A SRV or Service Record is a category of data in the DNS specifying information on available services. When looking up for a service, you must first lookup the SRV Record for the service to see which server actually handles it. Then it looks up the Address Record for the server to connect to its IP Address.
The SRV Record has a priority field similar to an MX Record's priority value. Clients always use the SRV Record with the lowest priority value first, and only fall back to other SRV Records if the connection with this Record's host fails. If a service has multiple SRV records with the same priority value, clients use the weight field to determine which host to use. The weight value is relevant only in relation to other weight values for the service, and only among SRV Records with the same priority value.
Newer Internet Protocols such as SIP (Session Initiation Protocol) and XMPP (Extensible Messaging and Presence Protocol) often require SRV support from clients.
There you would find 7 fields:
- Service Record Name: The Service Record Name consists of the symbolic Service Name and the Protocol Name of the desired service.
- Service Name: This is the symbolic name of the desired service. For example, chat, sip, etc.
- Protocol Name: The protocol of the desired service, usually either TCP or UDP.
- Priority: The priority of the target host. A lower value indicates higher priority.
- Weight: A relative weight for Records with the same Priority.
- Port: The TCP or UDP port on which the service is to be found.
- Target: The canonical hostname of the machine providing the service. If the Server providing the service is managed by you, then you may set the Value in the format - service.yourdomainname.com. If you are using another Internet Service Provider's Server, then you need to enter a Fully Qualified Domain Name like service.isp.com. Note that a Fully Qualified Domain Name always ends with a "." in the end.
- TTL: This is the Time To Live for this Record, in seconds. Any Server which once queries this Record will query it again after this time interval. The ideal TTL is 86400, which is 1 day. It can not be set to less than 14400, i.e. 4 hours.