TweetFollow Us on Twitter

SNMP Primer for OSX Leopard

Volume Number: 24 (2008)
Issue Number: 08
Column Tag: Network Management

SNMP Primer for OSX Leopard

An article on "Understanding, Configuring, and Customizing SNMP on OS X Leopard" that might change the way you think about your Mac.

by Noah Gift

Introduction

If you haven't done any work with SNMP before, you might be thinking, "who cares?" SNMP has gotten the somewhat deserved reputation as being complex to administer and understand. As a result, many people don't care, and there are not that many articles that talk about it.

This article will attempt to present the information in a gentle enough manner that no experience with SNMP will be required. At the same time, we will dive into a few obscure, but absolutely fascinating things that be done with SNMP that few delve into. It is quite fun to let a cat out of the bag, so let's dive into SNMP and see how it might just change the way you think about your Mac.

Backwards First

Because SNMP can be incredibly boring to talk about, let's save the obligatory overview of what SNMP is, the history of SNMP, configuring SNMP etc. Instead we are going to immediately do something useful. Just follow these steps on a OS X Leopard machine that is not a production server.

Step 1:  cp /etc/snmp/snmpd.conf /etc/snmp/snmpd.conf.bak.052108
Step 2: echo "rocommunity public" > /etc/snmp/snmpd.conf
Step 3:  Restart the snmpd daemon by simply typing the executable:
mini# snmpd
Step 4:  Query the disk usage of your machine via snmp:
mini# snmpdf –v 2c –c public localhost
Description            size (kB)          Used        Available    Used%
Physical memory        1310720            821440      489280         62%
Swap space               65532                 0       65532          0%
/                     77814832          51153396    26661436         65%
/dev                         1                 1           0        100%
/dev                         1                 1           0        100%
/net                         0                 0           0          0%
/home/net                    0                 0           0          0%

Hopefully these simple four steps did the equivalent of dropping a suitcase nuke on your preconceptions about SNMP. Let's now go over what we did in detail now. In the first step we simply made a backup of the snmp configuration file, and added a date stamp on it.

In the second step we overwrote the whole snmpd.conf file by simply echoing out one line of configuration syntax; this effectively erases pages upon pages of almost completely useless configuration data you will never use. This in fact is one of the biggest stumbling blocks to understanding new technology, too much information.

In the third step we told snmpd to start the daemon so we can query it. (We will get into launchd configurations later). Finally in the fourth step, we ran a specialized snmp tool that is effectively the same as running df –k on a local machine.

Backwards First Explained

Now that we have removed some of the magic behind SNMP, and brought it down to the level of complexity of ssh or tar let's fill in a few of the details. The very first detail to understand is the cryptic syntax for the snmpdf command. If we look at the command again, we can break it down into the following sections:

snmpdf –v [SNMP Version] –c [Name of community string] IP Address or hostname

Fortunately, all of the Net–SNMP command line tools follow this same syntax, so you will only need to understand one tool to understand the rest. The –v option refers to which version of SNMP you wish to use. The available options are 1, 2c, and 3. Versions 1 and 2c are not encrypted so they are only safe to use in a secure firewalled environment, and version 3 requires more explanation and setup. If you just want to setup snmp at home behind a firewall, then 2c is the version you will most likely want to use.

The next flag –c refers to the community string. With SNMP versions 1 and 2c, the authentication system revolves around setting community strings to grant access to the snmp daemon. There are ways to set community strings to allow read access, which is what we did in the section above, and also to allow read/write access. In this case, we went with a convention and used public as the ro, or read only community string.

Finally, we just typed in the name of the local machine. We could have typed in the name of any machine on the internet though, if they had snmp running and had the configuration we setup. This is really almost 80% of what someone needs to know about SNMP, we will cover a few of other things later. Given that we are now SNMP experts, let's write some code to monitor a home network for disk usage problems.

