DNS SIP Server Name Resolution

If a DNS name is given for a proxy/registrar address, the IP addresses associated with that name is discovered as specified in RFC3263 .

If a port is given, the only lookup is an A record. If no port is given, NAPTR and SRV records are tried before falling back on A records if NAPTR and SRV records return no results. If no port is given, and none is found through DNS, port 5060 is used. If the registration type is TLS, port 5061 is used.

CAUTION: Failure to resolve a DNS name is treated as signaling failure that causes a failover.
The following configuration causes the phone to build an SRV request based on the address you provide, including all subdomains. Use the format:
  • voIpProt.SIP.outboundProxy.address ="sip.example.com
  • voIpProt.SIP.outboundProxy.port ="0"

This SRV request produces a list of servers ordered by weight and priority, enabling you to specify sub-domains for separate servers, or you can create partitions of the same system. Please note that while making SRV queries and transport is configured as TCP, the phone adds the prefix <_service._proto.> to the configured address/FQDN but does not remove the sub-domain prefix, for example sip.example.com becomes _sip._tcp.sip.example.com . A single SRV query can be resolved into many different servers, session border controllers (SBCs), or partitions ordered by weight and priority, for example, voice.sip.example.com and video.sip.example.com . Alternatively, use DNS NAPTR to discover what services are available at the root domain.