Autossh systemd service. See https://avizard.blogspot.com/2021/01/aggressive-yet-sane-persistent-ssh-with.html

autossh_systemd_unit.service

[Unit]
Description=AutoSSH tunnel to remote signald Unix socket
After=network-online.target
# Note: user systemd instances do not see system-level targets like
# network-online.target by default, so this ordering hint is not
# guaranteed to work in a user unit. For reliable boot ordering,
# either run this as a system unit, or ensure the user session is
# started after the network via PAM/loginctl-linger configuration.
 
[Service]
# Set to 0 for boot-time use. The default (30s) causes autossh to
# exit immediately if the very first SSH attempt fails within 30
# seconds, which is unhelpful at boot when the remote may not be ready yet.
# With GATETIME=0, autossh retries on all failures including the
# first. Verify SSH works manually before enabling this service.
Environment="AUTOSSH_GATETIME=0"
 
# With -M 0, AUTOSSH_POLL does not perform active connection probing
# (that requires a non-zero monitor port). It does cap the maximum
# backoff sleep in autossh's internal restart loop; the default is
# 600s (10 minutes). 30s keeps the ceiling tight during outages.
Environment="AUTOSSH_POLL=30"
 
Environment="SSOCK=/var/run/signald/signald.sock"
 
ExecStart=/usr/bin/autossh -M 0 \
  -o "ServerAliveInterval 10" \
  -o "ServerAliveCountMax 3" \
  -o "ExitOnForwardFailure yes" \
  -o "StreamLocalBindUnlink yes" \
  -L ${SSOCK}:${SSOCK} \
  -N remote.server
 
# SIGHUP prods autossh out of any backoff sleep so it retries
# immediately. This is documented in the autossh README.
# Use SIGUSR1 instead to force-kill and restart the SSH child.
ExecReload=/usr/bin/kill -HUP $MAINPID
 
# systemd waits 60 seconds before restarting, matching Linux's
# hardcoded TCP_TIMEWAIT_LEN. This ensures the old connection's
# port state has fully expired before the next connection attempt.
Restart=always
RestartSec=60
 
# Sends SIGTERM to all processes in the cgroup on stop.
KillMode=control-group
 
[Install]
WantedBy=default.target