SIP auto-configuration

SIP can be auto-configured using DHCP both in IPv4 and in IPv6

From RFC 3361: Dynamic Host Configuration Protocol (DHCP-for-IPv4) Option for Session Initiation Protocol (SIP) Servers

This document specifies a DHCP option that allows SIP clients to locate a local SIP server that is to be used for all outbound SIP requests, a so-called outbound proxy server. (SIP clients MAY contact the address identified in the SIP URL directly, without involving a local SIP server. However in some circumstances, for example, when firewalls are present, SIP clients need to use a local server for outbound requests.) This is one of many possible solutions for locating the outbound SIP server; manual configuration is an example of another.

The SIP server DHCP option carries either a 32-bit (binary) IPv4 address or, preferably, a DNS RFC 1035 fully-qualified domain name to be used by the SIP client to locate a SIP server.

The DHCP IPv4 code for this option is 120.
IN DHCP6 the option code is OPTION_SIP_SERVER_D (21)
RFC 3361: SIP option for DHCP IPv4
RFC 3319: SIP option for DHCP IPv6