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 (SIP) and the (XMPP) often require SRV support by network elements.

A SRV record has the form:

An example SRV record in textual form that might be found in a zone file might be the following:

This points to a server named sipserver.example.com listening on TCP port 5060 for (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 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.

The first three records share a priority of 10, so the weight field's value will be used by clients to determine which server (host and port combination) to contact. The sum of all three values is 100, so bigbox.example.com will be used 60% of the time. The two hosts smallbox1 and smallbox2 will be used for 20% of requests each, with half of the requests that are sent to smallbox1 (i.e. 10% of the total requests) and the remaining half to smallbox2. If bigbox is unavailable, these two remaining machines will share the load equally, since they will each be selected 50% of the time.

If all three servers with priority 10 are unavailable, the record with the next lowest priority value will be chosen, which is backupbox.example.com. This might be a machine in another physical location, presumably not vulnerable to anything that would cause the first three hosts to become unavailable.

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