Writing a Disk Space Monitoring System in Python

Since we are now SNMP experts, and have bragged to all of our friends how we can monitor the disk usage of our machines remotely, we got thrown into a consulting gig "by accident." Our task is to write a simple nightly monitoring script that checks all of the machines on a local network to make sure they do not have critical disk space issues. This client has several Final Cut Pro Suites where the editors constantly overfill their RAID volumes, and it is our job to write code that will prevent it before it happens and send the company owner an email when disk space on any volume exceeds 80%.

With Python and SNMP this is actually a trivial, yet very useful, problem to solve. The first step is to write some code that flags disk usage that exceeds 80%.

Listing 1: snmpdf_alert.py

#!/usr/bin/env python

""" Parses the output of snmpdf and performs a regular expression match that searches for 80–99% disk usage pattern. If a match is found it prints out the volume that exceeds our "quota"."""

from subprocess import PIPE, Popen
import re
def snmpdf(machine):
    """Returns snmpdf output as file object"""
    p = Popen("snmpdf –v 2c –c public %s" % machine,
                shell=True,
                stdout=PIPE,
                stderr=PIPE)
    return p.stdout
def parse(file):
    """Parses file object and determines if critical match
    between 80–99% disk usage is met.
    Returns collected results with new stamp line."""
    collection = []
    pattern = "[8–9][0–9]%"
    outline = (line.split() for line in file)
    flag = (" ".join(row) for row in outline \
        if re.search(pattern, row[–1]))
    for line in flag:
        newline = "%s DISK USAGE CRITICAL" % line
        collection.append(newline)
    if len(collection) > 0:
        return collection
if __name__ == "__main__":
    #prints results
    out = snmpdf("localhost")
    result = parse(out)
    if result:
        for line in result:
            print line

All this script does is to take the results from the snmpdf command and look for volumes that have between 80–99 % utilization. If you are dealing with editing on a local RAID volume, often exceeding 80% capacity will cause performance problems. If we run this script on a machine with utilization problems, we get something that looks like this:

mini# python2.5 snmpdf_alert.py
/ 97349872 88678460 8671412 91% DISK USAGE CRITICAL

We can see that this machine's root volume or "/", is at 91% capacity. This is obviously a big problem that we need to address very soon.

In order to turn this into something that manages a network and emails a warning, it would be fairly easy to just run this in a cron job or via launchd, and mail an alert message if the output of the script was not None, which is what happens when there is no match for our regular expression.

If you are new to Python some of the code may look a little weird, so here are a few things to remember. First, whitespace is significant, so indentations are there to control the flow of the program. Second, if you are coming from another language, Python is fairly easy to pick up. If you would like a reasonable introduction to the language please refer to tutorial listed in references.

Getting Closer to 80% Knowledge of SNMP

There are few things we glossed over in the first parts of the article because they were boring, but let's get them out of the way. This should bring you a bit closer, if not all the way, to knowing 80% SNMP.

First, in order to get the SNMP daemon to run upon startup you will need to modify the plist for:

/System/Library/LaunchDaemons/org.net–snmp.snmpd.plist

And make it look as follows:

Listing 2: /System/Library/LaunchDaemons/org.net–snmp.snmpd.plist

<?xml version="1.0" encoding="UTF–8"?>
<!DOCTYPE plist PUBLIC "–//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList–1.0.dtd">
<plist version="1.0">
<dict>
    <key>Disabled</key>
    <false/>
    <key>KeepAlive</key>
    <true/>
    <key>Label</key>
    <string>org.net–snmp.snmpd</string>
    <key>OnDemand</key>
    <false/>
    <key>Program</key>
    <string>/usr/sbin/snmpd</string>
    <key>ProgramArguments</key>
    <array>
        <string>snmpd</string>
        <string>–f</string>
    </array>
    <key>RunAtLoad</key>
    <true/>
    <key>ServiceIPC</key>
    <false/>
</dict>
</plist>

What this does is to tell the snmpd daemon to start up upon reboot and then stay alive. Arguably, this could be set to OnDemand instead, but this is one of the newer features of launchd and it hasn't been fully tested with snmp much.

