How to setup reliable ssh tunnel

January 09, 2019

Used tools are autossh and systemd user unit. Might be useful for Raspberry Pi in combination with http://serveo.net

Install autossh:

% sudo apt install autossh

Create ssh tunnel configuration in the ~/.ssh/config:

Host example
    HostName example.net
    RemoteForward 22 localhost:22
    ServerAliveInterval 30
    ServerAliveCountMax 3
    ExitOnForwardFailure yes

Before going any further, run next command to authenticate host and update ~/.ssh/known_hosts:

% ssh example

Create systemd template unit file ~/.config/systemd/user/autossh-tunnel@.service:

[Unit]
Description=AutoSSH to '%I'
After=network.target

[Service]
Environment="AUTOSSH_GATETIME=0"
ExecStart=/usr/bin/autossh -M 0 -N %I
ExecStop=/bin/kill $MAINPID

[Install]
WantedBy=default.target

Reload configuration and start service:

% systemctl --user daemon-reload
% systemctl --user start autossh-tunnel@example

Enable autostart for the service:

% systemctl --user enable autossh-tunnel@example

Allow (optionally) to run services without session been opened:

% sudo loginctl enable-linger username