DNS SRV Record

When you enter a server such as example.com without a port, a SIP client has to work out which machine to contact, on which port, and over which transport. RFC 3263 describes the procedure, and DNS SRV records are its core.

An SRV record, defined in RFC 2782, has a name of the form _service._proto.domain. For SIP the service is _sip or _sips and the protocol is _udp, _tcp or _tls. A lookup for _sip._tls.example.com might return sip1.example.com on port 5061 with priority 10 and sip2.example.com on port 5061 with priority 20. The client tries the lowest priority first and uses weights to spread load among equals. Providers rely on this to move or add servers without asking customers to change settings.

If no SRV record exists, the client falls back to an ordinary A or AAAA lookup of the domain and the default port for the chosen transport. If you specify a port explicitly, SRV lookup is skipped.

SIPdia performs a DNS SRV lookup when the Server field has no port. Enter host:port to bypass it, which is useful on a PBX that listens on a non-standard port and has no SRV records.