Sip Server Domain Name Resolution - Yealink SIP-T2XP Administrator's Manual

Administrator's Guide for SIP-T2xP IP Phones

SIP Server Domain Name Resolution

If a domain name is configured for a SIP server, the IP address(es) associated with that
domain name will be resolved through DNS as specified by RFC 3263. The DNS query
involves NAPTR, SRV and A queries, which allows the IP phone to adapt to various
deployment environments. The IP phone performs NAPTR query for the NAPTR pointer
and transport protocol (UDP, TCP and TLS), the SRV query on the record returned from
the NAPTR for the target domain name and the port number, and the A query for the IP
addresses.
If an explicit port (except 0) is specified and the transport type is set to DNS-NAPTR, A
query will be performed only. If a SIP server port is set to 0 and the transport type is set to
DNS-NAPTR, NAPTR and SRV queries will be tried before falling to A query. If no port is
found through the DNS query, 5060 will be used.
The following details the procedures of DNS query for the IP phone to resolve the
domain name (e.g., yealink.pbx.com) of working server into the IP address, port and
transport protocol.
NAPTR (Naming Authority Pointer)
First, the IP phone sends NAPTR query to get the NAPTR pointer and transport protocol.
Example of NAPTR records:
order
IN NAPTR 90
IN NAPTR 100
166
pref flags service
50 "s" "SIP+D2T"
50 "s" "SIP+D2U"
regexp replacement
"" _sip._tcp.yealink.pbx.com
"" _sip._udp.yealink.pbx.com