STUN
STUN (Session Traversal Utilities for NAT, RFC 5389) lets a client discover its public IP and port as seen through a NAT, along with the NAT type.
Purpose
It provides public reflexive address discovery for endpoints behind NAT—the foundation of ICE and WebRTC NAT traversal.
Mechanism
The client sends a Binding Request to a STUN server, which replies with the source IP:port it observed (the reflexive public address). Additional attributes can test the NAT's type and behavior.
Use cases
- WebRTC NAT traversal
- VoIP SIP
- P2P public address discovery
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