The Session Traversal Utilities for NAT (STUN) prococol is used in Network Address Translation (NAT) traversal for real-time voice, video, messaging, and related interactive IP application communications. This light-weight, client-server protocol allows applications passing through a NAT to obtain the public IP address for the UDP connections the application uses to connect to remote hosts.
STUN support is provided at the SIP connector level, using the STUN for Java project. The STUN for Java project provides a Java implementation of the STUN Protocol (RFC 3489), which allows each SIP connector to select whether it should use STUN to discover a public IP address, and then use this address in the SIP messages sent through the connector.
To make a SIP connector STUN-enabled, three attributes must be appended to the <connector> child element in theserver.xml or <socket-binding> child element in standalone-sip.xml file. The properties are:
- useStun=”true”Enables STUN support for this connector. Ensure that the
ipAddressattribute is not set to127.0.0.1. - stunServerAddress=”<Public_STUN_Server>”STUN server address used to discover the public IP address of this SIP Connector. See Table 8.1, “Public STUN Servers”for a suggested list of public STUN servers.
- stunServerPort=”3478″STUN server port of the STUN server used in the
stunServerAddressattribute. Both TCP and UDP protocols communicate with STUN servers using this port only.
Note
A complete list of available SIP connector attributes and their descriptions is located in theSection 2.3.1, “Configuring SIP Connectors and Bindings” section of this guide.
A number of public STUN servers are available, and can be specified in the stunServerAddress. Depending on the router firmware used, the STUN reply packets’ MAPPED_ADDRESS may be changed to the router’s WAN port. To alleviate this problem, certain public STUN servers provide XOR_MAPPED_ADDRESS support. Table 8.1, “Public STUN Servers”provides a selection of public STUN servers.
Table 8.1. Public STUN Servers
| Server Address | XOR Support | DNS SRV Record |
|---|---|---|
| stun.ekiga.net | Yes | Yes |
| stun.fwdnet.net | No | Yes |
| stun.ideasip.com | No | Yes |
| stun01.sipphone.com | Yes | No |
| stun.softjoys.com | No | No |
| stun.voipbuster.com | No | No |
| stun.voxgratia.org | No | No |
| stun.xten.com | Yes | Yes |
| stunserver.org | Yes | Yes |
Note
For more information about NAT traversal best practices, refer to Section 9.2, “NAT Traversal”.
Mobicents provide Extensions for applications or external systems to interact with the Mobicents SIP Servlets container as well as Extensions not defined in the specification in the JSR 289 specification that can prove useful and might be proposed for inclusion in a next release of the SIP Servlets specification
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