There are four tricks in our arsenal that we’re going to use to jailbreak internal hosts behind a restrictive customer firewall:

• forward proxies (e.g. squid)
• TLS-terminating reverse proxies (e.g. nginx or stunnel)
• reverse tunnels (e.g. ssh -R)
• corkscrew

— Gabriella Gonzalez

I’ve lost count of how many times I’ve needed to do some kind of creating a socket between machines behind VPNs/Firewalls. This is a pretty useful summary of all the useful techniques.