Monitoring wireless devices via ping

[Rob Nelson]

I am monitoring a variety of Wireless devices via ping, over a VPN:

[ Nagios ] -> PIX <- VPN tunnel -> PIX -> devices

When the devices lose power during storms or whatnot (we have the head end 
on UPS's but not every building, so 4 out of 70 devices are on UPS), they 
become unreachable from the Nagios monitoring server. However, from a local 
machine at the site, every device is reachable - and once pinged, are 
available over the VPN to Nagios. By using debugs, I am *sure* that the VPN 
sessions are not stale or dying - after all, I am able to use the VPN to 
get to our remote server.

I have some theories on this below, but has anyone seen anything similar to 
this? I'd really like to know why it happens.






So here's my theories....

The wireless devices, when you ping them after a reboot, NEVER respond on 
the first ping. My suspicion is that the wireless devices do not talk 
unless they're *used*. And of course, if a host on a remote network pings 
them first, they aren't going to talk at all. My support for this is that 
any access points supporting a user at the time of power loss, such as the 
central office in each site, continues to be reachable across the VPN, and 
the others come back slowly as people turn their machines on, reboot, come 
home from work, etc.

My workaround has been to do an "nmap -sP <ip>.1-105" on a 5 minute 
schedule on the server at each site. It's a PITA, but it's worth it if it 
keeps Nagios from throwing 70 errors every time the power goes out for 3 
seconds.

Rob Nelson
Network Administrator, Capitol Broadband
C: 919-369-1874
rob at capband.net 



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