329 pause  19:07 ?        00:00:00 /usr/lib/autossh/autossh -M 0 -p 22 ...
$ cat /var/run/mytunnel.pid
446

This behaviour prevents from stopping autossh using start-stop-daemon or kill it using the pid from the pidfile.

Is there any reason to this behaviour ?

How to workaround it and make autossh's pid matches the pidfile ?

Have you tried setting the `AUTOSSH_PIDFILE` variable and having `autossh` write the pid file itself? — Eric Renouf, Dec 04 '15 at 19:31
That's because `-f` tells autossh to drop into the background. (i.e. fork). Try removing it. — BraveNewCurrency, Dec 05 '15 at 18:00
removing `-f`` option does not work well neither: two processes are then created by autossh, and pidfile's pid does not match the root process. See more info in the answer I found. — lauhub, Dec 06 '15 at 17:17

score 3 · Answer 1 · edited Apr 13 '17 at 12:13

The solution I found is inspired by this answer.

Actually, the AUTOSSH_PIDFILE variable could not be used by autossh (because start-stop-daemon runs in a different environment).

So the workaround is to use :

$ sudo start-stop-daemon --background --name mytunnel --start --exec /usr/bin/env AUTOSSH_PIDFILE="/var/run/mytunnel.pid" /usr/lib/autossh/autossh -- -M 0 -p 22 user@server -f -T -N -R 31022:localhost:31222

/usr/bin/env AUTOSSH_PIDFILE="/var/run/mytunnel.pid" correctly defines the necessary environment variable
--make-pidfile and --pidfile are no longer required by start-stop-daemon
sudo start-stop-daemon --pidfile /var/run/mytunnel.pid --stop now works to kill autossh
--background option makes the ssh's -f optional (using -f or not does not change anything if --background is used)

The reason for the behaviour is not completely clear to me. However, it seems that autossh automatically creates several processes to handle correctly ssh instances when it does not see AUTOSSH_PIDFILE variable.

Edit:

When using it from a service init script (in /etc/init.d/servicename), the syntax has to be modified:

sudo start-stop-daemon --background --name mytunnel --start --exec /usr/bin/env -- AUTOSSH_PIDFILE="/var/run/mytunnel.pid" /usr/lib/autossh/autossh -M 0 -p 22 user@server -f -T -N -R 31022:localhost:31222

Notice the -- that must come just after the /usr/bin/env command (it was after the /usr/lib/autossh/autossh from command line).

TheChymera · Answer 2 · 2022-05-24T15:06:15.800

Just an update for a working solution from 7 years into the future as I recently came across this exact same issue.

The AUTOSSH_PIDFILE variable for some reason will make the program fail without any output, including when using -v. What I found would work is simply foregoing -f on autossh and using --background on the start-stop-daemon. Apparently this also necessitates the -N flag on autossh, which is as best I can tell undocumented.

Could be a new development or upstream issues, but in any case the answer by OP did not work for me, but this did:

start-stop-daemon --start --background --user myuser --quiet --make-pidfile --pidfile "/var/run/myhost_autossh.pid" --exec autossh -- -M 0 -N myhost

autossh pid is not equal to the one in pidfile when using start-stop-daemon

2 Answers2