The goal is to make a shell script or C program that acts as a TCP beacon and on demand forms a client-initiated SSH redirection pipe (kind of a reversed way).
The initial communication is TCP and initiated by the client (behind the firewall), then there is no need to open any ports: the firewall accepts all outgoing traffic.
The beacon is a TCP connection that is initiated in 10 minute intervals. Normally it hangs up after 5 seconds. But if the server responds and sends a specific string into the TCP pipe, the client will try to establish a SSH-connection to server port 22.
Any idea about how to implement the TCP beacons? Thanks.