python net-snmp加载mibs

Question

I'm using net-snmp's python libraries to do some long queries on various switches. I would like to be able to load new mibs -- but I cannot find any documentation on how to do this.

PySNMP appears to be rather complicated and requires me to create Python objects for each mib (which doesn't scale for me); so I'm stuck with net-snmp's libraries (which aren't bad except for the loading mib thing).

I know I can use the -m and -M options with the net-snmp command-line tools, and there's documentation on pre-compiling the net-snmp suite ( ./configure , make etc.) with all the mibs (and I assume into the libraries too); if the Python libraries do not offer the ability to load mibs, can I at least configure net-snmp to provide my python libraries access to the mibs without having to recompile?

Answer 1

You technically don't have to initialize or export any environment variables if you configure net-snmp properly.

(Noting that I'm on Ubuntu 12.04.1 LTS so I really didn't have to compile net-snmp from source, and even though I'll cover the entirety of what I did for completeness, this should really only apply if you want to set up some MIBs to be automatically slurped in by net-snmp or its Python bindings.)

First I did sudo apt-get install libsnmp-base libsnmp-python libsnmp15 snmp

This will install net-snmp and its libraries as well as the Python bindings. It also installs some default MIBs (only for for net-snmp ) in /usr/share/mibs/netsnmp/ . If you want to grab a bunch of other IETF/IANA MIBs, do:

sudo apt-get install snmp-mibs-downloader

Which, as you'd expect, will download a ton of other standard MIBs (including IF-MIB and such) into /var/lib/mibs/iana , /var/lib/mibs/ietf and also /usr/share/mibs/iana and /usr/share/mibs/ietf . The snmp-mibs-downloader package also gives you the /usr/bin/download-mibs command if you want to download the MIBs again.

Next, Use the snmpconf command to set up your net-snmp environment:

$ snmpconf -h
/usr/bin/snmpconf [options] [FILETOCREATE...]
options:
  -f           overwrite existing files without prompting
  -i           install created files into /usr/share/snmp.
  -p           install created files into /home/$USER/.snmp.
  -I DIR       install created files into DIR.
  -a           Don't ask any questions, just read in current
               current .conf files and comment them
  -r all|none  Read in all or none of the .conf files found.
  -R file,...  Read in a particular list of .conf files.
  -g GROUP     Ask a series of GROUPed questions.
  -G           List known GROUPs.
  -c conf_dir  use alternate configuration directory.
  -q           run more quietly with less advice.
  -d           turn on debugging output.
  -D           turn on debugging dumper output.

I used snmpconf -p and walked through the menu items. The process basically looks for existing snmp.conf files ( /etc/snmp/snmp.conf by default) and will merge those in with the newly created config file that will get put in /home/$USER/.snmp/snmp.conf specified by the -p option. From there on out you really only need to tell snmpconf where to look for MIBs, but there are a number of useful options that are provided by the script for generating configuration directives in snmp.conf .

You should have a mostly working environment after you finish up with snmpconf . Here's what my (very bare-bones) /home/$USER/.snmp/snmp.conf looks like:

###########################################################################
#
# snmp.conf
#
#   - created by the snmpconf configuration program
#

###########################################################################
# SECTION: Textual mib parsing
#
#   This section controls the textual mib parser.  Textual
#   mibs are parsed in order to convert OIDs, enumerated
#   lists, and ... to and from textual representations
#   and numerical representations.

# mibdirs: Specifies directories to be searched for mibs.
#   Adding a '+' sign to the front of the argument appends the new
#   directory to the list of directories already being searched.
#   arguments: [+]directory[:directory...]

mibdirs : +/usr/share/mibs/iana:/usr/share/mibs/ietf:/usr/share/mibs/netsnmp:/home/$USERNAME/.snmp/mibs/newmibs

# mibs: Specifies a list of mibs to be searched for and loaded.
#   Adding a '+' sign to the front of the argument appends the new
#   mib name to the list of mibs already being searched for.
#   arguments: [+]mibname[:mibname...]

mibs +ALL

Some gotchas:

1. When net-snmp loads this config file it doesn't do a recursive directory search, so you have to give an absolute path to the directory where the MIBs live.
2. If you choose to tell net-snmp to load all 300+ MIBs in those directories, it could slow down your SNMP queries, and there are bound to be some things dumped to STDERR because of the fact that some MIBs would either be out of date, wrong, or trying to import definitions from MIBs that don't exist or weren't downloaded by the package. Your options are: tell snmpconf how you want those errors to be handled, or figure out what's missing or out of date and download the MIB yourself. If you go for the latter, you may find yourself going down a MIB rabbithole, so keep that in mind. Personally I'd suggest that you load them all, and then work backwards to only load the given MIBs that would make sense for polling a particular device.
3. The order of the directories that you specify in the search path in snmp.conf is important, especially if some MIBs reference or depend on other MIBs. I made one error I was getting go away simply by taking a MIB file in the iana directory and moving it into the ietf directory. I'm sure there's a way to programmatically figure out which MIBs depend on the others and make them happily coexist in a single directory but I didn't want to waste a bunch of time trying to figure this out.

The moral of the story is, if you've got a proper snmp.conf, you should just be able to do this:

$ python
>>> import netsnmp
>>> oid = netsnmp.VarList(netsnmp.Varbind('dot1qTpFdbPort'))
>>> res = netsnmp.snmpwalk(oid, Version=2, DestHost='10.0.0.1', Community='pub')
>>> print res
('2', '1')
>>>

FYI I omitted a bunch of STDERR output but again you can configure your environment to dump STDERR to a logfile if you wish via snmp.conf configuration directives.

Hope this helps.

Answer 2

I found an answer after all. From the snmpcmd(1) man page:

   -m MIBLIST
          Specifies a colon separated  list  of  MIB  modules  (not
          files)  to load for this application.  This overrides (or
          augments) the environment variable  MIBS,  the  snmp.conf
          directive  mibs,  and the list of MIBs hardcoded into the
          Net-SNMP library.

The key part here is that you can use the MIBS environment variable the same way you use the -m command line option...and that support for this is implemented at the library level. This means that if you define the MIBS environment variable prior to starting Python, it will affect the behavior of the netsnmp library:

$ python 
Python 2.7.2 (default, Oct 27 2011, 01:40:22) 
[GCC 4.6.1 20111003 (Red Hat 4.6.1-10)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> import netsnmp
>>> os.environ['MIBS'] = 'UPS-MIB:SNMPv2-SMI'
>>> oid = netsnmp.Varbind('upsAlarmOnBattery.0')
>>> netsnmp.snmpget(oid, Version=1, DestHost='myserver', Community='public')
('0',)
>>> 

Note that you must set os.environ['MIBS'] before calling any of the netsnmp module functions (because this will load the library and any environment changes after this will have no affect).

You can (obviously) also set the environment variable outside of Python:

$ export MIBS='UPS-MIB:SNMPv2-SMI'
$ python
>>> import netsnmp
>>> oid = netsnmp.Varbind('upsAlarmOnBattery.0')
>>> netsnmp.snmpget(oid, Version=1, DestHost='myserver', Community='public')
('0',)
>>>