HP-UX C SIP Stack Programmer's Guide (Novembery 2007)

Working with DNS 329

WORKING WITH DNS

INTRODUCTION SIP uses DNS procedures to allow a client to resolve a SIP Uniform Resource

Identifier (URI) into the IP address, port, and transport protocol of the next hop

to contact. The SIP Stack DNS feature is implemented in the Resolver and

Transmitter layers of the SIP Stack and propagates upwards to all layers.

The DNS queries that the SIP Stack performs are non-blocking. This means that

when the SIP Stack is waiting for a response from a DNS server, the SIP Stack

can perform other operation, such as message sending, receiving and processing,

call handling, and so on.

By default, the SIP Stack is compiled with basic DNS functionality that

includes:

 A/AAAA queries to resolve IP addresses of hosts given as a

domain name.

 NAPTR queries to resolve URI addresses, given as Tel URIs.

The SIP Stack enables a second mode of operation, an enhanced DNS mode.

When using the enhanced DNS mode, the behavior of the SIP Stack will comply

with the client DNS procedures defined in RFC 3263 and RFC 3824. This

includes usage of NAPTR and SRV DNS queries, and the ability to maintain a

list of resolved addresses that the application will be able to try one after the

other, in case of send failure.

CONFIGURING DNS

ARAMETERS

To work with DNS servers, the SIP Stack requires the following information:

 A list of DNS servers—tells the SIP Stack which DNS servers

the SIP Stack should work with.