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Service Location (SRV) Resource Records

Friday, October 28, 2011

DNS servers must provide support for Service Location (SRV) resource records described in RFC 2052 in order for Active Directory to function properly.  SRV resource records map the name of a service to the name of a server offering that service. Active Directory clients and domain controllers use SRV records to locate AD related services as well as other functions. Additional, SRV resource records can be used to locate non-AD services and applications.

Here is an example of a SRV Resource Record.

_ldap._tcp.itgeared.com, 600 IN SRV 0 100 389 SERVER1.itgeared.com

The SRV Record Components


ComponentExampleExplanation
Service_ldapThe name of the service.
Protocol_tcpThe protocol used for this service.
Nameanitkb.comThe domain name that this record refers to.
TTL600The time-to-live in seconds.
ClassINThe standard DNS Internet class.
Resource RecordSRVIdentifies the record as SRV.
Priority0Identifies the priority. If multiple records exist, the client will connect using the record with the lowest value.
Weight100A load-balancing mechanism used when multiple records exist with the same priority; the clients choose the record with the higher weight value.
Port389The port used by this service.
TargetSERVER1.anitkb.comThe host that provides the service.


In this example, the information in this record states that is a client is searching for a LDAP server in the itgeared.com domain, the client should connect to SERVER1.itgeared.com. The next step for the client is to do a DNS lookup for SERVER1.itgeared.com, unless it already has this information stored in its DNS cache.

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