With autossh you can establish an SSH reverse tunnel from a given system, provided it can reach some other machine via SSH outside its own network. You can also do that with ssh alone, but autossh comes with added features that are worthwhile exploring and using.
Ubuntu 14.04 LTS and older (Upstart)
To use autossh with Upstart, you need two files: /etc/init/autossh.conf and /etc/init/autossh.override. The former is the main Upstart script, the latter is a file providing customizable settings for the former.
/etc/init/autossh.conf:
description "Establish persistent SSH tunnel"
start on local-filesystems and net-device-up IFACE=eth0 and started ssh
stop on [016]
respawn
respawn limit 5 60 # respawn max 5 times in 60 seconds
script
# exec 2>>/tmp/autossh.log
# set -x
export AUTOSSH_POLL
export AUTOSSH_FIRST_POLL
export AUTOSSH_LOGFILE
export AUTOSSH_LOGLEVEL
export AUTOSSH_GATETIME
sleep 5
autossh -M $AUTOSSH_MONIPORT -- \
-4Nngi $SSH_IDENTITY $SSH_OPTIONS \
-o 'StrictHostKeyChecking=no' \
-o 'UserKnownHostsFile=/dev/null' \
-o 'PasswordAuthentication=no' \
-o 'PubkeyAuthentication=yes' \
-o 'ServerAliveInterval 60' \
-o 'ServerAliveCountMax 3' \
-o 'BatchMode=yes' \
$SSH_CONNECTION_HOST
end script
If you ever run into trouble, uncomment the two commented out lines in the script and have a look at the /tmp/autossh.log afterward.
/etc/init/autossh.override:
setuid user setgid usergroup env SSH_CONNECTION_HOST=user@host.domain.tld env SSH_IDENTITY=/home/user/.ssh/id_rsa env SSH_OPTIONS="-R 10022:localhost:22" env AUTOSSH_MONIPORT=10023 env AUTOSSH_POLL=60 env AUTOSSH_FIRST_POLL=30 env AUTOSSH_LOGFILE=/var/log/autossh.log env AUTOSSH_LOGLEVEL=7 env AUTOSSH_GATETIME=0
Some remarks:
SSH_CONNECTION_HOSTis the host to which you want to connect on the outside.SSH_OPTIONSgives the arguments forsshas started byautossh; here we forward port 22 fromlocalhost(the machine running the Upstart script) to port 10022 atlocalhostforhost.domain.tldAUTOSSH_LOGFILEmake sure this file is writable byuserorusergroup
Use start autossh, stop autossh and restart autossh to control this Upstart service. If you decided to name your file differently, filename.conf means you need to pass filename as the service name.
Ubuntu 16.04 LTS and newer (systemd)
With the introduction of systemd to Ubuntu, we need to provide a unit file on these newer Ubuntu versions.
/etc/systemd/system/autossh.service:
[Unit] Description=Establish persistent SSH tunnel Requires=ssh.service Wants=network-online.target After=network-online.target [Service] User=user Group=usergroup Environment=AUTOSSH_POLL=60 AUTOSSH_FIRST_POLL=30 AUTOSSH_LOGFILE=/var/log/autossh.log AUTOSSH_LOGLEVEL=7 AUTOSSH_GATETIME=0 ExecStart=/usr/bin/autossh -M 10023 -- -4Nngi /home/user/.ssh/id_rsa -R 10022:localhost:22 -o 'StrictHostKeyChecking=no' -o 'UserKnownHostsFile=/dev/null' -o 'PasswordAuthentication=no' -o 'PubkeyAuthentication=yes' -o 'ServerAliveInterval 60' -o 'ServerAliveCountMax 3' -o 'BatchMode=yes' user@host.domain.tld RestartSec=6 Restart=always [Install] WantedBy=multi-user.target
This unit file combines the settings from what was the .override in Upstart directly into the unit. If you wanted to separate most of the settings out, you could use the EnvironmentFile stanza with the respective file containing variable assignments.
To have systemd re-read its unit files, run systemctl daemon-reload. To verify the status (also after starting), run systemctl status autossh.service. To start or restart the service, run systemctl restart autossh.service. And last but not least to enable the service to start at boot time, run systemctl enable autossh.service.
// Oliver
PS: beware of the -f switch of autossh. Neither Upstart nor systemd like them particularly. In case you decide to use them you need to let these init systems know how many times a fork() happens, so that it can figure out the PID of the resulting daemon process.
