Optimizing the Presence Detection with SNMP

So now, as described in this post, I've got some rudimentary presence detection going, and can generate events when my wife and I (or rather, our phones) come and go. However, I felt like the ssh-based method I was using was a bit of a kludge, and I really wanted a better way to go about it.

At some point in the past few weeks, I read a bit about SNMP, or Simple Network Management Protocol. It's not really all that simple. From what I read, however, it looked like a pretty useful way to get information from a device, and I stumbled across a few posts mentioning that it could be extended, like this one.


Intrigued, I decided to try installing snmpd on my OpenWRT router, a process which wasn't trivial, since my router was already pretty crammed full of stuff. See <<this post>> for more on that effort.

Eventually, though, I was able to get it running, and took a closer look at "extend" directive.  I noticed that by default, it was running /usr/bin/cat, with the argument /proc/sys/fs/file_nr, basically dumping the number of open file handles. I manually ran the command and took note of the output.

Then, a quick Google search turned up this Windows binary for snmpwalk functionality. I ran it, searched through the output, and lo and behold, I saw a string that looked suspiciously like the output to the script.  Running the program with the -op and -os args allowed me to trim down the output to just the parts I cared about. This was definitely the results of the script. Getting excited, i changed the configuration of snmpd to call my own script, the cleverly named 'test.sh' and crossed my fingers.

--test.sh--
#/bin/sh
echo "Hello, World"

--Output of snmpwalk--
C:\work>snmpwalk -r:192.168.1.1 -os:1.3.6.1.4.1.2021.8.1 -op:1.3.6.1.4.1.2021.8.2
SnmpWalk v1.01 - Copyright (C) 2009 SnmpSoft Company
[ More useful network tools on http://www.snmpsoft.com ]
OID=.1.3.6.1.4.1.2021.8.1.1.1, Type=Integer, Value=1
OID=.1.3.6.1.4.1.2021.8.1.2.1, Type=OctetString, Value=filedescriptors
OID=.1.3.6.1.4.1.2021.8.1.3.1, Type=OctetString, Value=/mnt/sda1/test.sh /proc/sys/fs/file-nr
OID=.1.3.6.1.4.1.2021.8.1.100.1, Type=Integer, Value=0
OID=.1.3.6.1.4.1.2021.8.1.101.1, Type=OctetString, Value=Hello, World
OID=.1.3.6.1.4.1.2021.8.1.102.1, Type=Integer, Value=0
OID=.1.3.6.1.4.1.2021.8.1.103.1, Type=OctetString, Value=Total: 7

Success!

After that, I wrote a quick script to output presence information for the two MAC addresses I cared about separated by a comma, and sure enough, the result was right there in OID.1.3.6.1.4.1.2021.8.1.101.1. That presence script, if you're interested, looks like this (with MAC's obscured to protect the innocent):

--presence.sh--
#!/bin/sh
mac1=`iw wlan0 station dump | grep -c '00:00:00:00:00:00'`
mac2=`iw wlan0 station dump | grep -c '00:00:00:00:00:01'`
macsum=$((mac1 + mac2 * 2))
echo $mac1,$mac2
exit $macsum

In case you were wondering, I'm using the exit code as a quick and dirty bitmask for the two connected users I'm curious about.

Now it's just a matter of getting a python or perl library that talks SNMP, and integrating this back into my presence script should be a piece of cake. The best news is that when I run this command, as opposed to ssh, it returns immediately, without any appreciable lag, which makes me feel better about running it more often, and having a more dynamic presence detection routine.

So there you have it.  Using SNMP for something a little out of the ordinary.  I'm sure there are better ways to accomplish the same thing, and I may not actually end up with this method for my presence detection after all is said and done. I'd really like to switch to a more event-driven rather than a polled system (If you have any suggestions, I'd love to hear them!).