SNMP-Based Master/Subagent
3.1 SNMP-Based Master/Subagent Overview
The Master Agent is the main component of Solstice Enterprise Agent technology. It runs as a daemon process and listens to User Datagram Protocol (UDP) port 161 for SNMP requests. The Master Agent also opens another port to receive SNMP trap notifications from various subagents. These traps are forwarded to various managers, as determined by the configuration files.
3.1.1 Invoking the Master Agent
When the system is initially booted, a system startup script file invokes the Master Agent if the /etc/snmp/conf/snmpdx.rsrc configuration file contains neither blank lines nor comment lines. Upon the Master Agent's invocation, it reads its various configuration files and appropriate actions are performed by activating subagents, determining the subtree OID for various subagents, populating its own MIBs. The Master Agent provides the following functionality:
Invokes subagents
Communicates with subagents
Registers subagents
Sends requests to subagents
Receives responses from subagents
Traps notifications from subagents
Note - If you have snmpd (part of Domain Manager) or some other SNMP agent using port 161 running, the Solstice Enterprise Agents cannot run.
3.1.2 Invoking the Subagent
A subagent can be invoked in the following ways:
By the Master Agent - the Master Agent may invoke all agents with the agent resource file existing under SEA_SNMPConfiguration_Directory. The agents are invoked as specified in the Master Agent resource file. If the Master Agent successfully invokes a subagent, it creates a row entry in the subagent table and fills in the MIB variables with appropriate values for that subagent. A subagent is then registered with the Master Agent and is then ready to receive SNMP requests from the Master Agent.
Manually or automatically at boot time - system administrators/users may manually invoke subagents that have no agent resource file; or, startup scripts can invoke the subagents when the system is booted. Such agents can be invoked only after the Master Agent has been invoked. Additionally, these agents must have been created using the Solstice Enterprise Agent Software Development Kits (SDKs) and linked with the appropriate libraries. This allows the subagents to register dynamically with the Master Agent.
3.1.3 Communicating With the Subagent
Any communication from subagents to the Master Agent is done through UDP port 161. The topic of sending traps to the Master Agent from the subagents is discussed in "3.1.6 Trap Notification".
Note - The Master Agent communicates on separate ports for each subagent.
The Master Agent also checks to be sure that registered subagents are up and running, based on the following conditions:
Time-out mechanism for each Get, Get Next, and Set request is sent to each subagent by the Master Agent.
No activity between the Master Agent and the subagents. The Master Agent (based on watch_dog time as defined in the agent resource file) is used to determine whether a particular subagent is active. This is accomplished by sending an SNMP Get request to the subagent if there has been no activity between the subagent and the Master Agent for some specific configured period of time.
3.1.4 Registering the Subagent
To register the subagent, the Master Agent binds it to the MIB. The Master Agent then determines its present location, using one of the following methods:
Static method - The Master Agent reads agent resource files. This resource file contains an entry for each subagent.
Dynamic method - The Master Agent receives the information from the subagents. The subagent sends a SET request containing the MIB objects needed to register with the Master Agent, through the use of the registration API.
The binding policy relates to the registration of SNMP object identifiers (OIDs). It involves decision-making on the part of the Master Agent when dispatching SNMP requests to various subagents. The Master Agent supports the binding policy, as shown in Table 3-1.
Table 3-1 Binding Policy
Type of Registration | Method of Registration |
|---|---|
Individual variable registration | A subagent can manage individual variables |
Row registration | A subagent can manage each row or multiple rows |
Table registration | A subagent can register full and partial tables; partial table registration means that some columns of a table can be registered; for example, if a table has columns c1-c5, a subagent can then register a partial table managing c3 and c5 columns (only) of that table |
Duplicate registration | NOT allowed |
Overlapping registration | In the case of overlapped registration, the Master Agent dispatches requests on the basis of best OID match |
3.1.5 Sending Requests
The Master Agent supports the forwarding of SNMP requests (Get, Get Next, and Set) in two modes. The mode is indicated by providing an optional argument with the command-line invocation. These modes are:
Group mode - Multiple variables can be included in each request from the Master Agent to the subagents. This results in one send-request per agent.
Split mode - Each variable in the incoming request results in one send-request to each subagent.
Following is a list of allowable send requests.
SET - The set is implemented in a multi-phase fashion. First, all the varbinds in the set request are retrieved using a Get request. If successful, a Set request is sent to the respective subagents. If the Set request succeeds, a SUCCESS response is sent to the manager. If the Set request does not succeed, another Set request is sent to the subagents, with original values. This cancels the failed Set request. Any other SNMP requests being processed are completed before the Set request can be initiated.