Website: http://grappl.io/

Donations: http://grappl.io/donate

Grappl is a tool used to host servers behind closed ports, and it's powered by this.

Relay servers work like routers, establishing software-level tunnels and transmitting data.

This is the code that runs in Grappl's relay servers. If you compile/run it, you can create your own working Grappl relay server, that can be connected to using the Grappl API or using the Advanced GUI.