Configuring STUN

STUN (Session Traversal Utilities for NAT) is a protocol used to discover a computer's public IP address and port. It is used for NAT hole punching, a technique supported by FriendNet to establish direct connections between peers, even when they have firewalls and are behind NATs.

FriendNet servers (version 1.2.0 and later) have a built-in STUN server that runs on the same addresses that the server is configured to listen on. By default, it will try to guess its own public listening IP and port to send to clients who query the server for STUN servers to use. However, this guess can sometimes be incorrect, leaving clients without a working STUN server to use for NAT hole punching.

It is recommended to manually configure the server's public listening IP and port as the STUN server address to send to clients.

To configure the STUN server addresses to send to clients, edit server.json and add a stun_servers value:

{
    "listen": [
        "0.0.0.0:20038"
    ],
    "stun_servers": [
        "my.friendnet.server:20038"
    ]
}

For example, if your FriendNet server has the public IP 1.2.3.4 and you were listening on 0.0.0.0:20038 (wildcard address, port 20038), then your server.json should look something like this:

{
    "listen": [
        "0.0.0.0:20038"
    ],
    "stun_servers": [
        "1.2.3.4:20038"
    ]
}

If you'd like to provide different STUN servers (like if your server only has an IPv6 address, but you want to enable IPv4 STUN), you can provide 3rd party STUN servers in the stun_servers value:

{
	"stun_servers": [
        "stun.l.google.com:19302",
        "stun.cloudflare.com:3478"
	]
}

If multiple STUN servers are provided, the client will try all of them simultaneously and use the response from the first one that responds.