SRV Record — Service Location

What Is an SRV Record?

An SRV record (Service record) specifies the hostname and port for a specific service on your domain. Unlike A or CNAME records, SRV records include priority, weight, and port information, enabling sophisticated service discovery and load balancing.

BIND Zone File Syntax

; Format: _service._protocol.domain.  TTL  IN  SRV  priority weight port target
_sip._tcp.example.com.       3600    IN    SRV    10 60 5060 sipserver.example.com.
_sip._tcp.example.com.       3600    IN    SRV    10 40 5060 sipbackup.example.com.
_xmpp-server._tcp.example.com.    3600    IN    SRV    5 0 5269 xmpp.example.com.
_ldap._tcp.example.com.      3600    IN    SRV    0 100 389 ldap.example.com.

Field Breakdown

Field	Description
_service	Service name preceded by underscore (e.g., _sip, _xmpp-server, _ldap)
_protocol	Transport protocol: _tcp or _udp
priority	Lower values are tried first (like MX priority)
weight	Relative weight for load balancing among same-priority records
port	TCP or UDP port number the service listens on
target	Hostname providing the service (must not be a CNAME)

Priority and Weight

Priority works like MX records — clients connect to the lowest-priority servers first and fail over to higher-priority servers.

Weight distributes traffic among servers with the same priority. In the SIP example above, both servers have priority 10. The server with weight 60 receives approximately 60% of the traffic, while the server with weight 40 receives approximately 40%.

To disable a service, set the target to . (a single dot):

_sip._tcp.example.com.    IN    SRV    0 0 0 .

Common SRV Record Uses

• SIP (Voice over IP): _sip._tcp and _sip._udp
• XMPP (Jabber messaging): _xmpp-client._tcp and _xmpp-server._tcp
• LDAP (Directory services): _ldap._tcp
• Kerberos authentication: _kerberos._tcp and _kerberos._udp
• CalDAV/CardDAV: _caldavs._tcp and _carddavs._tcp
• Minecraft servers: _minecraft._tcp

Microsoft Active Directory

Active Directory relies heavily on SRV records for service discovery. Domain controllers register SRV records for LDAP, Kerberos, and Global Catalog services, allowing domain-joined machines to automatically locate the nearest domain controller:

_ldap._tcp.dc._msdcs.example.com.    IN    SRV    0 100 389 dc1.example.com.
_kerberos._tcp.example.com.           IN    SRV    0 100 88  dc1.example.com.

Limitations

SRV records are only useful for protocols and applications that explicitly support SRV lookups. Standard HTTP web browsers do not query SRV records to find web servers — they use A and AAAA records. The HTTPSSVC (HTTPS) record type is an emerging alternative for HTTP-based service discovery.