TURN server
also: Traversal Using Relays around NAT, relay server
A TURN (Traversal Using Relays around NAT) server is a network relay that helps clients behind firewalls or NATs establish peer-to-peer connections by forwarding traffic when direct communication is blocked.
TURN servers act as intermediaries for network traffic when two peers cannot communicate directly due to restrictive firewalls, Network Address Translation (NAT), or symmetric NAT configurations. Instead of a direct peer-to-peer connection, data flows through the TURN server, which relays packets between clients.
TURN is commonly used in VoIP applications, video conferencing, and WebRTC to ensure connectivity works across various network topologies. For example, two users on restrictive corporate networks might use a TURN server to establish a video call when their direct IP addresses aren't reachable to each other.
Common TURN server implementations on Linux include coturn and restund. A TURN server listens on specific ports (typically UDP 3478, TCP 3478, and others) and authenticates clients before relaying their traffic.