SRV record
Encyclopedia
A Service record is a specification of data in the Domain Name System
defining the location, i.e. the hostname
and port number, of servers for specified services. It is defined in RFC 2782, and its type code is 33. Some Internet protocols such as the Session Initiation Protocol
(SIP) and the Extensible Messaging and Presence Protocol
(XMPP) often require SRV support by network elements.
_service._proto.name TTL class SRV priority weight port target
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.
This points to a server named sipserver.example.com listening on TCP port 5060 for Session Initiation Protocol
(SIP) protocol services. The priority given here is 0, and the weight is 5.
As in MX record
s, the target host in SRV records point to an address record (A or AAAA record). Hostname alias (CNAME record
) cannot be used as valid target.
If a service has multiple SRV records with the same priority value, clients use the weight field to determine which host to use. The weight value is relevant only in relation to other weight values for the service, and only among records with the same priority value.
In the following example, both the priority and weight fields are used to provide a combination of load balancing and backup service.
_sip._tcp.example.com. 86400 IN SRV 10 60 5060 bigbox.example.com.
_sip._tcp.example.com. 86400 IN SRV 10 20 5060 smallbox1.example.com.
_sip._tcp.example.com. 86400 IN SRV 10 10 5060 smallbox2.example.com.
_sip._tcp.example.com. 86400 IN SRV 10 10 5066 smallbox2.example.com.
_sip._tcp.example.com. 86400 IN SRV 20 0 5060 backupbox.example.com.
The first four records share a priority of 10, so the weight field's value will be used by clients to determine which server (host and port combination) to contact. The sum of all four values is 100, so bigbox.example.com will be used 60% of the time. The two hosts smallbox1 and smallbox2 will be used for 20% of requests each, with half of the requests that are sent to smallbox2 (i.e. 10% of the total requests) going to port 5060 and the remaining half to port 5066. If bigbox is unavailable, these two remaining machines will share the load equally, since they will each be selected 50% of the time.
If all four servers with priority 10 are unavailable, the record with the next highest priority value will be chosen, which is backupbox.example.com. This might be a machine in another physical location, presumably not vulnerable to anything that would cause the first four hosts to become unavailable.
The load balancing provided by SRV records is inherently limited, since the information is essentially static. Current load of servers is not taken into account.
(dig) or nslookup
.
$ dig SRV _sip._tcp.example.com
$ host -t SRV _sip._tcp.example.com
$ nslookup -type=srv _sip._tcp.example.com
$ nslookup
> set type=srv
> _sip._tcp.example.com
communications protocol
s:
In Microsoft
Windows 2000
clients use SRV records to find the domain controller
for a given service. SRV Records are also used by Outlook 2007, 2010 and Macintosh 10.6 mail to locate the Exchange Autodiscover service.
Domain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
defining the location, i.e. the hostname
Hostname
A hostname is a label that is assigned to a device connected to a computer network and that is used to identify the device in various forms of electronic communication such as the World Wide Web, e-mail or Usenet...
and port number, of servers for specified services. It is defined in RFC 2782, and its type code is 33. Some Internet protocols such as the Session Initiation Protocol
Session Initiation Protocol
The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...
(SIP) and the Extensible Messaging and Presence Protocol
Extensible Messaging and Presence Protocol
Extensible Messaging and Presence Protocol is an open-standard communications protocol for message-oriented middleware based on XML . The protocol was originally named Jabber, and was developed by the Jabber open-source community in 1999 for near-real-time, extensible instant messaging , presence...
(XMPP) often require SRV support by network elements.
Record format
An SRV record has the form:_service._proto.name TTL class SRV priority weight port target
- service: the symbolic name of the desired service.
- proto: the transport protocol of the desired service; this is usually either TCPTransmission Control ProtocolThe Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
or UDPUser Datagram ProtocolThe User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...
. - name: the domain name for which this record is valid.
- TTL: standard DNS time to liveTime to liveTime to live is a mechanism that limits the lifespan of data in a computer or network. TTL may be implemented as a counter or timestamp attached to or embedded in the data. Once the prescribed event count or timespan has elapsed, data is discarded. In computer networking, TTL prevents a data...
field. - class: standard DNS class field (this is always IN).
- priority: the priority of the target host, lower value means more preferred.
- weight: A relative weight for records with the same priority.
- port: the TCP or UDP port on which the service is to be found.
- target: the canonical hostname of the machine providing the service.
An example SRV record in textual form that might be found in a zone file
Zone file
A Domain Name System zone file is a text file that describes a DNS zone. A DNS zone is a subset, often a single domain, of the hierarchical domain name structure of the DNS. The zone file contains mappings between domain names and IP addresses and other resources, organized in the form of text...
might be the following:
_sip._tcp.example.com. 86400 IN SRV 0 5 5060 sipserver.example.com.
This points to a server named sipserver.example.com listening on TCP port 5060 for Session Initiation Protocol
Session Initiation Protocol
The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...
(SIP) protocol services. The priority given here is 0, and the weight is 5.
