"JSTUN" is a java based STUN (Simple Traversal of User Datagram Protocol (UDP) Through Network Address Translation (NAT)) implementation. STUN provides a mean for applications to discover the presence and type of firewalls or NATs between them and the public internet. Additionally, in presence of a NAT STUN can be used by applications to learn the public Internet Protocol (IP) address assigned to the NAT.

So far, most of the message headers and attributes as standardized in RFC 3489 are part of "JSTUN". The current "JSTUN" version also includes a STUN-Client to discover firewalls and NATs.