![]() STUN does the best it can in such a hostile environment. The result of this lack of standardization has been a proliferation of devices whose behavior is highly unpredictable, extremely variable, and uncontrollable. The problems in STUN have to do with the lack of standardized behaviors and controls in NATs. The problems with STUN are not design flaws in STUN. You can use a DH2i NAT test to determine if your site is behind a Symmetric NAT device.Ĭlosing words (also from the obsolete RFC 3489) Furthermore, only the external host that receives a packet can send a UDP packet back to the internal host. If the same host sends a packet with the same source address and port, but to a different destination, a different mapping is used. Symmetric: A symmetric NAT is one where all requests from the same internal IP address and port, to a specific destination IP address and port, are mapped to the same external IP address and port.Specifically, an external host can send a packet, with source IP address X and source port P, to the internal host only if the internal host had previously sent a packet to IP address X and port P. Port Restricted Cone: A port restricted cone NAT is like a restricted cone NAT, but the restriction includes port numbers.Unlike a full cone NAT, an external host (with IP address X) can send a packet to the internal host only if the internal host had previously sent a packet to IP address X. Restricted Cone: A restricted cone NAT is one where all requests from the same internal IP address and port are mapped to the same external IP address and port.Furthermore, any external host can send a packet to the internal host, by sending a packet to the mapped external address. Full Cone: A full cone NAT is one where all requests from the same internal IP address and port are mapped to the same external IP address and port.Various types of NAT (still according to the RFC) STUN servers are generally attached to the public Internet. STUN Server: A STUN Server (also just referred to as a server) is an entity that receives STUN requests, and sends STUN responses.A STUN client can execute on an end system, such as a user’s PC/ router, or can run in a network element, such as a conferencing server. STUN Client: A STUN client (also just referred to as a client) is an entity that generates STUN requests.The service name is _stun._udp or _stun._tcp STUN may use DNS SRV records to find STUN servers attached to a domain.Support as of 2015 is fairly good, but legacy devices may lack it. STUN is not universally supported by VOIP devices yet.The server will also require users to carry out tests on a secondary IP and port number. STUN operates on TCP and UDP port 3478.STUN enables a device to find out its public IP address and the type of NAT service it’s sitting behind.Note: The STUN RFC states: This protocol is not a cure-all for the problems associated with NAT.
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