Ssh – Is autossh redundant with systemd

autosshdebiansystemd

When creating a persistent reverse SSH tunnel, is autossh useful on a system running systemd? Typically, autossh is run by a service with the option -M set to zero which disables monitoring of the link. This means that ssh has to exit before autossh will restart it. From the man page:

Setting the monitor port to 0 turns the monitoring function off, and
autossh will only restart ssh upon ssh's exit. For example, if you are
using a recent version of OpenSSH, you may wish to explore using the
ServerAliveInterval and ServerAliveCountMax options to have the SSH
client exit if it finds itself no longer connected to the server. In
many ways this may be a better solution than the monitoring port.

It seems that the systemd service itself is capable of doing this with a service file that contains these options:

Type=simple
Restart=always
RestartSec=10

So is autossh redundant when run by a systemd service? Or is it doing other things that help to keep the SSH connection up?

Thanks.

Best Answer

Since trying both is relatively easy, test the reliability of both approaches and confirm for yourself.

I think you'll find that autossh is better suited for the job. autossh designed to run in the foreground, while systemd is designed primarily for background services that aren't attached to a particular TTY.

Also, autossh has at least one feature specific to the task:

Periodically (by default every 10 minutes), autossh attempts to pass traffic on the monitor forwarded port. If this fails, autossh will kill the child ssh process (if it is still running) and start a new one;

So autossh is doing more than keeping a process running, it's confirming that the ssh connection is actually working.

Related Solutions

Debian – Systemd Service stops on its user exit – debian 9 stretch

You might try configuring systemd to not kill user processes on logout for the wanted user:

sudo loginctl enable-linger teamspeak

systemd – Restarting Service Only as a Specific User

To achieve that the user techops can control the service publicapi.service without giving a password, you have different possiblities. Which one is suitable for you cannot be answered as you have to choose on your own.

The classical sudo approach is maybe the most used, as it is there for a long time. You would have to create e.g. the file as follows.
Note that the drop-in directory /etc/sudoers.d is only active when #includedir /etc/sudoers.d is set in /etc/sudoers. But that should be the case if you are using a modern Ubuntu distribution. As root execute:

cat > /etc/sudoers.d/techops << SUDO
techops ALL= NOPASSWD: /bin/systemctl restart publicapi.service
techops ALL= NOPASSWD: /bin/systemctl stop publicapi.service
techops ALL= NOPASSWD: /bin/systemctl start publicapi.service
SUDO

Now you should be able to run the systemctl commands as user techops without giving a password by prepending sudo to the commands.

sudo systemctl start publicapi.service
sudo systemctl stop publicapi.service
sudo systemctl restart publicapi.service

The second method would be to use PolKit (was renamed from PolicyKit) to allow the user techops to control systemd services. Depending on the version of polit, you can give normal users control over systemd units.
To check the polkit version, just run pkaction --version.

with polkit version 0.106 and higher, you can allow users to control specific systemd units.
To do so, you could create a rule as root:

cat > /etc/polkit-1/rules.d/10-techops.rules << POLKIT
polkit.addRule(function(action, subject) {
    if (action.id == "org.freedesktop.systemd1.manage-units" &&
        action.lookup("unit") == "publicapi.service" &&
        subject.user == "techops") {
        return polkit.Result.YES;
    }
});
POLKIT

with polkit version 0.105 and lower: you can allow users to control systemd units. This unfortunately includes all systemd units and you might not want to do this. Not sure if there is a way to limit access to specific systemd units with version 0.105 or lower, but maybe someone else can clarify.
To enable this, you could create a file as root:
```
cat > /etc/polkit-1/localauthority/50-local.d/org.freedesktop.systemd1.pkla << POLKIT
[Allow user techops to run systemctl commands]
Identity=unix-user:techops
Action=org.freedesktop.systemd1.manage-units
ResultInactive=no
ResultActive=no
ResultAny=yes
POLKIT
```

In both cases you can run systemctl [start|stop|restart] publicapi.service as user techops without giving a password. In the latter case ( polkit <= 0.105 ) the user techops could control any systemd unit.

A third option would be to make the service a user service, which does not need sudo or polkit configurations. This puts everything under the control of the user and only works if your actual service that is started with /home/techops/publicapi start can run without root privileges.

First you have to enable lingering for the user techops. This is needed to startup the user service on boot. As root execute:

loginctl enable-linger techops

Next you have to move the systemd unit file into the techops user directory. As user techops execute the commands as follows.

mkdir -p ~/.config/systemd/user

cat > ~/.config/systemd/user/publicapi.service << UNIT
[Unit]
Description=public api startup script

[Service]
Type=oneshot
RemainAfterExit=yes
EnvironmentFile=-/etc/environment
WorkingDirectory=/home/techops
ExecStart=/home/techops/publicapi start
ExecStop=/home/techops/publicapi stop

[Install]
WantedBy=default.target
UNIT

Note that the WantedBy has to be default.target as there is no multi-user.target in the user context.

Now reload the configuration and enable the service. Again as user techops execute the commands.

systemctl --user daemon-reload
systemctl --user enable publicapi.service
systemctl --user start publicapi.service

In general you should place your systemd units in /etc/systemd/system/ not directly in /etc/systemd/system/multi-user.target.wants. When you execute systemctl enable publicapi.service a symbolic link will be created in etc/systemd/system/multi-user.target.wants or whatever target is specified for that unit.

As already mentioned, if the service/process itself can be run without root privileges you should consider adding User=techops to your unit file to run the process with a non-privileged user account.

Best Answer

Related Solutions

Debian – Systemd Service stops on its user exit – debian 9 stretch

systemd – Restarting Service Only as a Specific User

Related Question