start autossh reverse tunnel automatically when network comes up

I connect from my home PC thru an Internet server I own to a restricted client (that I also own) using a reverse ssh tunnel.

I have a script that sets up the restricted client to middleman leg of this ssh tunnel. It works fine if I execute the script manually. Here it is:

#!/bin/sh autossh -M 12399 -oPubkeyAuthentication=yes -oPasswordAuthentication=no -oLogLevel=error -oUserKnownHostsFile=/dev/null -oStrictHostKeyChecking=no -i ~/.ssh/named_id_rsa -R 12345:localhost:22 [email protected] -p 22 

I want to have it run automatically whenever the network is up on this machine.

To accomplish this I thought about saving it as: /etc/network/if-up.d/reverse_ssh_tunnel.sh

I know I would have to remove "~/" and specify the full path to the private key as home/username/.ssh/named_id_rsa.

I suspect I'll have to use su -c my_script_name username, but I can't get that to work. Solving this part of my question is the most important. It needs to be fully automatic & unattended.

To connect from my home, I do two manual steps:

A. ssh home -> middleman
B. ssh middleman -> restricted client

Can A & B be done in a single step? (This would be nice, but not essential.)

In summary:

1. Ideally, I'd like to do A & B in one step, manually.
2. And I need the restricted client -> middleman autossh connection to be fully automatic.