Set up public SSH

This chapter will focus on creating an out-of-band backup SSH tunnel (non-WireGuard, non-Traefik based), using a public gateway port on the sentry. This backdoor connection will ensure that you retain access to the Raspberry Pi even during emergency maintenance cycles, including restarting Traefik, restarting Docker, or even rebooting the system (the persistent tunnels will be restarted on boot).

Ensure the SSH server is secure

During the rpi-imager configuration, you should have selected the option for SSH to Allow public-key authentication only. You should double-check that this setting was applied:

cat /etc/ssh/sshd_config | grep "^PasswordAuthentication"

PasswordAuthentication no

If correctly setup, it should show that PasswordAuthentication is disabled (thus requring the use of keys rather than passwords).

Open the firewall for SSH (port 2220)

Install dependencies

AutoSSH is used to create a reliable SSH tunnel.

sudo apt-get update
sudo apt-get install -y autossh

Enable SSH GatewayPorts on the sentry

To expose a public reverse tunnel via SSH, you must enable the non-default GatewayPorts setting in the server’s SSH config, which allows reverse tunnels to be published to the WAN interface of the sentry:

sentry sshd-config GatewayPorts=yes

## This is the default setting:
sentry sshd-config GatewayPorts=no

Make a one-time transient tunnel

Forward the public sentry port 2220 to the local Raspberry Pi port 22:

sentry ssh-expose 2220 22

To check the status:

sentry ssh-expose

## Active tunnels:
HOST            PUBLIC_PORT  LOCAL_PORT   TYPE
----            -----------  ------------ ----
sentry          2220         22           transient

With the tunnel active, you can ssh to the Pi from anywhere, going through the reverse tunnel public port:

ssh -p 2220 pi@pi.example.com

To stop the service later:

sentry ssh-expose 2220 22 --close

Make a persistent tunnel

To make a persistent connection that will survive a reboot, you must enable the systemd linger privilege for the pi user:

sudo loginctl enable-linger ${USER}

To create a persistent connection, use the the --persistent flag:

sentry ssh-expose 2220 22 --persistent

## Active tunnels:
HOST            PUBLIC_PORT  LOCAL_PORT   TYPE
----            -----------  ------------ ----
sentry          2220         22           persistent

Reconfigure SSH clients

On your personal workstation, you can create a new config for accessing the Pi through the public sentry reverse tunnel:

cat <<EOF >> ~/.ssh/config
Host pi.example.com
    User pi
    Port 2220
    ControlMaster auto
    ControlPersist yes
    ControlPath /tmp/ssh-%u-%r@%h:%p
EOF

Test the public SSH connection to the Pi

ssh -t pi.example.com w

The w command prints a list of the currently logged in users, and their remote IP address (-t is important to ensure the current connection is included):

18:24:29 up 5 days, 33 min,  1 user,  load average: 0.10, 0.07, 0.08
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
ryan     pts/2    127.0.0.1        18:24    0.00s  0.01s  0.01s w