Set up the host to connect to and port base to open:
export HOSTNAME=foo.bar
export JUMPHOST=jumphost.baz
export PORTBASE=1234
Create the autossh user:
sudo useradd --system --create-home --shell /usr/sbin/nologin autossh
Create the SSH key:
sudo -u autossh ssh-keygen -t ed25519
sudo -u autossh sed -i -e "s/$/ (generated on $(date +%Y-%m-%d))/" ~autossh/.ssh/id_ed25519.pub
Add the SSH key to the jumphost:
PUBKEY=$(sudo cat ~autossh/.ssh/id_ed25519.pub)
cat <<EOF
# ${HOSTNAME}: ports ${PORTBASE}4, ${PORTBASE}6
no-pty,no-X11-forwarding,permitopen="localhost:${PORTBASE}4",permitopen="localhost:${PORTBASE}6",command="/bin/echo do-not-send-commands" ${PUBKEY}
EOF
Connect to the host as autossh to add the SSH keys to known_hosts:
sudo -u autossh ssh -4 ${JUMPHOST}
sudo -u autossh ssh -6 ${JUMPHOST}
Download and fix the config files:
curl -sSL https://gist.githubusercontent.com/tdussa/f0920a329bd91b58ae58c9b4c83d45f0/raw/autossh@hostname-v4 | \
sed -e "s/-R 4:/-R ${PORTBASE}4:/" -e "s/TARGET_HOST=.*/TARGET_HOST=${JUMPHOST}/" -e "s/autossh@hostname-/autossh@${JUMPHOST}-/" | \
sudo tee /etc/default/autossh@${JUMPHOST}-v4
curl -sSL https://gist.githubusercontent.com/tdussa/f0920a329bd91b58ae58c9b4c83d45f0/raw/autossh@hostname-v6 | \
sed -e "s/-R 6:/-R ${PORTBASE}6:/" -e "s/TARGET_HOST=.*/TARGET_HOST=${JUMPHOST}/" -e "s/autossh@hostname-/autossh@${JUMPHOST}-/" | \
sudo tee /etc/default/autossh@${JUMPHOST}-v6
Download the unit file:
curl -sSL https://gist.githubusercontent.com/tdussa/f0920a329bd91b58ae58c9b4c83d45f0/raw/autossh@.service | \
sudo tee /etc/systemd/system/autossh@.service
Enable and start the service:
systemctl enable autossh@${JUMPHOST}-v4.service
systemctl enable autossh@${JUMPHOST}-v6.service
systemctl start autossh@${JUMPHOST}-v4.service
systemctl start autossh@${JUMPHOST}-v6.service
Verify all is good
systemctl status autossh@${JUMPHOST}-v4.service
systemctl status autossh@${JUMPHOST}-v6.service
journalctl -u autossh@${JUMPHOST}-v4
journalctl -u autossh@${JUMPHOST}-v6