Start N processes with one systemd service file

daemonsystemd

I found this systemd service file to start autossh to keep up a ssh tunnel: https://gist.github.com/thomasfr/9707568

[Unit]
Description=Keeps a tunnel to 'remote.example.com' open
After=network.target

[Service]
User=autossh
# -p [PORT]
# -l [user]
# -M 0 --> no monitoring
# -N Just open the connection and do nothing (not interactive)
# LOCALPORT:IP_ON_EXAMPLE_COM:PORT_ON_EXAMPLE_COM
ExecStart=/usr/bin/autossh -M 0 -N -q -o "ServerAliveInterval 60" -o "ServerAliveCountMax 3" -p 22 -l autossh remote.example.com -L 7474:127.0.0.1:7474 -i /home/autossh/.ssh/id_rsa

[Install]
WantedBy=multi-user.target

Is there a way to configure systemd to start several tunnels in one service.

I don't want to create N system service files, since I want to avoid copy+paste.

All service files would be identical except "remote.example.com" would be replace with other host names.

1.5 year later …

I asked this question roughly 1.5 year ago.

My mind has changed. Yes, it's nice, that you can do this with systemd, but I will use configuration-management in the future.

Why should systemd implement a template language and substitute %h? .. I think it makes no sense.

Several months later I think this looping and templating should be solved on a different level. I would use Ansible or TerraForm for this now.

Best Answer

Well, assuming that the only thing changing per unit file is the remote.example.com part, you can use an Instantiated Service.

From the systemd.unit man page:

Optionally, units may be instantiated from a template file at runtime. This allows creation of multiple units from a single configuration file. If systemd looks for a unit configuration file, it will first search for the literal unit name in the file system. If that yields no success and the unit name contains an "@" character, systemd will look for a unit template that shares the same name but with the instance string (i.e. the part between the "@" character and the suffix) removed. Example: if a service getty@tty3.service is requested and no file by that name is found, systemd will look for getty@.service and instantiate a service from that configuration file if it is found.

Basically, you create a single unit file, which contains a variable (usually %i) where the differences occur and then they get linked when you "enable" that service.

For example, I have a unit file called /etc/systemd/system/autossh@.service that looks like this:

[Unit]
Description=AutoSSH service for ServiceABC on %i
After=network.target

[Service]
Environment=AUTOSSH_GATETIME=30 AUTOSSH_LOGFILE=/var/log/autossh/%i.log AUTOSSH_PIDFILE=/var/run/autossh.%i.pid
PIDFile=/var/run/autossh.%i.pid
#Type=forking
ExecStart=/usr/bin/autossh -M 40000 -NR 5000:127.0.0.1:5000 -i /opt/ServiceABC/.ssh/id_rsa_ServiceABC -l ServiceABC %i

[Install]
WantedBy=multi-user.target

Which I've then enabled

[user@anotherhost ~]$ sudo systemctl enable autossh@somehost.example.com
ln -s '/etc/systemd/system/autossh@.service' '/etc/systemd/system/multi-user.target.wants/autossh@somehost.example.com.service'

And can intereact with 

[user@anotherhost ~]$ sudo systemctl start autossh@somehost.example.com
[user@anotherhost ~]$ sudo systemctl status autossh@somehost.example.com
autossh@somehost.example.service - AutoSSH service for ServiceABC on somehost.example
   Loaded: loaded (/etc/systemd/system/autossh@.service; enabled)
   Active: active (running) since Tue 2015-10-20 13:19:01 EDT; 17s ago
 Main PID: 32524 (autossh)
   CGroup: /system.slice/system-autossh.slice/autossh@somehost.example.com.service
           ├─32524 /usr/bin/autossh -M 40000 -NR 5000:127.0.0.1:5000 -i /opt/ServiceABC/.ssh/id_rsa_ServiceABC -l ServiceABC somehost.example.com
           └─32525 /usr/bin/ssh -L 40000:127.0.0.1:40000 -R 40000:127.0.0.1:40001 -NR 5000:127.0.0.1:5000 -i /opt/ServiceABC/.ssh/id_rsa_ServiceABC -l ServiceABC somehost.example.com

Oct 20 13:19:01 anotherhost.example.com systemd[1]: Started AutoSSH service for ServiceABC on somehost.example.com.
[user@anotherhost ~]$ sudo systemctl status autossh@somehost.example.com
[user@anotherhost ~]$ sudo systemctl status autossh@somehost.example.com
autossh@somehost.example.com.service - AutoSSH service for ServiceABC on somehost.example.com
   Loaded: loaded (/etc/systemd/system/autossh@.service; enabled)
   Active: inactive (dead) since Tue 2015-10-20 13:24:10 EDT; 2s ago
  Process: 32524 ExecStart=/usr/bin/autossh -M 40000 -NR 5000:127.0.0.1:5000 -i /opt/ServiceABC/.ssh/id_rsa_ServiceABC -l ServiceABC %i (code=exited, status=0/SUCCESS)
 Main PID: 32524 (code=exited, status=0/SUCCESS)

Oct 20 13:19:01 anotherhost.example.com systemd[1]: Started AutoSSH service for ServiceABC on somehost.example.com.
Oct 20 13:24:10 anotherhost.example.com systemd[1]: Stopping AutoSSH service for ServiceABC on somehost.example.com...
Oct 20 13:24:10 anotherhost.example.com systemd[1]: Stopped AutoSSH service for ServiceABC on somehost.example.com.

As you can see, all instances of %i in the unit file get replaced with somehost.example.com.

There's a bunch more specifiers that you can use in a unit file though, but I find %i to work best in cases like this.

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