Next, we need to talk a little bit about the snmpwalk command, OIDs, and MIBs. The very short explanation of an OID is that it is a string of numbers, with a human readable name, that lives inside of a MIB file. This whole system is a hierarchically–assigned namespace for the SNMP protocol to keep track of what an agent can provide when you query it.

Let's take a look at how the snmpwalk command is used to shed some light on this. Here is a basic snmp query of our local machine again using the OID sysdescr:

mini# snmpwalk –v 2c –c public localhost sysdescr
SNMPv2–MIB::sysDescr.0 = STRING: Darwin mini.local 9.2.2
Darwin Kernel Version 9.2.2: Tue Mar 4 21:17:34 PST 2008;
root:xnu–1228.4.31~1/RELEASE_I386 i386

By examining the command we ran you may notice the syntax is identical to the syntax of the snmpdf command, with the exception of the word sysdescr. This word is an OID, and it lives in the MIB–2 file. You can refer to the references for an actual breakdown of this OID and where it lives in the larger MIB structure if you are curious, but if just want to use SNMP you only need to know important OID's to query.

As our last mention of this topic, it is important to know that the snmpwalk command retrieves all of the OIDs that are listed below it in the hierarchical structure. To get the superset of the OID that holds sysdescr, we could type in this:

mini# snmpwalk –v 2c –c public localhost system
SNMPv2–MIB::sysDescr.0 = STRING: Darwin mini.local 9.2.2 Darwin Kernel
Version 9.2.2: Tue Mar 4 21:17:34 PST 2008; root:xnu–1228.4.31~1/RELEASE_I386 i386
SNMPv2–MIB::sysObjectID.0 = OID: NET–SNMP–MIB::netSnmpAgentOIDs.255 DISMAN–EVENT–MIB::sysUpTimeInstance = Timeticks: (1892500) 5:15:25.00 [OUTPUT SHORTENED FOR SPACE]

This gives us not only the sysdescr OID, but many others as well. For writing a monitoring system you almost never query something in this manner though, you typically just find out the important lower level OIDs, and then write some scripts using them.

This is all we are going to cover about the very nitty gritty details about how SNMP is implemented. If you are curious, you do a Google search for these terms and you should get all of the reference material you need.

A Final Trick: Extending the Net–SNMP Agent with Python and OSAScript

At this point we have the basics covered for configuring, configuring, scripting, and querying SNMP on the Mac. As a final trick, we are going to extend the Net–SNMP agent on our Mac and do something fun.

We are going to write a script that tells iTunes to start up and play something on "Party Shuffle" if we query a specific OID. In order to do that we need double our already massive configuration file, or add one more line. Let's make our /etc/snmp/snmpd.conf look like this:

rocommunity public
exec PlayItunes /usr/bin/python /tmp/playitunes.py

Next, we need to write that script so that it executes when we run. Here is what that script looks like:

Listing 3: playitunes.py

#!/usr/bin/env python
from subprocess import Popen, PIPE
cmd = """osascript<<END
tell application "iTunes"
play playlist "Party Shuffle"
end tell
END"""
def play_iTunes():
     Popen(cmd, shell=True, stdout=PIPE, stderr=PIPE)
     print "Started ITunes Party Shuffle"
play_iTunes()

Next we need to send the snmpd daemon a HUP to tell it to reread it's configuration file. Do a ps –ef | grep snmpd and then give the PID a kill –1. This is what it looks like when I do it:

mini# ps –ef | grep snmpd                                   
    0    26     1   0   0:01.82 ??         0:02.26 snmpd –f
    0   515   355   0   0:00.00 ttys000    0:00.00 grep snmpd
mini# kill –1 26