As in MX record
MX record
A mail exchanger record is a type of resource record in the Domain Name System that specifies a mail server responsible for accepting email messages on behalf of a recipient's domain, and a preference value used to prioritize mail delivery if multiple mail servers are available...
s, the target host in SRV records point to an address record (A or AAAA record). Hostname alias (CNAME record
CNAME record
A CNAME record or Canonical Name record is a type of resource record in the Domain Name System that specifies that the domain name is an alias of another, canonical domain name. This helps when running multiple services from a single IP address...
) cannot be used as valid target.
Provisioning for high service availability
The priority field determines the precedence of use of the record's data. Clients always use the SRV record with the lowest-numbered priority value first, and fallback to other records of equal or higher priority if the connection to the host fails.If a service has multiple SRV records with the same priority value, clients use the weight field to determine which host to use. The weight value is relevant only in relation to other weight values for the service, and only among records with the same priority value.
In the following example, both the priority and weight fields are used to provide a combination of load balancing and backup service.
_sip._tcp.example.com. 86400 IN SRV 10 60 5060 bigbox.example.com.
_sip._tcp.example.com. 86400 IN SRV 10 20 5060 smallbox1.example.com.
_sip._tcp.example.com. 86400 IN SRV 10 10 5060 smallbox2.example.com.
_sip._tcp.example.com. 86400 IN SRV 10 10 5066 smallbox2.example.com.
_sip._tcp.example.com. 86400 IN SRV 20 0 5060 backupbox.example.com.
The first four records share a priority of 10, so the weight field's value will be used by clients to determine which server (host and port combination) to contact. The sum of all four values is 100, so bigbox.example.com will be used 60% of the time. The two hosts smallbox1 and smallbox2 will be used for 20% of requests each, with half of the requests that are sent to smallbox2 (i.e. 10% of the total requests) going to port 5060 and the remaining half to port 5066. If bigbox is unavailable, these two remaining machines will share the load equally, since they will each be selected 50% of the time.
If all four servers with priority 10 are unavailable, the record with the next highest priority value will be chosen, which is backupbox.example.com. This might be a machine in another physical location, presumably not vulnerable to anything that would cause the first four hosts to become unavailable.
The load balancing provided by SRV records is inherently limited, since the information is essentially static. Current load of servers is not taken into account.
Retrieving a SRV record
SRV records may be queried with standard network administration tools, such as the Domain Information GroperDomain Information Groper
Domain Information Groper is a network administration command-line tool for querying Domain Name System name servers for any desired DNS records....
(dig) or nslookup
Nslookup
nslookup is a network administration command-line tool available for many computer operating systems for querying the Domain Name System to obtain domain name or IP address mapping or for any other specific DNS record.-Background:...
.
$ dig SRV _sip._tcp.example.com
$ host -t SRV _sip._tcp.example.com
$ nslookup -type=srv _sip._tcp.example.com
$ nslookup
> set type=srv
> _sip._tcp.example.com
Usage
SRV records are common in conjunction with the following standardizedInternet standard
In computer network engineering, an Internet Standard is a normative specification of a technology or methodology applicable to the Internet. Internet Standards are created and published by the Internet Engineering Task Force .-Overview:...
communications protocol
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
s:
- CalDAVCalDAVCalendaring Extensions to WebDAV, or CalDAV, is an Internet standard allowing a client to access scheduling information on a remote server. It extends WebDAV specification and uses iCalendar format for the data. The protocol is defined by RFC 4791...
and CardDAVCardDAVCardDAV is an address book client/server protocol designed to allow users to access and share contact data on a server.The CardDAV protocol was developed by the IETF and has been published as RFC 6352.... - Client SMTP Authorization
- IMPS
- Kerberos
- LDAPLightweight Directory Access ProtocolThe Lightweight Directory Access Protocol is an application protocol for accessing and maintaining distributed directory information services over an Internet Protocol network...
- SIPSession Initiation ProtocolThe Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...
- XMPPExtensible Messaging and Presence ProtocolExtensible Messaging and Presence Protocol is an open-standard communications protocol for message-oriented middleware based on XML . The protocol was originally named Jabber, and was developed by the Jabber open-source community in 1999 for near-real-time, extensible instant messaging , presence...
- DNS Service Discovery (DNS-SD)
In Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
Windows 2000
Windows 2000
Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the...
clients use SRV records to find the domain controller
Domain controller
On Windows Server Systems, a domain controller is a server that responds to security authentication requests within the Windows Server domain...
for a given service. SRV Records are also used by Outlook 2007, 2010 and Macintosh 10.6 mail to locate the Exchange Autodiscover service.
See also
- MX recordMX recordA mail exchanger record is a type of resource record in the Domain Name System that specifies a mail server responsible for accepting email messages on behalf of a recipient's domain, and a preference value used to prioritize mail delivery if multiple mail servers are available...
- a DNS RR type used for locating the SMTP server - List of DNS record types
- IETF draft using SRV records to locate whois servers
External links
- RFC 2782 - definition of the SRV resource record
- Men & Mice's DNS Glossary - SRV Record
- Rick van Rein's articles on SRV resource records
- Comprehensive list of defined SRV service types
- draft-andrews-http-srv-01.txt - Use of SRV records in conjunction with HTTP and URIs (Expired Internet-Draft)
- PHP Client for HTTP+SRV - PHP class extending the Zend Framework to support SRV records in its HTTP client
- RFC 6186 - Use of SRV Records for Locating Email Submission/Access Services