Overview of the SNMP Master Agent

The SNMP master agent function is available on the SnmpV3AdaptorServer. As the SnmpV3AdaptorServer is trilingual, a master agent can answer requests in SNMPv1, SNMPv2 or SNMPv3. The master agent function is based on the functional blocks of the Java DMK 5.0 SNMP toolkit, particularly:

• SnmpV3AdaptorServer

• SnmpProxy

• SnmpUsmProxy

• SnmpTrapForwarder

SnmpV3AdaptorServer

The SnmpV3AdaptorServer makes it possible to register an SnmpMibAgent that handles all varbinds whose OIDs are contained below a given node of the global OID tree. For each SNMP request, all varbinds that are relevant to the part of the OID tree handled by the SnmpMibAgent are grouped into an SnmpMibRequest and passed to the SnmpMibAgent. The concrete SnmpMibAgent implementation can be either a local MIB generated by the mibgen tool, or a remote MIB, configured with an SnmpProxy object.

The SnmpV3AdaptorServer enables you to register overlapping MIBs. The actual value returned to the manager is the value implemented by the deepest MIB registered for that value. This makes it possible to register, for example, a default SnmpMibAgent to which all unhandled OIDs are forwarded. An example of such a default subagent can be the Solaris subagent snmpdx, all the OIDs of which are neither handled locally nor handled by known subagents. This makes it possible to forward them to an SnmpProxy object that forwards to snmpdx and is registered at the root of the OID tree. This only requires registering such an SnmpProxy object at the root of the OID tree.

The SnmpV3AdaptorServer supports MIB scoping, namely, context names, and makes it possible to register a MIB within a given scope, that is, for a given context name. The SnmpV3AdaptorServer also makes it possible to extract context names from an SNMPv1 or SNMPv2 community string. The master agent function can thus be spawned over several context names. A specific context name can be dedicated to a specific subagent, enabling you to register a single SnmpProxy object at the root of the OID tree for a given context name. The context name can also be shared between several subagents, and possibly local MIBs, as described above.

SnmpProxy

The SnmpProxy object makes it possible to get the actual values requested by the manager from a MIB implemented in a remote SNMP subagent. The SnmpProxy objects are able to get values from SNMP agents implementing either of the SNMPv1 or SNMPv2 protocols. This makes it possible to get values from legacy SNMP agents in a way that is transparent to the manager.

The SnmpProxy object extends the SnmpMibAgent abstract class, and as such can be registered in the SnmpV3AdaptorServer just like a local MIB generated by the mibgen tool. In a master agent application, one SnmpProxy must be created per subagent. When creating the SnmpProxy object, you must provide it with the following parameters to enable it to access the underlying subagent:

• Subagent's IP address

• Subagent's port number

• Security parameters, namely community strings, used for communicating with the subagent

• Version of SNMP that must be used to access the subagent, namely either SNMPv1 or SNMPv2

Then you must register that proxy with the SnmpV3AdaptorServer, once for each MIB implemented by the subagent that you want to mirror in the master agent. This is demonstrated in Example 21–1.

Example 21–1 Registering SnmpProxy in the SnmpV3AdaptorServer

A subagent implements the following three MIBs:

• mib#1 (root OID=x.y.z.1)

• mib#2 (root OID=x.y.z.2)

• mib#3 (root OID=x.y.z.3)

If you want mib#1 and mib#3 to be accessible from the master agent managers, then you need to register your SnmpProxy object for both mib#1 and mib#3:

		SnmpV3AdaptorServer snmpAdaptor = ....;
     SnmpProxy mySubAgentProxy = new SnmpProxy(....);
     SnmpOid oids[] = new SnmpOid(2);
     oids[0] = new SnmpOid("x.y.z.1");
     oids[1] = new SnmpOid("x.y.z.3");
     snmpAdaptor.addMib(mySubAgentProxy,oids);

Once you have done this, all varbinds contained in an SNMP request whose OIDs appear to be below x.y.z.1 or x.y.z.3 are passed to the registered mySubAgentProxy. The mySubAgentProxy proxy gets the actual values from the remote agent it is proxying. OIDs that appear to be contained below x.y.z.2 are not forwarded, and where applicable, are handled locally, because the SnmpProxy is not registered for those OIDs.

SnmpUsmProxy

The SnmpUsmProxy object performs the same function as the SnmpProxy, except that it gets values from SNMP agents implementing the SNMPv3 protocol.

The SnmpUsmProxy object extends the SnmpUsmMibAgent abstract class, and as such can be registered in the SnmpV3AdaptorServer just like a local MIB generated by the mibgen tool. In a master agent application, one SnmpUsmProxy must be created per subagent. When creating the SnmpUsmProxy object, you must provide it with the following parameters to enable it to access the underlying subagent:

• Subagent's IP address

• Subagent's port number

• Security parameters, namely context names, the user name and the security level, used for communicating with the subagent

• Request context, namely the context engine ID and the context name

The SnmpUsmProxy receives requests in any version of SNMP, and always forwards them as SNMPv3 requests.

SnmpTrapForwarder

An SnmpTrapForwarder object makes it possible to listen to traps emitted by a subagent, convert them into the master agent's manager's protocol or protocols, and forward them to the manager or managers as if they originated from the master agent itself. One or more SnmpTrapForwarder objects must be instantiated, and the subagents must be configured to send their traps to these objects. This is the responsibility of the main application code. This is explained in detail in Trap Forwarding.