http://www.net-snmp.org/wiki/index.php/TUT:snmpget
Basic Example
There are three main elements to such a request -where to retrieve this information from,the administrive information associated with the request,and what information is actually required:
% snmpget -v 2c -c demopublic test.net-snmp.org SNMPv2-MIB::sysUpTime.0 SNMPv2-MIB::sysUpTime.0 = Timeticks: (586731977) 67 days, 21:48:39.77
In this example, test.net-snmp.org is the host name of the agent to query, using version 2 of the SNMP protocol, and the community string "demopublic". The OID being requested is
sysUpTime.0 from the MIB module SNMPv2-MIB.
The same basic command can also be used to retrieve a single element from within a table:
% snmpget -v 2c -c demopublic test.net-snmp.org SNMPv2-MIB::sysORDescr.1 SNMPv2-MIB::sysORDescr.1 = STRING: The Mib module for SNMPv2 entities
The
snmptranslate tutorial described several ways to specifyan OID, and most of that discussion applies here too (as well as most of the otherNet-SNMP command-line tools). The one significant difference is that
snmpget(and most of the other tools) will apply "random-lookup" by default, so it is notstrictly necessary to specify the name of the MIB. The two commands above couldequally well be given as:
% snmpget -v 2c -c demopublic test.net-snmp.org sysUpTime.0 SNMPv2-MIB::sysUpTime.0 = Timeticks: (586731977) 67 days, 21:48:39.77 % snmpget -v 2c -c demopublic test.net-snmp.org sysORDescr.1 SNMPv2-MIB::sysORDescr.1 = STRING: The Mib module for SNMPv2 entities
and this form will be used throughout the rest of these tutorials.
Similarly, the options discussed in theCustomized Output Formats tutorialcan be used to control how the results should
be displayed.
SNMP Versions and Administration
Both the original SNMPv1 and the later SNMPv2c use the clear-text "community string" as a de-facto password, to indicate whether a particular request should be authorised or not. The SNMPv1 equivalent of the first example would be almost identical:
% snmpget -v 1 -c demopublic test.net-snmp.org sysUpTime.0 SNMPv2-MIB::sysUpTime.0 = Timeticks: (586731977) 67 days, 21:48:39.77
SNMPv3 uses a significantly different authentication mechanism,typically based around usernames and passwords, and allows bothproper validation of SNMP requests, and even encryption of the trafficbetween the management client and the SNMP agent.See the
SNMPv3 Options tutorial for details.
The default version used will depend on how the software was configured whenit was first compiled. Typically, the Net-SNMP suite will probably use SNMPv3by default, but it is safest to always specify the version explicitly.
Failed Requests
The examples above show the successful retrieval of information from thetarget system. But what about requests that are not successful? How does SNMP handle failed requests?
A common mistake when using the snmpget command is to forget the index (or "instance subidentifier") of the data being requested. This is less likely when retrieving a value from within a table, where it is natural to include the index as part of
the OID. But for scalar objects, there is only one value, so it doesn't seem necessary to specify an index - surely the MIB object name alone should be sufficient?
However SNMP is consistent in requiring an instance for all MIB objects - even scalar objects. In this case, the instance subidentifier is always a simple
.0 (zero), as shown in the first example above.
Omitting this results in an error:
% snmpget -v 1 -c demopublic test.net-snmp.org sysUpTime Error in packet Reason: (noSuchName) There is no such variable name in this MIB. This name doesn't exist: sysUpTime
Note that SNMPv2c gives a slightly more informative message:
% snmpget -v 2c -c demopublic test.net-snmp.org sysUpTime SNMPv2-MIB::sysUpTime = No Such Instance currently exists
The other likely cause of failure is that the agent does not support the requested MIB object at all. With SNMPv1, the error is exactly the same ("noSuchName").SNMPv2c uses a slightly different indication of this situation:
% snmpget -v 2c -c demopublic test.net-snmp.org .1.3.6.1.2.1.1.99.0 SNMPv2-MIB::system.99.0 = No Such Object available on this agent at this OID
SNMPv3 reports problems in the same way as SNMPv2c.
Timeouts
Another possible type of failure is that the request may timeoutwithout returning any information at all.Assuming that the remote system is actually running an SNMP agent,the most likely cause of this would be the access control settingsof the remote agent.See
the FAQ entriesWhy doesn't the agent respond? andHow
do I configure access control? for further details.
Multiple Values
All the examples so far have worked with a single value. But snmpget can also retrieve several MIB values in a single request:
% snmpget -v 2c -c demopublic test.net-snmp.org sysUpTime.0 sysLocation.0 SNMPv2-MIB::sysUpTime.0 = Timeticks: (586903243) 67 days, 22:17:12.43 SNMPv2-MIB::sysLocation.0 = UCDavis
This works in the same way for all versions SNMP.The difference between SNMPv1 and the later versions becomes apparent when one of the OIDs being requested is not valid. SNMPv2c (and SNMPv3) will display what information they can:
% snmpget -v 2c -c demopublic test.net-snmp.org sysUpTime.0 sysLocation # No instance .0 SNMPv2-MIB::sysUpTime.0 = Timeticks: (586903243) 67 days, 22:17:12.43 SNMPv2-MIB::sysLocation = No Such Instance currently exists
while the equivalent SNMPv1 request will simply fail:
% snmpget -Cf -v 1 -c demopublic test.net-snmp.org sysUpTime.0 sysLocation Error in packet Reason: (noSuchName) There is no such variable name in this MIB. This name doesn't exist: sysLocation
(Note that the -Cf flag is needed to prevent snmpget from automatically correcting this problem, and retrying the request - thus defeating the point of this example!)