Network Working Group A. Gulbrandsen
Request for Comments: 2782 Troll Technologies
Obsoletes: 2052 P. Vixie
Category: Standards Track Internet Software Consortium
A DNS RR for specifying the location of services (DNS SRV)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document describes a DNS RR which specifies the location of the
server(s) for a specific protocol and domain.
Overview and rationale
Currently, one must either know the exact address of a server to
contact it, or broadcast a question.
The SRV RR allows administrators to use several servers for a single
domain, to move services from host to host with little fuss, and to
designate some hosts as primary servers for a service and others as
Clients ask for a specific service/protocol for a specific domain
(the word domain is used here in the strict RFC 1034 sense), and get
back the names of any available servers.
Note that where this document refers to "address records", it means A
RR's, AAAA RR's, or their most modern equivalent.
Gulbrandsen, et al. Standards Track [Page 1]RFC 2782 DNS SRV RR February 2000
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY"
used in this document are to be interpreted as specified in [BCP 14].
Other terms used in this document are defined in the DNS
specification, RFC 1034.
In general, it is expected that SRV records will be used by clients
for applications where the relevant protocol specification indicates
that clients should use the SRV record. Such specification MUST
define the symbolic name to be used in the Service field of the SRV
record as described below. It also MUST include security
considerations. Service SRV records SHOULD NOT be used in the absence
of such specification.
If a SRV-cognizant LDAP client wants to discover a LDAP server that
supports TCP protocol and provides LDAP service for the domain
example.com., it does a lookup of
as described in [ARM]. The example zone file near the end of this
memo contains answering RRs for an SRV query.
Note: LDAP is chosen as an example for illustrative purposes only,
and the LDAP examples used in this document should not be considered
a definitive statement on the recommended way for LDAP to use SRV
records. As described in the earlier applicability section, consult
the appropriate LDAP documents for the recommended procedures.
The format of the SRV RR
Here is the format of the SRV RR, whose DNS type code is 33:
_Service._Proto.Name TTL Class SRV Priority Weight Port Target
(There is an example near the end of this document.)
The symbolic name of the desired service, as defined in Assigned
Numbers [STD 2] or locally. An underscore (_) is prepended to
the service identifier to avoid collisions with DNS labels that
occur in nature.
Gulbrandsen, et al. Standards Track [Page 2]RFC 2782 DNS SRV RR February 2000
Some widely used services, notably POP, don't have a single
universal name. If Assigned Numbers names the service
indicated, that name is the only name which is legal for SRV
lookups. The Service is case insensitive.
The symbolic name of the desired protocol, with an underscore
(_) prepended to prevent collisions with DNS labels that occur
in nature. _TCP and _UDP are at present the most useful values
for this field, though any name defined by Assigned Numbers or
locally may be used (as for Service). The Proto is case
The domain this RR refers to. The SRV RR is unique in that the