Hole punching coordinator for UltraGrid

This utility serves as a meeting point for UltraGrid clients, that need to connect to each other, but don't have publicly routable IP addresses.

Dependencies

There is only one dependency - asio.

On Ubuntu and other Debian based distributions it can be installed like this:

apt install libasio-dev

On Fedora/RHEL:

dnf install asio-devel

Building

mkdir build && cd build
cmake ..
make

Usage

./nat-helper [-h/--help] [-p/--port <port>]

If no port is specified, 12558 is used.

Protocol description

Clients connect to the nat-helper server, identify themselves with a name, and join a room. Once two clients enter the same room, nat-helper forwards name, sdp description string, and all candidate address pairs between the two clients.

All communication is done via messages that have the following structure:

<HEADER><MSG_BODY>

HEADER: 5B string containing length of MSG_BODY, null-termination optional
MSG_BODY: content of message, length determined by header, max 2048B

After establishing connection to the nat-helper server, following messages are sent and received in that order:

1. Client sends its name
2. Client sends room name to join
3. Client sends its sdp description
4. Client receives the name of the other client in the room
5. Client receives the sdp description of the other client

After that the client sends and receives sdp candidate pairs as they are discovered.