From OpenNMS

The DnsMonitor enables OpenNMS to monitor the ability of a DNS server to resolve names, and to report on the latency of this service.

Configuration

The default version of poller-configuration.xml shipped with OpenNMS has the DnsMonitor configured and enabled. Usually no configuration is necessary.

Service Parameters

Like all monitors, the service tag can take a number of parameters:

name: the name of the service in OpenNMS, this must be the same as the service name specified in capsd-configuration.xml.
interval: how often (in milliseconds) to perform a poll using this monitor
user-defined: whether or not this service is user-defined, used primarily in the UI
status: whether or not the monitor is enabled

Monitor-Specific Parameters

DnsMonitor also takes the following monitor-specific parameters:

port: The UDP port on which the monitor should contact the DNS servers that it is monitoring
lookup: The DNS name that the service monitor should attempt to resolve. Defaults to localhost. All DNS servers should answer for this name, but some do not. Specify a name here for which you know your DNS servers will answer.
fatal-response-codes: Available starting from OpenNMS 1.6.10. A comma-separated list of numeric DNS response codes that will be considered fatal if present in the server's response. Default value is 2, which corresponds to Server Failed. A list of codes and their meanings is found in RFC 2929.