SRV record

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. A SRV record has the form: 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. IN: standard DNS class field (this is always IN). SRV: Type of Record (this is always SRV). 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 might be the following: 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. The priority field determines the precedence of the use of the record's data. Clients should use the SRV records with the lowest-numbered priority value first, and fall back to records of higher value if the connection fails. If a service has multiple SRV records with the same priority value, clients should load balance them in proportion to the values of their weight fields. In the following example, both the priority and weight fields are used to provide a combination of load balancing and backup service.