DNS Agent

The DNS agent updates the canonical name (CNAME) mapping in the domain name server when failing over applications across subnets (performing a wide area failover.)

If your failover target and source nodes are on the same subnet, then you do not need to use the DNS resource.

If, however, the failover target and source nodes reside on different subnets, you need to use the DNS agent. The agent updates the name server and allows clients to connect seamlessly to the failed over instance of the application service.

Entry Points

• Monitor---If the online lock file exists, monitor queries the name servers for the CNAME record for the alias and reports back ONLINE if the response from at least one of the name servers contains the same canonical name associated with the alias as specified in the HostName attribute. If not, the monitor reports the resource as OFFLINE.
• Online---Queries the authoritative name server of the domain for CNAME records and updates the CNAME record on the name server with the specified alias to canonical name mapping. It adds a new CNAME record if a related record is not found. Creates an online lock file if online was successful.
• Offline---Removes the online lock file, which the online entry point created.
• Open---Removes the online lock file if the online lock file exists, and the CNAME record on the name server does not contain the expected alias or canonical name mapping.
• Clean---Removes the online lock file, if present.

State Definitions

• ONLINE---Online lock exists and the CNAME RR is as expected.
• OFFLINE---Either the lock does not exist, or the expected record is not found.
• UNKNOWN---Problem with configuration.