Finally, we are ready to "query" our machine and make it trigger our custom script. There is a standard OID that responds to custom scripts and it is used in the snmpwalk call below:

mini# snmpwalk –v 2c –c public localhost .1.3.6.1.4.1.2021.8
UCD–SNMP–MIB::extIndex.1 = INTEGER: 1
UCD–SNMP–MIB::extNames.1 = STRING: PlayItunes
UCD–SNMP–MIB::extCommand.1 = STRING: /usr/bin/python
UCD–SNMP–MIB::extResult.1 = INTEGER: 0
UCD–SNMP–MIB::extOutput.1 = STRING: Started ITunes Party Shuffle
UCD–SNMP–MIB::extErrFix.1 = INTEGER: noError(0)
UCD–SNMP–MIB::extErrFixCmd.1 = STRING: 

When we run the command, and see the output, we can see the print statement that we included with our script. If we actually take a look at our machine, we will notice that ITunes indeed pops up, and starts on Party Shuffle.

This is a slightly silly example for extending a Net–SNMP agent, but if you are a home power user, maybe it isn't. Who doesn't want to brag and tell your spouse, you turned on iTunes via SNMP to the downstairs computer?

Conclusion

We covered a lot of ground in this article, and hopefully got you to understand how SNMP might be useful in your computing ecosystem. There were quite a few things we glossed over, mostly the boring stuff, but they aren't entirely necessary for you to start hacking around with SNMP.

We covered about 80% of the most important material, and I will leave the remaining 20% for you to pick up on your own. If you do plan on learning more about SNMP theory it would make sense to read a book or two on the subject or read a few articles on Wikipedia on SNMP. For casual use of SNMP though, you have more than your fair share to explore with the ideas from this article.

One final word of caution though is to make sure that you only use SNMP version 1, and 2 if you are communicating on a secure LAN behind a firewall. There have been some high profile break–ins of machines that have occurred by using insecure versions of SNMP over the internet. If you do need to remotely query or a control a machine across a the internet you must use SNMP v3 to be secure.

Bibliography and References

Noah Gift. "Using Net–SNMP and IPython". IBM Developerworks,AIX and Unix, http://www.ibm.com/developerworks/aix/library/au-netsnmpnipython/.

Noah Gift. "Python and Applescript". O'Reilly. http://www.oreilly.com/pub/a/mac/2007/05/08/using-python-and-applescript-to-get-the-most-out-of-your-mac.html

Noah Gift and Jeremy Jones. Python For Unix and Linux Systems Administration. O'Reilly . ISBN: 0596515820

Guido van Rossum. Python Tutorial: http://docs.python.org/tut.

OID Value/Description of sysdescr: http://www.alvestrand.no/objectid/1.3.6.1.2.1.1.1.html


Noah Gift has been a Mac user since his family bought a Macintosh Performa 6300 in 1992, and started connected to BBS networks immediately and then eventually the World Wide Web in 1993 when it become open to the public. He is the co-author of Python For Unix and Linux Systems Administration by OReilly. Noah has a couple of decades of experience in the Television and Film industry starting off as an editor for ABC Network News as a teenager. He contributed to the first feature animated film for Disney Feature Animation and Sony Imageworks. He also had stints at Turner Studios and Caltech, where he worked for the Nobel Prize-winning President as a Mac expert. He has a Masters degree in CIS, and is LPI and ACSA certified. He also organizes PyAtl, the local Python programmers user group in Atlanta. Currently Noah is consultant, writer and speaker, specializing in OS X/Unix, Linux, Python, and Web development for his company, Giftcs, www.giftcs.com. Many of his projects and writing are available at www.noahgift.com. He can be contacted at noah.gift@giftcs.com

