How can a reverse-SSH connection be launched and maintained on Ubuntu boot with systemd?

My mum has a wee laptop that needs, when it boots up, to reverse-SSH connect itself to a server (so I can help out when needed).

I'm struggling to get the connection to persist when it is started by systemd.

In /reverse_SSH.sh I've got something like this:

#!/bin/bash
while true; do
    ssh -R 19123:localhost:22 [email protected]
    sleep 1000
done

In /etc/systemd/system/reverse_SSH.service I've got something like this:

[Unit]
Description=reverse-SSH

[Service]
Type=fork
ExecStart=/reverse_SSH.sh

[Install]
WantedBy=multi-user.target

When I run sudo systemctl start reverse_SSH, the SSH connection does appear to happen, and the MOTD of the server can be seen in the output of sudo systemctl status reverse_SSH, but the SSH connection doesn't seem to persist and I can't access the machine on the server with a command like ssh sonnyboy@localhost -p 19123.

What nonsense am I doing wrong? Thanks!

Solution 1:

I would prefer to use the autossh package for such service. So first install it:

sudo apt update && sudo apt install autossh

Then create configuration entry for your connection, by creating a new file within the directory /etc/ssh/ssh_config.d/. Let's call it reverse.ssh.www.example.org.conf. Here is an example:

sudo nano /etc/ssh/ssh_config.d/reverse.ssh.www.example.org.conf

# This file is loaded by /etc/ssh/ssh_config
Host reverse.ssh.www.example.org
    HostName www.example.org
    IdentityFile /root/.ssh/your_passles_id_rsa
    User mum
    Port 22
    RemoteForward 19123 127.0.0.1:22
    GatewayPorts yes
    Compression yes

Note the SSH key is possession of the root user who will manage the connection by our service.
The last two options are not mandatory.

Finally create the service:

sudo nano /etc/systemd/system/reverse-autossh-www-example-org.service

[Unit]
Description=Keeps a resident tunnel to www.example.org open
#After=network.target
After=network-online.target

[Service]
User=root
ExecStart=/usr/bin/autossh -M 0 -N -q -o "ServerAliveInterval 60" -o "ServerAliveCountMax 3" reverse.ssh.www.example.org
ExecStop=/usr/bin/killall -s KILL autossh
#ExecStop=/bin/kill $MAINPID
Restart=always
RestartSec=3
#Environment=AUTOSSH_GATETIME=0

[Install]
WantedBy=multi-user.target

Enable and start the service:

sudo systemctl daemon-reload
sudo systemctl enable reverse-autossh-www-example-org.service
sudo systemctl start reverse-autossh-www-example-org.service

I've used this configuration for last two years in order to solve similar task - it works like a charm. However, here are few references:

https://www.freedesktop.org/software/systemd/man/systemd.service.html
https://linuxize.com/post/using-the-ssh-config-file/
https://gist.github.com/thomasfr/9707568

How can a reverse-SSH connection be launched and maintained on Ubuntu boot with systemd?

Solution 1:

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