Guide
SRV records: finding the host and port of a service
Most DNS lookups answer “what address?”. An SRV record answers “where, exactly, is this service?” with a host name and a port, so the service can run on any machine and any port without the clients being reconfigured. Internet telephony, chat, calendars, directory services and game servers rely on it. The page looks up the SRV records for a service name and shows how a client would choose among them.
The name and the record
An SRV name has three parts: an underscore-prefixed service, an underscore-prefixed protocol and the domain. _sip._tcp.example.com asks for SIP over TCP at example.com. The answer gives four numbers and a name: priority, weight, port and the target host. RFC 2782 defines the format. The target needs its own A or AAAA record, because the SRV record carries a name and not an address. The page offers the common service names as presets, so you do not have to remember the underscores.
How clients choose
A client sorts the records by priority and takes the lowest. If several records share that priority, it picks one at random, with probability proportional to weight. If the chosen server cannot be reached, it tries another from the same group, and then moves to the next priority. The page makes this concrete in a column “Share of its priority group”: records with weights 60 and 20 show 75 % and 25 %. A weight of 0 gives a record very little chance of being chosen while others of the same priority exist.
The explicit “no service” answer
A target of a single dot means the service is deliberately unavailable at this domain (RFC 2782). A client should stop looking instead of falling back to a default port. The page recognises this and states it. A name with no SRV records at all is different: the client may use its built-in default, if it has one. Browsers do not use SRV to find websites, so adding records for _http._tcp does not change how a site is reached.
Where you will see SRV
SIP and SIPS for voice calls, XMPP for chat servers, IMAP, submission and CalDAV/CardDAV for account autoconfiguration, LDAP and Kerberos in Active Directory networks, Minecraft Java servers (so players can connect without typing a port), Matrix federation and Microsoft’s autodiscover. Each of these has the service and protocol names built into the preset list.
A worked example
Press Sample to look up _sip._tcp.sip2sip.info. When this guide was reviewed the page showed one record: priority 100, weight 100, a share of its priority group of 100 %, port 5060 and target proxy.sipthor.net, with a TTL of 300 seconds. A client connecting would therefore resolve proxy.sipthor.net and open TCP port 5060. The NAPTR Lookup for the same domain shows the earlier step in which SIP chooses between transports.
Limits
The lookup is for the name you enter. The page does not guess a service from a domain, test whether the port is open or check that the target answers. Some resolvers are slow for names that do not exist, so an empty result can take a moment to appear. Port numbers can be checked against the Port Number Lookup.
Related tools
- NAPTR Lookup shows the rewrite step that SIP uses before SRV.
- Port Number Lookup identifies the registered use of a port.
- A Record Lookup resolves the target host of an SRV record.
- DNS Lookup lists SRV beside the other record types.
Checked against the current version of the SRV Record Lookup on .