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Adobe Dreamweaver CC 2018 18.0.0.10136 -...
Dreamweaver CC 2018 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Dreamweaver customer). Adobe Dreamweaver CC 2018 allows you to... Read more
Adobe Lightroom 20170919-1412-ccb76bd] -...
Adobe Lightroom is available as part of Adobe Creative Cloud for as little as $9.99/month bundled with Photoshop CC as part of the photography package. Lightroom 6 is also available for purchase as a... Read more
Adobe Photoshop CC 2018 19.0.0 - Profess...
Photoshop CC 2018 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Photoshop customer). Adobe Photoshop CC 2018, the industry standard... Read more
Adobe Muse CC 2017 2018.0.0 - Design and...
Muse CC 2018 is available as part of Adobe Creative Cloud for as little as $14.99/month (or $9.99/month if you're a previous Muse customer). Adobe Muse 2018 enables designers to create websites as... Read more
Adobe Animate CC 2017 18.0.0.107 - Anima...
Animate CC 2018 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Flash Professional customer). Animate CC 2018 (was Flash CC) lets you... Read more
Hopper Disassembler 4.3.0- - Binary disa...
Hopper Disassembler is a binary disassembler, decompiler, and debugger for 32- and 64-bit executables. It will let you disassemble any binary you want, and provide you all the information about its... Read more
GraphicConverter 10.5.1 - $39.95
GraphicConverter is an all-purpose image-editing program that can import 200 different graphic-based formats, edit the image, and export it to any of 80 available file formats. The high-end editing... Read more
Delicious Library 3.7 - Import, browse a...
Delicious Library allows you to import, browse, and share all your books, movies, music, and video games with Delicious Library. Run your very own library from your home or office using our... Read more
Duet 1.6.7.0 - Use your iPad as an exter...
Duet is the first app that allows you to use your iDevice as an extra display for your Mac using the Lightning or 30-pin cable. Note: This app requires a $14.99 iOS companion app. Version 1.6.7.0:... Read more
iExplorer 4.1.5 - View and transfer file...
iExplorer is an iPhone browser for Mac lets you view the files on your iOS device. By using a drag and drop interface, you can quickly copy files and folders between your Mac and your iPhone or... Read more

Wheels of Aurelia (Games)
Wheels of Aurelia 1.0.1 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.1 (iTunes) Description: | Read more »
Halcyon 6: Starbase Commander guide - ti...
Halcyon 6 is a well-loved indie RPG with stellar tactical combat and some pretty good writing, too. It's now landed on the App Store, so mobile fans, if you're itching for a good intergalactic adventure, here's your game. Being a strategy RPG, the... | Read more »
Game of Thrones: Conquest guide - how to...
Fans of base building games might be excited to know that yet another entry in the genre has materialized - Game of Thrones: Conquest. Yes, you can now join the many kingdoms of the famed book series, or create your own, as you try to conquer... | Read more »
Halcyon 6: Starbase Commander (Games)
Halcyon 6: Starbase Commander 1.4.2.0 Device: iOS Universal Category: Games Price: $6.99, Version: 1.4.2.0 (iTunes) Description: An epic space strategy RPG with base building, deep tactical combat, crew management, alien diplomacy,... | Read more »
Legacy of Discord celebrates its 1 year...
It’s been a thrilling first year for fans of Legacy of Discord, the stunning PvP dungeon-crawling ARPG from YOOZOO Games, and now it’s time to celebrate the game’s first anniversary. The developers are amping up the festivities with some exciting... | Read more »
3 reasons to play Thunder Armada - the n...
The bygone days of the Battleship board game might have past, but naval combat simulators still find an audience on mobile. Thunder Armada is Chinese developer Chyogames latest entry into the genre, drawing inspiration from the explosive exchanges... | Read more »
Experience a full 3D fantasy MMORPG, as...
Those hoping to sink their teeth into a meaty hack and slash RPG that encourages you to fight with others might want to check out EZFun’s new Eternity Guardians. Available to download for iOS and Android, Eternity Guardians is an MMORPG that lets... | Read more »
Warhammer Quest 2 (Games)
Warhammer Quest 2 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Dungeon adventures in the Warhammer World are back! | Read more »
4 of the best Halloween updates for mobi...
Halloween is certainly one of our favorite times for mobile game updates. Many popular titles celebrate this spooky season with fun festivities that can stretch from one week to even the whole month. As we draw closer and closer to Halloween, we'... | Read more »
Fire Rides guide - how to swing to succe...
It's another day, which means another Voodoo game has come to glue our hands to our mobile phones. Yes, it's been an especially prolific month for this particular mobile publisher, but we're certainly not complaining. Fire Rides is yet another... | Read more »

