Know any port knocking as a service companies? If there was a reliable way to get a udp connection between hosts that I didn't have to write myself, I'd be tempted to take this on.
I think the term you're looking for is UDP hole punching, port knocking is performing a special sequence of connections (i.e. try TCP on 8100 then UDP on 4000 then TCP on 2000) to open up an additional port (like SSH) to a certain IP.
I'd be curious if there exists such a company or even any good open source libraries that can tie into other servers.
