A Service record (SRV record) is a specification of data in the Domain Name System defining the location, i.e. the hostname and port number, of servers for specified services. It is defined in RFC 2782, and its type code is 33. Some Internet protocols such as the Session Initiation Protocol (SIP) and the Extensible Messaging and Presence Protocol (XMPP) often require SRV support by network elements.
Record Format
A SRV record has the form:
_service._proto.name. TTL class SRV priority weight port target.
- service: the symbolic name of the desired service.
- proto: the transport protocol of the desired service; this is usually either TCP or UDP.
- name: the domain name for which this record is valid, ending in a dot.
- TTL: standard DNS time to live field.
- class: standard DNS class field (this is always IN).
- priority: the priority of the target host, lower value means more preferred.
- weight: A relative weight for records with the same priority, higher value means higher chance of getting picked.
- port: the TCP or UDP port on which the service is to be found.
- target: the canonical hostname of the machine providing the service, ending in a dot.
An example SRV record in textual form that might be found in a zone file might be the following:
_sip._tcp.example.com. 86400 IN SRV 0 5 5060 sipserver.example.com.
This points to a server named sipserver.example.com listening on TCP port 5060 for Session Initiation Protocol (SIP) protocol services. The priority given here is 0, and the weight is 5.
As in MX records, the target in SRV records must point to hostname with an address record (A or AAAA record). Pointing to a hostname with a CNAME record is not a valid configuration.