Chapter 3 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
A system startup script file invokes the Master Agent when the system
is initially booted. 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.
-
GET and GETNEXT -
Sends requests to subagents.
-
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.
-
GET RESPONSE - Receives
responses from subagents.
-
TRAP - Receives notifications
from subagents.
3.1.6 Trap Notification
The subagents send the traps to the Master Agent; the Master Agent the
decides which managers will receive the trap. This decision is configurable.
3.2 Description of the Subagent
Subagents do not interact with the network manager directly. Instead,
they communicate with the Master Agent. The management responsibility of the
subagent can be offered to the Master Agent; however, it is up to the Master
Agent to accept or ignore the offers.
After the subagents have registered with the Master Agent, they wait
for the SNMP requests from the Master Agent. If a request is received, it
sends the appropriate response. Additionally, the subagents can send SNMP
traps.
The subagent is composed of four major components: the SNMP driver,
Service API stack, subagent application, and MIB database. The Master Agent
is responsible for receiving and sending the SNMP Protocol Data Units (PDUs).
It also encodes and decodes the PDUs. The SP (Stack) dispatches the received
requests appropriately or calls the applicable call back functions.
The system interface module is the implementation of the call back)
of all the variables managed by the subagent. The MIB compiler automatically
generates this information.
Except for the system dependent interface and the MIB database, the
other components are reusable by other subagents and are provided in a library.
3.2.1 Establishment
This step involves the effort by the subagents to announce their presence
and describe how to communicate with them. The transport used is UDP. The
port on which the subagents listen for the SNMP request is configurable, as
mentioned in "4.4 Agents Access Control File" on "4.4 Agents Access Control File".
This establishment can also be done dynamically through a handshaking protocol.
3.2.2 Maintenance
The subagent periodically checks for existence of the Master Agent.
The dynamic subagents (not invoked by the Master Agent) periodically determine
whether the Master Agent is active. The runtime library reregisters the subagent's
subtree with the Master Agent in the event of any interruptions.
3.2.3 Termination
Termination involves the effort to inform the Master Agent that the
dynamic agent is about to exit. The Master Agent can then remove the row entry
from its subagent table in the memory.
3.3 Using the Master Agent
snmpdx [-h] [-p port_number] [-r filename]
[-a filename] [-c dirname] [-i filename] [-o filename] [-y]
[-m GROUP|SPLIT] [-d debug_level]
The command line arguments are shown in Table 3-2.
Table 3-2 Master Agent Command Line Arguments
|
Argument
|
Description
|
|
-a filename
|
A full path (default) of the access control
file is: /etc/snmp/conf/snmdx.acl; see "4.4 Agents Access Control File" for more information
|
|
-c dirname
|
The full path of the (default) directory containing
the agent resource files; the default directory is: /etc/snmp/conf
|
|
-d debug_level
|
Used for debugging purposes; depending on
the debug_level (0-4), it prints a specific
amount of information; the default debug_level
is 0
|
|
-h
|
Command usage
|
|
-i filename
|
The full path of the PID used by the Master
Agent for recovery after a crash; it contains tuples of the UNIX process ID,
port number, resource name, and agent name (default files) is: /var/snmp/snmpdx.st
|
|
-o filename
|
This file contains the tuple (enterprise_name, oid); for example,
(Sun Microsystems,1.3.1.6.1.4.32); the file (default) is
used as a base for lookup in the trap-filtering and forwarding process is: /etc/snmp/conf/enterprises.oid
|
|
-m
GROUP | SPLIT
|
The forwarding of SNMP requests mode; the default is GROUP; see "3.1.5 Sending Requests"for
a description of the two modes
|
|
-p port_number
|
The port number; the default port number is
161; for example, -p 1234
|
|
-r filename
|
The full path of the resource file name used
by the Master Agent; stores information about the subagents that the Master
Agent invokes and manages; the default resource file is /etc/snmp/conf/snmpdx.rsrc; see "4.2 Agents Resource Configuration Files"for
more information
|
|
-y
|
A recovery indicator signals when invoking the Master Agent process and then
invokes the recovery module; the recovery process discovers which subagents
in the previous session are still active; those subagents not active are re-spawned
by the Master Agent
|
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