DhcpMonitor

Use this monitor to check the availability and functionality of DHCP servers. The DhcpMonitor is run by Pollerd and opens a background process to listen for incoming DHCP responses. A DHCP server is tested by sending a DISCOVER message. If the DHCP server responds with an OFFER, the service is marked as up. The background listening process is started only if the DhcpMonitor is used.

Make sure no DHCP client is running on the Horizon server and using port UDP/67 and UDP/68. If UDP/67 and UDP/68 are already in use, you will find warning messages in your log files. You can test if a process is listening on UDP/68 with sudo ss -lnpu sport = :68.

The use of the DhcpMonitor previously required the installation of an additional package. The functionality of this additional plugin has been rolled into the core Horizon service and is no longer a separate install.

Monitor facts

Class Name

org.opennms.netmgt.poller.monitors.DhcpMonitor

This monitor implements the Common Configuration Parameters.

DhcpMonitor configuration

Table 1. Optional DhcpMonitor parameters in poller-configuration.xml.
Parameter	Description	Default
macAddress	The MAC address that Horizon uses for a DHCP request.	00:06:0D:BE:9C:B2
relayMode	Puts the poller in relay mode.	false
myIpAddress	This parameter will usually be set to the IP address of the Horizon server, if relayMode is set to true. In relay mode, the DHCP server being polled will unicast its responses directly back to the IP address specified by myIpAddress rather than broadcasting its responses. This lets DHCP servers be polled even though they are not on the same subnet as the Horizon server, and without the aid of an external relay.	127.0.0.1
extendedMode	When extendedMode is false, the DHCP poller sends a DISCOVER and expects an OFFER in return. When extendedMode is true, the DHCP poller first sends a DISCOVER. If it receives no valid response it sends an INFORM. If that returns no valid response, it sends a REQUEST. OFFER, ACK, and NAK are all considered valid responses in extendedMode.	false
requestIpAddress	This parameter applies only to REQUEST queries sent to the DHCP server when extendedMode is true. The query requests the specified IP address.	127.0.0.1

Figure 1. Visualization of DHCP message flow in broadcast mode

Figure 2. Visualization of DHCP message flow in relay mode

Example: testing DHCP server in the same subnet

Example configuration on how to configure the monitor in poller-configuration.xml. The monitor tries to send a maximum of three DISCOVER messages and waits three seconds for the DHCP server OFFER message.

Examples use CentOS/RHEL path name. For Debian/Ubuntu, use /var/lib/opennms/rrd/response.

Configure a DHCP service in poller-configuration.xml

<service name="DHCP" interval="300000" user-defined="false" status="on">
 <parameter key="retry" value="2" />
 <parameter key="timeout" value="3000" />
 <parameter key="relayMode" value="false"/>
 <parameter key="extendedMode" value="false"/>
 <parameter key="rrd-repository" value="/opt/opennms/share/rrd/response" />
 <parameter key="rrd-base-name" value="dhcp" />
 <parameter key="ds-name" value="dhcp" />
</service>

<monitor service="DHCP" class-name="org.opennms.netmgt.poller.monitors.DhcpMonitor"/>

Example: testing DHCP server in a different subnet in extended mode

You can use the same monitor in poller-configuration.xml as in the above example.

Configure DhcpMonitor to test DHCP server in a different subnet. The OFFER from the DHCP server is sent to myIpAddress.

<service name="DHCP" interval="300000" user-defined="false" status="on">
 <parameter key="retry" value="2" />
 <parameter key="timeout" value="3000" />
 <parameter key="relayMode" value="true"/>
 <parameter key="extendedMode" value="false"/>
 <parameter key="myIpAddress" value="1.2.3.4"/>
 <parameter key="rrd-repository" value="/opt/opennms/share/rrd/response" />
 <parameter key="rrd-base-name" value="dhcp" />
 <parameter key="ds-name" value="dhcp" />
</service>

<monitor service="DHCP" class-name="org.opennms.netmgt.poller.monitors.DhcpMonitor"/>

If in extendedMode, the time required to complete the poll for an unresponsive node increases by a factor of three. It is a good idea to limit the number of retries to a small number.