A Service (SRV) record is a type of Domain Name System (DNS) record that provides information about available services on a specific domain. SRV records are used to indicate the location of a service, such as a mail server, in a network. Unlike other DNS records that map domain names to IP addresses, SRV records map domain names to hostnames and port numbers associated with specific services.

Key points about SRV records:

Service Specification: An SRV record specifies the following information:

  • The service being offered (e.g., SIP, LDAP, XMPP)
  • The transport protocol used by the service (e.g., TCP, UDP)
  • The priority of the target host
  • The weight of the target host (used for load balancing)
  • The port number on which the service is available
  • The target hostname (the domain name of the server providing the service)

Protocol and Service Type: SRV records are often used for various network services, including email (SMTP), instant messaging (XMPP), Voice over IP (VoIP), and more. The “_service._protocol” format is used to define an SRV record. For example, “_sip._tcp.example.com” indicates a SIP service offered over TCP on the domain example.com.

Load Balancing: The “priority” and “weight” fields in an SRV record allow for load balancing of services across multiple servers. Servers with higher priority values are tried first, and within the same priority level, servers with higher weights are selected more often.

Port Number: SRV records include the port number on which the service is available. This allows clients to connect to the appropriate service port when making requests.

Format: SRV records have a specific format in DNS, which includes the service, protocol, priority, weight, port, and target hostname. The format is similar to this example:

   _service._protocol.priority.weight.port targethostname

Example: An example of an SRV record for an XMPP (Jabber) service might look like this:

   _xmpp-client._tcp.example.com. IN SRV 5 0 5222 xmpp-server.example.com.

DNS Query: To retrieve an SRV record, a client performs a DNS query for the appropriate “_service._protocol” domain, and the response provides the necessary details to locate the service.

SRV records play a crucial role in allowing clients to discover and connect to various network services efficiently. They are particularly useful for services that might be distributed across multiple servers and ports, ensuring proper load balancing and redundancy for improved service availability.