Wandering Thoughts archives


Some notes on using snmpwalk to poke at devices with SNMP

Suppose, not hypothetically, that you're exploring what some device exposes over SNMP. You're probably using snmpwalk (or perhaps snmpbulkwalk). Snmpwalk is probably completely clear to people who use it all of the time, but it's not clear to people like me who touch it only once in a while. So here are some notes from my recent experiences (for future me, among others), assuming general background in reading SNMP.

To start with, many of the interesting snmpwalk options are covered in the snmpcmd manual page because they're common across all of the 'snmp*' commands. Generally you'll want to specify some SNMP version (often '-v2c', but some old devices may require '-v1') and the public community ('-c public'). If you have collected vendor MIBs for your device and put them in a directory, you'll want to add that directory to the search list and then load some or all MIB modules:

snmpwalk -M +/tmp/mibs -m +VENDOR-MIB1:VMIB2 -v2c -c public ....

Often '-m ALL' is what you want, which tells snmpwalk to load all of the MIBs it can find. Generally you don't want to use -M without a + or a -, because vendor MIBs will try to import things from standard MIBs and fail if the standard MIB search path has been scrubbed away (ask me how I know).

As the snmpwalk manual page says, if you don't give it an OID to start from it starts from SNMPv2-SMI::mib-2,, and only looks at things underneath that. If you're exploring a device's SNMP information, this is probably not what you want. If you want to see everything, start from OID 1:

snmpwalk [...] example.com .1

(The leading '.' forces snmpwalk to consider this a fully qualified OID and start from the root.)

If you're pretty certain that what you're looking for is in the proper vendor-specific OID space, you can start from there:

snmpwalk [...] example.com .

You can further restrict this to a specific vendor OID, ege, and this can be useful in various situations (there are some standard things under this 'vendor' OID prefix, such as a standard set of Net-SNMP MIBs). If you know the root identifier name of a MIB you're interested in, you can also specify where to start from as '<MIB>::<root>', eg 'ATEN-IPMI-MIB::aten'; however, this may require reading the MIB to find the identifier name.

(If you're a keen person you can remember that is also known as 'SNMPv2-SMI::enterprises', and actually it can be shortened to 'enterprises'.)

If you're lucky, you'll have a vendor MIB and it will be accurate. If you're not, you're trying to explore what information is there and you'll probably start wanting some snmpwalk -O output options. One useful one is '-Oa', which gets snmpwalk to display more things as strings (there's also '-OT', but really why bother). If you want to see what OIDs are being found under some level, you may want to de-clutter the output by forcing all numeric OIDs with '-On'.

As far as I know, there's no option to snmpwalk to tell it to exclude output for known MIBs and only give you unknown OIDs, which would make it convenient to fish for non-standard areas of the device's SNMP OID tree. The best you can do is snmpwalk things and fish for appropriate patterns, like:

snmpwalk -v2c -c public example.com enterprises |
  fgrep SNMPv2-SMI::enterprises.

It turns out that another way to find the specific vendor OID root is to query for the special OID SNMPv2-MIB::sysObjectID:

; snmpwalk -v2c -c public example.com sysObjectID
SNMPv2-MIB::sysObjectID.0 = OID: SNMPv2-SMI::enterprises.21317

(This special OID is if you need it in OID form, which you may do on some systems where installing SNMP tools doesn't also pull in standard MIBs for you. Thanks, Ubuntu.)

Note that this may not give you all of what the vendor stashes away in SNMP. On one of our Dells that I checked, this reports only about enterprises.674.10892.5, when there is other stuff at enterprises.674.10892.2 as well.

sysadmin/SnmpwalkNotes written at 22:15:07; Add Comment

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