Autossh tutorial for reverse SSH tunneling (using a systemd service)

AutoSSH.md

Prerequisites:

• the device you're trying to access (behind NAT);
• a public SSH server (you need root access + custom sshd configuration to forward ports on 0.0.0.0, otherwise you can use jump hosts);

Setup steps:

• install autossh on the device behind nat;
• generate a private ssh key pair to /etc/ssh/id_autossh;
• authorize the public key on the public server (you may optionally create a limited account);
• create /etc/systemd/system/autossh.service and /etc/ssh/autossh.vars and fill it with your specific setup variables;
• systemctl daemon-reload && systemctl restart autossh && systemctl enable autossh
• check systemctl status autossh && netstat -tlnp on the public server';

autossh.service

[Unit]
Description=AutoSSH Reverse Port Forwarding
Wants=network-online.target
After=network.target network-online.target
StartLimitInterval=0

[Service]
Type=simple
User=root
EnvironmentFile=/etc/ssh/autossh.vars
ExecStart=/usr/bin/autossh -M 0 -N -o "ServerAliveInterval 60" \
	-o "ServerAliveCountMax 3" -o StrictHostKeychecking=no -o ExitOnForwardFailure=yes \
	-o UserKnownHostsFile=/dev/null -p ${AUTOSSH_SSH_PORT} -l ${AUTOSSH_SSH_USER} \
	-i /etc/ssh/id_autossh ${AUTOSSH_SSH_HOST} \
	-R 0.0.0.0:${AUTOSSH_FORWARD_PORT}:127.0.0.1:22
Restart=always
RestartSec=30

[Install]
WantedBy=multi-user.target

autossh.vars

AUTOSSH_SSH_HOST=public-server.example.com
AUTOSSH_SSH_PORT=22
AUTOSSH_SSH_USER=autossh
AUTOSSH_FORWARD_PORT=10001