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SNMP Protocol Object Modification Using SetRequest Messages
The GetRequest-PDU, GetNextRequest-PDU,
and GetBulkRequest-PDU messages are the three members of
the SNMP Read class
of PDUsthey are used to let an SNMP
manager read MIB objects from an MIB agent. The opposite function is
represented by the SNMP Write class, which contains a single
member: the SNMP SetRequest-PDU message.
The use of this PDU is fairly obvious;
where one of the three Get PDUs specifies a variable whose value
is to be retrieved, the SetRequest-PDU message contains a specification
for variables whose values are to be modified by the network administrator.
Remember that SNMP
does not include specific commands to
let a network administrator control a managed device. This is in fact
the control method, by setting variables that affect the
operation of the managed device.
The set process is the complement
of the get process; the same basic idea, pretty much, but a reversal
in how the object values travel and what is done with them.
The process follows these steps (see Figure 275):
Verification of Object Modification Requests
- SNMP Manager Creates SetRequest-PDU:
Based on the information changes specified by the user through the SNMP
application, the SNMP software on the network management station creates
a SetRequest-PDU message. It contains a set of MIB object names
and the values to which they are to be set.
- SNMP Manager Sends SetRequest-PDU:
The SNMP manager sends the PDU to the device being controlled.
- SNMP Agent Receives and Processes
SetRequest-PDU: The SNMP agent receives and processes the
set request. It examines each object in the request along with the value
to which the object is to be set, and determines if the request should
or should not be honored.
- SNMP Agent Makes Changes and Creates
Response-PDU: Assuming that the information in the request
was correct (and any security provisions have been satisfied), the SNMP
agent makes changes to its internal variables. The agent creates a Response-PDU
to send back to the SNMP Manager, which either indicates that the request
succeeded, or contains error codes to indicate any problems with the
request found during processing.
- SNMP Agent Sends Response-PDU:
The agent sends the response back to the SNMP Manager.
- SNMP Manager Processes Response-PDU:
The manager processes the information in the Response-PDU to see the
results of the set.
Figure 275: SNMP Object Modification Process
The communication process for setting a MIB object value is very similar to that used for reading one. The main difference is that, of course, the object values are sent from the SNMP Manager to the SNMP Agent, carried in the SetRequest-PDU message.
Obviously, telling a device to change
a variable's value is a more significant request than just asking the
device to read the value. For this reason, the managed device must very
carefully analyze and verify the information in the request to ensure
that the request is valid. The checks performed include:
- Verifying the names of the objects to be changed.
- Verifying that the objects are allowed to be
modified (based on their Access or Max-Access object
- Checking the value included in the request to
ensure that its type and size are valid for the object to be changed.
This is also a place where general
protocol security issues become more important.
Key Concept: SNMP network management stations control the operation of managed devices by changing MIB objects on those devices. This is done using the SetRequest-PDU message, which specifies the objects to be modified and their values.
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Version 3.0 - Version Date: September 20, 2005
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