STUN

Session Traversal Utilities for NAT (STUN) is a standardized set of methods, including a network protocol, for traversal of network address translator (NAT) gateways in applications of real-time voice, video, messaging, and other interactive communications.

STUN is a tool used by other protocols, such as Interactive Connectivity Establishment (ICE), the (SIP), or WebRTC. It provides a tool for hosts to discover the presence of a network address translator, and to discover the mapped, usually public, (IP) address and port number that the NAT has allocated for the application's (UDP) connections to remote hosts. The protocol requires assistance from a third-party network server (STUN server) located on the opposing (public) side of the NAT, usually the public Internet.

Originally, STUN was an acronym for Simple Traversal of User Datagram Protocol (UDP) through Network Address Translators, but this title was changed in a specification of an updated set of methods published as RFC 5389, retaining the same acronym.

STUN is a tool for communications protocols to detect and traverse network address translators that are located in the path between two endpoints of communication. It is implemented as a light-weight client-server protocol, requiring only simple query and response components with a third-party server located on the common, easily accessible network, typically the Internet. The client side is implemented in the user's communications application, such as a (VoIP) phone or an instant messaging client.

The basic protocol operates essentially as follows. The client, typically operating inside a private network, sends a binding request to a STUN server on the public Internet. The STUN server responds with a success response that contains the IP address and port number of the client, as observed from the server's perspective. The result is obfuscated through exclusive or (XOR) mapping to avoid translation of the packet content by application layer gateways (ALGs) that perform deep packet inspection in an attempt to perform alternate NAT traversal methods.

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