check_icmp oddness

[Andreas Ericsson]

Israel Brewster wrote:
> I am showing some odd behavior with the check_icmp plugin. I am using  
> it to monitor a number of stations around the state, and for the most  
> part it appears to be working properly. However, with four of these  
> stations, check_icmp is showing consistently 60% loss-even though the  
> check_ping plugin shows them as fine, and just running a straight  
> ping on these sites also shows no packet loss. The only difference  
> between these sites and other sites we are monitoring is that they  
> are running linksys RV082 routers, while other sites are running  
> different model linksys routers. Would anyone be able to explain why  
> I am seeing this sort of behavior? Thanks.
> 

Perhaps I can shed some light on this.

check_icmp is designed to spew out packets very rapidly, while at the
same time trying to be polite to the target it's checking. It does this
by the simple expedient of waiting a, by default, very short maximum
time between two packets which gets even shorter if the targeted host
happens to answer to one of the probes being directed at it. When the
targeted host *does* answer, check_icmp decides it can shorten the default
interval and immediately send another probe, as the host it's targeting
apparently isn't suffering from congestion.

For *most* operating systems, this works quite nicely, and check_icmp
is capable of deciding the latency and loss very quickly.

Other operating systems thinks check_icmp sends packets too quickly and
drops a few of the probes. That's probably what you're seeing. The way
around it is to add the '-i' option with a suitable value, such as 0.3s,
which will force a minimum interval of 0.3 seconds between each packet.

Good luck

-- 
Andreas Ericsson                   andreas.ericsson at op5.se
OP5 AB                             www.op5.se
Tel: +46 8-230225                  Fax: +46 8-230231

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