TUT:snmpgetnext

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The other main SNMP operation for retrieving information is the GETNEXT request - a "fuzzy-matching" version of GET. Whereas snmpget retrieves the value associated with the specified OID, snmpgetnext takes this OID and retrieves the value associated with the next (valid) OID: 

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org SNMPv2-MIB::sysUpTime.0
   SNMPv2-MIB::sysContact.0 = Wes Hardaker wjhardaker@ucdavis.edu

OID Completion

One way to use this operation is by providing an incomplete OID - perhaps omitting the instance subidentifier, or giving an "internal" MIB object. The agent will determine the next complete instance, and return that OID together with the corresponding value: 

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org sysUpTime
   SNMPv2-MIB::sysUpTime.0 = Timeticks: (586978184) 67 days, 22:29:41.84
   

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org system
   SNMPv2-MIB::sysDescr.0 = HP-UX net-snmp B.10.20 A 9000/715
   

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org .1.3.6
   SNMPv2-MIB::sysDescr.0 = HP-UX net-snmp B.10.20 A 9000/715


This is particularly useful with MIB tables, where it may not be obvious what index values are valid. Issuing a GETNEXT request with the bare MIB name of one of the columns will return that entry from the first row of the table: 

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org tcpConnState
   TCP-MIB::tcpConnState.0.0.0.0.25.0.0.0.0.0 = INTEGER: 2

It's also possible to provide a partial index, and have the agent complete it: 

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org tcpConnState.127.0.0.1
   TCP-MIB::tcpConnState.127.0.0.1.631.0.0.0.0.0 = INTEGER: 2


Walking the MIB

Another way of using the GETNEXT operation is to issue a series of such requests, with each one using the OID from the previous result. This has the effect of "walking" through the MIB: 


   % snmpgetnext -v 2c -c demopublic test.net-snmp.org sysUpTime.0
   SNMPv2-MIB::sysContact.0 = Wes Hardaker wjhardaker@ucdavis.edu
   

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org sysContact.0
   SNMPv2-MIB::sysName.0 = net-snmp
   

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org sysName.0
   SNMPv2-MIB::sysLocation.0 = UCDavis


Again, this technique is particularly useful with MIB tables, where it will walk through successive rows of the table: 

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org sysORDescr
   SNMPv2-MIB::sysORDescr.1 = STRING: The MIB module to describe generic objects for network interface sub-layers
   % snmpgetnext -v 2c -c demopublic test.net-snmp.org sysORDescr.1
   SNMPv2-MIB::sysORDescr.2 = STRING: The MIB module for SNMPv2 entities
   % snmpgetnext -v 2c -c demopublic test.net-snmp.org sysORDescr.2
   SNMPv2-MIB::sysORDescr.4 = STRING: The MIB module for managing IP and ICMP implementations

Note that this works even if the indexes are not consecutive. However, the GETNEXT operation does not stop at the end of a table column (or even at the end of the whole table). If you issue a GETNEXT request on the last instance in a table column (or an index value greater than the last row of the table), then the agent will still return the next valid instance - typically the value of the next column from the first row of the table: 

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org sysORDescr.99
   SNMPv2-MIB::sysORUpTime.1 = Timeticks: (82) 0:00:00.82

or possibly the first instance from a completely different area of the MIB altogether: 

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org sysORUpTime.99
   IF-MIB::ifNumber.0 = INTEGER: 3

It's up to the client application (or person) making the request to recognise when this happens, and not to continue walking the MIB (unless that is explicitly what is required). The snmpwalk command has been designed to do precisely this task.


