16.3.2 Traps in the Agent and AgentV3 Examples

Before starting the SNMP agent again, edit the jdmk.acl file to replace the occurrences of yourmanager with the name of a host running an SNMP manager.

You can start the example SNMPv3 agent by replacing Agent with AgentV3.

Start the agent, specifying the InetAddressAcl file as a property:

$ java -classpath classpath 
-Djdmk.acl.file=examplesDir/current/Snmp/Agent/jdmk.acl \ 
       -classpath classpath:. Agent nbTraps

In these commands, nbTraps is the number of traps that the agent sends. Set it to a small integer to avoid too much output. If you omit this parameter, traps are sent continuously.

If you do not have an SNMP manager or a second host, do not copy the InetAddressAcl file or specify it as a property. In the absence of the trap-community definitions, the traps are addressed to the trap port on the local host. And even if no manager is running, we can still see the agent sending the traps. See 17.1.4 SNMP Trap Handler for details about receiving traps.

To Interact With the Trap Generator

1. Access this agent's HTML adaptor by pointing a web browser to the following URL: http://localhost:8082/. Click on the class=LinkTrapGenerator,ifIndex=1 MBean in the trapGenerator domain.

   Through the HTML adaptor, you can see the MBean representing the trap generator object. You can modify its attributes to change the table entry that it operates on and to change the interval between traps.

2. Change the trap interval to 10000 so that traps are sent every 10 seconds.

3. Go back to the agent view and click on the ifEntry.ifIndex=1 MBean in the ifTable domain. Set the reload period to 10, and click the Reload button.

   You should see the effect of the trap generator is to switch the value of the IfOperStatus variable. It is our implementation of the table entry that sends a trap when this status is changed.

4. Go back to the agent view and click on the name=Snmp MBean in the RFC1213_MIB domain. Scroll down to see the SnmpOutPkts and SnmpOutTraps variables.

   These variables should be the only nonzero values, if no manager has connected to the SNMP agent. The Snmp group shows information about the SNMP adaptor, and we can see how many traps have been sent since the agent was started.

5. Press Control-C when you have finished interacting with the agent.

The LinkTrapGenerator MBean is not manageable through the SNMP adaptor because it is not part of any MIB. It is an example of another MBean providing some control of the SNMP agent, and this control can be exercised by other managers connecting through other protocols. This shows that designing an SNMP agent application involves both the implementation of the MIB functionality and, if desired, the implementation of other dynamic controls afforded by the JMX architecture and the services of Java DMK.