SRV Records, or service records, are DNS records meant for various domain services. An SRV record typically defines a symbolic name and the transport protocol used as part of the domain name, and defines the priority, weight, port and target for the service in the record content.

An SRV record has the form: _service._proto.Name TTL class SRV priority weight port target

Where each field means the following:

Service – defines the symbolic service name. These always begin with underscore symbol.

Protocol – defines protocol name, common values are _tcp and _udp.

Priority – lower numbers mean higher priority in the range of 0-65535.

Weight – used when more than one service is listed with the same priority. If you have two SRV records with 0 (zero) priority but one of them has the weight of 1, and the other has the weight of 4 – the record with weight four will be delivered four times out of 5 by nameservers. The range of weight values is 0-65535.

Port – defines a port number that delivers the service on the target server. You can use, for example, port 2000 instead of more common port 80.

Host – defines the external domain for the target.

Target – Specifies the name of the host that will provide the service and requires a proper A or AAAA record. If the ‘Host’ entry is omitted, the target is considered as internal. A value of ‘.’ on the target blocks a service on your host.

TTL – time-to-live value which defines how long the caching nameservers should cache your SRV record before querying your DNS server for the new value again.

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.

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