STUN is a network protocol that recognizes the existence and type of firewalls and NAT routers and takes this information into consideration. It enables the uncomplicated use of devices (e.g. SIP telephones) and programs in networks that should receive information from the Internet.

STUN helps to identify the current public IP address of the line. This is necessary in order for the opposite terminal to correctly address and return your call data.

See RFC Standard (RFC 3489).

STUN messages are sent by the SwyxLinkManager at least every 10 seconds as long as there is no other data traffic circulating via the corresponding port. This ensures that the NAT router's NAT table (masquerading) cannot be destroyed again and that changes to the external IP address of the NAT router can be transmitted. This means that SwyxWare SIP trunks can also be operated with a DSL connection that is terminated every 24 hours by the IP provider and thus receives a new IP address.

Therefore, STUN messages, SIP logon, the SIP connection creation, and the voice data are sent via the NAT router.

If a firewall exists, it must be disabled for this type of communication. The rules required are described below and must be configured in the appropriate syntax in your firewall.