Monitoring wireless devices via ping
Rob Nelson
rob at capband.net
Fri Aug 8 20:44:22 CEST 2003
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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