sidedoor binary package in Ubuntu Bionic amd64

 sidedoor maintains an SSH connection or tunnel
 with a shell script daemon.
 .
 The primary use case is maintaining a remote port forward
 to the local SSH server (or another port). Thus, the local
 device can be accessed without using incoming connections
 that may be blocked by a NAT or firewall or otherwise
 impractical with mobile devices.
 .
 SSH clients can connect to the device via the reverse SSH proxy
 that sidedoor tunnels to. This proxy server can be untrusted
 and run by a third party or cloud service.
 .
 sidedoor enables SSH keepalives and retries SSH with
 exponential backoff. In order to reconnect as soon as possible,
 it resets the backoff when a network interface is brought up
 (or changed).
 .
 Refer to the sidedoor man page to configure and set SSH keys.

Source package