Price Scanner via MacPrices.net

Apple restocks full line of refurbished 13″ M...
Apple has restocked a full line of Apple Certified Refurbished 2017 13″ MacBook Pros for $200-$300 off MSRP. A standard Apple one-year warranty is included with each MacBook, and shipping is free.... Read more
13″ 3.1GHz/256GB MacBook Pro on sale for $167...
Amazon has the 2017 13″ 3.1GHz/256GB Space Gray MacBook Pro on sale today for $121 off MSRP including free shipping: – 13″ 3.1GHz/256GB Space Gray MacBook Pro (MPXV2LL/A): $1678 $121 off MSRP Keep an... Read more
13″ MacBook Pros on sale for up to $120 off M...
B&H Photo has 2017 13″ MacBook Pros in stock today and on sale for up to $120 off MSRP, each including free shipping plus NY & NJ sales tax only: – 13-inch 2.3GHz/128GB Space Gray MacBook... Read more
15″ MacBook Pros on sale for up to $200 off M...
B&H Photo has 15″ MacBook Pros on sale for up to $200 off MSRP. Shipping is free, and B&H charges sales tax in NY & NJ only: – 15″ 2.8GHz MacBook Pro Space Gray (MPTR2LL/A): $2249, $150... Read more
Roundup of Apple Certified Refurbished iMacs,...
Apple has a full line of Certified Refurbished 2017 21″ and 27″ iMacs available starting at $1019 and ranging up to $350 off original MSRP. Apple’s one-year warranty is standard, and shipping is free... Read more
Sale! 27″ 3.8GHz 5K iMac for $2098, save $201...
Amazon has the 27″ 3.8GHz 5K iMac (MNED2LL/A) on sale today for $2098 including free shipping. Their price is $201 off MSRP, and it’s the lowest price available for this model (Apple’s $1949... Read more
Sale! 10″ Apple WiFi iPad Pros for up to $100...
B&H Photo has 10.5″ WiFi iPad Pros in stock today and on sale for $50-$100 off MSRP. Each iPad includes free shipping, and B&H charges sales tax in NY & NJ only: – 10.5″ 64GB iPad Pro: $... Read more
Apple iMacs on sale for up to $130 off MSRP w...
B&H Photo has 21-inch and 27-inch iMacs in stock and on sale for up to $130 off MSRP including free shipping. B&H charges sales tax in NY & NJ only: – 27″ 3.8GHz iMac (MNED2LL/A): $2179 $... Read more
2017 3.5GHz 6-Core Mac Pro on sale for $2799,...
B&H Photo has the 2017 3.5GHz 6-Core Mac Pro (MD878LL/A) on sale today for $2799 including free shipping plus NY & NJ sales tax only . Their price is $200 off MSRP. Read more
12″ 1.2GHz Space Gray MacBook on sale for $11...
Amazon has the 2017 12″ 1.2GHz Space Gray Retina MacBook on sale for $100 off MSRP. Shipping is free: 12″ 1.2GHz Space Gray MacBook: $1199.99 $100 off MSRP Read more

Jobs Board

Commerce Engineer, *Apple* Media Products -...
Commerce Engineer, Apple Media Products (New York City) Job Number: 113028813New York City, New York, United StatesPosted: Sep. 20, 2017Weekly Hours: 40.00 Job Read more
US- *Apple* Store Leader Program - Apple (Un...
US- Apple Store Leader Program Job Number: VariousUnited StatesPosted: Oct. 19, 2017Retail Store Job Summary Learn and grow as you explore the art of leadership at Read more
Product Manager - *Apple* Pay on the *Appl...
Job Summary Apple is looking for a talented product manager to drive the expansion of Apple Pay on the Apple Online Store. This position includes a unique Read more
*Apple* Retail - Multiple Positions - Farmin...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Frameworks Engineer, *Apple* Watch - Apple...
Job Summary Join the team that is shaping the future of software development for Apple Watch! As a software engineer on the Apple Watch Frameworks team you will Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.