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SRV record

SRV

An SRV record publishes where a particular service runs — including its port, which no other record type carries.

01.

What an SRV record carries

An SRV record, defined in RFC 2782, names the host and port for a specific service. It is the only common record type that includes a port number, which is why protocols that do not use a fixed port rely on it.

The record name encodes the service and protocol as underscore-prefixed labels: _sip._tcp.example.com. The underscores prevent collisions with real hostnames.

02.

Priority and weight

Each record carries a priority and a weight. Priority works like MX: lower numbers are tried first. Weight distributes traffic among records that share a priority, in proportion to the weight values.

A client is expected to honour both. This gives finer control than MX records, which have priority but no weight.

How clients split traffic_sip._tcp.example.com

Records

10sip1.example.com
10sip2.example.com
10sip3.example.com
20backup.example.com

Share of new connections

Illustrative records. Priority orders the tiers; within a tier, weight sets each target's share of new connections. A weight of zero is tried only when every weighted target is exhausted.
03.

Where you will meet it

SRV records are used by SIP, XMPP, Minecraft servers, Microsoft Active Directory and various autoconfiguration mechanisms.

Email does not use SRV for delivery — that is what MX is for. Some mail clients do use SRV records to discover submission and IMAP settings automatically, which is a different job.

Related records

MX recordNames the mail servers that receive email for a domain.A recordMaps a hostname to an IPv4 address.TXT recordHolds arbitrary text, used for verification and email policy.