Monitoring a Linksys WAG200G using SNMP

I have been using a Linksys WAG200G as a wireless access point since December 2007. I’m not using it for my broadband connection as I have a separate firewall and router already on my network. It has been running reliably without any problems since installed and it occurred to me that it had been some time since I had used the device’s administration page or reviewed Cisco’s patch history for it.

Using the web interface, the installed firmware was shown to be version 1.0.9, which was some way behind the current 1.1.9 release. I couldn’t find the release notes for any versions prior to 1.1.5 so I decided to upgrade the firmware to be certain that any known vulnerabilities had been patched.

After exploring the device’s web interface, I remembered that the little router supported SNMP. I didn’t have a NMS when it was installed so I had left this feature unconfigured. Now that I have a Nagios console it was time to activate the SNMP management. I set the device name to the same name that it’s IP resolves to in my DNS (wap101). I then set the monitoring IP address and trap target address to that of my NMS. Finally, I set the read community to public, and the write community to private.

From a command prompt on my NMS, I dumped a list of the management functions supported by the WAG200G using this command…

snmpwalk -v1 -c public -m ALL .1

My Linksys uses for it’s Ethernet interface. Change it to your device’s IP address if you are going to try it yourself. Redirecting the output to a file is useful for future reference.

A sample output of snmpwalk looks like this

IF-MIB::ifInErrors.1 = Counter32: 0
IF-MIB::ifInErrors.2 = Counter32: 0
IF-MIB::ifInErrors.3 = Counter32: 0
IF-MIB::ifInErrors.4 = Counter32: 0
IF-MIB::ifInErrors.5 = Counter32: 0

My WAG200G is only used as a WLAN access point, so I apologise now for not covering anything to do with monitoring ADSL or anything other than the Ethernet and WLAN interfaces in the Host and Service Definition file for my WAG200G. If you want to monitor more, just pick the relevant items required from the MIBs reported by snmpwalk and add them to your Nagios services. Think about the outputs and what conditions they need for alerts if any. Most of mine only need to alert if the result increases from zero. This is the list of services I am only interested in monitoring:-

  • PING
  • Uptime
  • eth0 IN Discarded Packets
  • eth0 IN Errors
  • eth0 IN Unknown Protocols
  • eth0 OUT Discarded Packets
  • eth0 OUT Errors
  • eth0 Operational Status
  • wlan0 IN Discarded Packets
  • wlan0 IN Errors
  • wlan0 IN Unknown Protocols
  • wlan0 OUT Discarded Packets
  • wlan0 OUT Errors
  • wlan0 Operational Status

I found that Nagios doesn’t like non-unique service descriptions, which is why my descriptions take the form shown above. Click here to view my Host and Services Definitions for the WAG200G.

The host definition inherits from the generic-switch template and looks like this…

# Define the switch that we'll be monitoring
define host{
use generic-switch ; Inherit default values from a template
host_name wap101 ; The name we're giving to this switch
alias Linksys WAG200G ; A longer name associated with the switch
address ; IP address of the switch
hostgroups switches ; Host groups this switch is associated with

Each service inherits from the generic-service template and looks something like this…

# Monitor Port 4 (wlan0) number of errors in via SNMP
define service{
use generic-service ; Inherit values from a template
host_name wap101
service_description wlan0 IN Errors
check_command check_snmp!-C public -o ifInErrors.4 -c 0 -m IF-MIB

I used the documentation on check_snmp to prevent critical warnings for zero values (-c 0). In time, if any of my services start seeing errors I can change them to use a warning range and a critical range instead.

My Ubuntu 9.10 package install of Nagios was missing the command snmp_check. I added the following code to the bottom of my /etc/nagios-plugins/config/snmp.cfg to get SNMP working as the vital command was missing for some reason.

define command{
command_name check_snmp
command_line $USER1$/check_snmp -H $HOSTADDRESS$ $ARG1$

One Response to “Monitoring a Linksys WAG200G using SNMP”

  1. Mariusz says:

    Super article, everything works OK

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