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An SRV record or Service record is a category of data in the Internet Domain Name System specifying information on available services. It is defined in RFC 2782. Newer internet protocols such as SIP and XMPP often require SRV support from clients. Client implementations of older protocols (e.g. LDAP, SMTP) may have SRV support added to it. On the Internet, the Domain Name System (DNS) stores and associates many types of information with domain names; most importantly, it serves as the phone book for the internet, translating human-friendly domain names and computer hostnames, e. ...
The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. ...
Extensible Messaging and Presence Protocol, or XMPP, is an open, XML-based protocol for near real-time extensible messaging and presence events. ...
In computer networking, the Lightweight Directory Access Protocol, or LDAP (IPA: ), is a networking protocol for querying and modifying directory services running over TCP/IP. A directory is a set of information with similar attributes organized in a logical and hierarchical manner. ...
Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. ...
Record format
An SRV record holds the following information: - Service: the symbolic name of the desired service.
- Protocol: this is usually either TCP or UDP.
- Domain name: the domain for which this record is valid.
- TTL: standard DNS time to live field.
- Class: standard DNS class field (this is always IN).
- Priority: the priority of the target host.
- 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 hostname of the machine providing the service.
An example SRV record might look like this: The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data using Stream Sockets. ...
The User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. ...
Time to live (sometimes abbreviated TTL) is a limit on the period of time or number of iterations or transmissions in computer and computer network technology that a unit of data (e. ...
_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 SIP protocol connections. The priority given here is 0, and the weight is 5. The three letter abbreviation SIP can refer to the following: Soft Input Panel, the main input method on Pocket PC Session Initiation Protocol, an IETF standard, one of the leading signaling protocols for Voice over IP (VoIP), along with H.323 Server Interface Pod, a Dell proprietary hardware unit that...
Load balancing with SRV The priority field is similar to an MX record's priority value. Clients always use the SRV record with the lowest priority value first, and only fall back to other records if the connection with this record's host fails. Thus a service may have a designated "fallback" server, which will only be used if the primary server fails. Only another SRV record, with a priority field value higher than the primary server's record, is needed. An MX record or Mail exchange record is a type of resource record in the Domain Name System (DNS) specifying how Internet e-mail should be routed. ...
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 20 5060 smallbox2.example.com. _sip._tcp.example.com. 86400 IN SRV 10 20 5066 smallbox2.example.com. ;multiple instances can be configured with another port _sip._tcp.example.com. 86400 IN SRV 20 0 5060 backupbox.example.com. The first three records share a priority 10, so the weight field's value will be used by clients to determine which host to contact. The sum of all three values is 100, so bigbox.example.com will be used 60% of the time. The other two hosts, smallbox1 and smallbox2, will be used for 20% of requests each. 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 three hosts 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 three hosts to become unavailable.
It should be noted that 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.
Usage SRV records are often used by Microsoft Windows 2000 clients to find the Domain Controller for a given service—which is controversial as it is not standardized usage. Microsoft Corporation, (NASDAQ: MSFT, HKSE: 4338) is a multinational computer technology corporation with global annual revenue of US$44. ...
Windows 2000 (also referred to as Win2K) is a preemptible, interruptible, graphical and business-oriented operating system that was designed to work with either uniprocessor or symmetric multi-processor 32-bit Intel x86 computers. ...
On Windows Server Systems, the domain controller (DC) is the server that responds to security authentication requests (logging in, checking permissions, etc. ...
Further, SRV records are common in conjunction with the following standardised protocols: The word standard has several meanings: Originally, standard referred to a conspicuous object used as a rallying point in battle. ...
In the field of telecommunications, a communications protocol is the set of standard rules for data representation, signalling, authentication and error detection required to send information over a communications channel. ...
Extensible Messaging and Presence Protocol, or XMPP, is an open, XML-based protocol for near real-time extensible messaging and presence events. ...
Official logo of the Jabber Software Foundation Jabber is a collection of open, real-time communication technologies built on the Extensible Messaging and Presence Protocol (XMPP). ...
The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. ...
In computer networking, the Lightweight Directory Access Protocol, or LDAP (IPA: ), is a networking protocol for querying and modifying directory services running over TCP/IP. A directory is a set of information with similar attributes organized in a logical and hierarchical manner. ...
See also The LOC record (RFC1876) is a means for expressing location information in the Domain Name System. ...
An MX record or Mail exchange record is a type of resource record in the Domain Name System (DNS) specifying how Internet e-mail should be routed. ...
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