Multiple Variables

Just as with snmpget, it's also possible to run a single GETNEXT request with several OIDs, and it will retrieve the next valid instance for each of these: 

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org sysUpTime system
   SNMPv2-MIB::sysUpTime.0 = Timeticks: (586978184) 67 days, 22:29:41.84
   SNMPv2-MIB::sysDescr.0 = HP-UX net-snmp B.10.20 A 9000/715

Once again, this is particularly useful with columns of a table, as this can be used to walk through the table one row at a time: 

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org sysORDescr sysORID sysORUpTime
   SNMPv2-MIB::sysORDescr.1  = STRING: The MIB module to describe generic objects for network interface sub-layers
   SNMPv2-MIB::sysORID.1     = IF-MIB::ifMIB 
   SNMPv2-MIB::sysORUpTime.1 = Timeticks: (82) 0:00:00.82
   % snmpgetnext -v 2c -c demopublic test.net-snmp.org sysORDescr.1 sysORID.1 sysORUpTime.1
   SNMPv2-MIB::sysORDescr.2  = STRING: The MIB module for SNMPv2 entities
   SNMPv2-MIB::sysORID.2     = SNMPv2-MIB::snmpMIB 
   SNMPv2-MIB::sysORUpTime.2 = Timeticks: (81) 0:00:00.81
      etc

This is basically the purpose of the snmptable command.

One thing to be aware of is that there may potentially be "holes" in the table, where a particular column does not have a value in certain rows. The GETNEXT request will step over these, and return the value of that column from the next row where it does exist. The next iteration would need to adjust to this if the walk is to keep stepping through a row at a time: 

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org sysORDescr.2 sysORID.2 sysORUpTime.2
   SNMPv2-MIB::sysORDescr.4  = STRING: The MIB module for managing IP and ICMP implementations
   SNMPv2-MIB::sysORID.5     = UDP-MIB::udpMIB
   SNMPv2-MIB::sysORUpTime.4 = Timeticks: (83) 0:00:00.83
   % snmpgetnext -v 2c -c demopublic test.net-snmp.org sysORDescr.4 sysORID.4 sysORUpTime.4
   SNMPv2-MIB::sysORDescr.5  = STRING: The MIB module for managing UDP implementations
   SNMPv2-MIB::sysORID.5     = UDP-MIB::udpMIB
   SNMPv2-MIB::sysORUpTime.5 = Timeticks: (82) 0:00:00.82

[This is a somewhat artificial example, but serves to illustrate the idea]


Problems

If a GETNEXT request is given the very last valid OID (or one later than this), then the agent needs to indicate that there are no further values to retrieve. With SNMPv1, this uses the same error as a GET request for a non-existant OID: 

   % snmpgetnext -v 1 -c demopublic test.net-snmp.org .2.0.9999
   Error in packet.
   Reason: (noSuchName) There is no such variable name in this MIB.
   Failed object: joint-iso-ccitt.0.9999

SNMPv2 appears similar, but uses a different exception to indicate this situation: 

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org .2.0.9999
   joint-iso-ccitt.0.9999 = No more variables left in this MIB View (It is past the end of the MIB tree)

But as with the GET requests, the difference between these two versions really becomes apparent when one OID fails in a multi-OID GETNEXT request: 

   % snmpgetnext -Cf -v 1 -c demopublic test.net-snmp.org .2.0.9999 system
   Error in packet.
   Reason: (noSuchName) There is no such variable name in this MIB.
   Failed object: joint-iso-ccitt.0.9999
   

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org .2.0.9999 system
   joint-iso-ccitt.0.9999 = No more variables left in this MIB View (It is past the end of the MIB tree)
   SNMPv2-MIB::sysDescr.0 = HP-UX net-snmp B.10.20 A 9000/715

Once again, SNMPv3 works in the same way as SNMPv2c.


Unsupported MIB Objects

One other thing to be aware of is that it is the agent which determines what OID should be returned, not the client application. If the agent does not happen to implement a particular MIB object (or group of objects) that the client knows about, then the agent will return the next OID that it does implement, even if this is not the entry that was actually required: 

   % snmpgetnext -v 2c -c demopublic test.net-snmp.org enterprises.hp
   UCD-SNMP-MIB::memIndex.0 = INTEGER: 0

The same holds if the agent is configured to only reveal a subset of the full MIB. Any objects that are outside the authorised view will be skipped, as if they had not been implemented at all.

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