Understanding service checks

[Dirk H. Schulz]

Hi Arno,

Arno Lehmann schrieb:

> Hello,
>
> On 3/28/2006 8:01 PM, Dirk H. Schulz wrote:
>
>> Hi folks,
>>
>> today both our dns servers went out of function.
>
>
> Funny day, eh?

Ah, yes - oh, ah, no, definitely not. :-)

>
>> As a result every service check in nagios timed out after 10 sec. and 
>> was CRITICAL.
>>
>> EVERY one of several hundred!
>>
>> Since service check definitions depend on host definitions and host 
>> definitions contain ip addresses I would not have expected a dns 
>> outage to have any effect on nagios' service checks.
>>
>> Can anyone explain why there IS an effect? I would like to understand 
>> it more deeply.
>
>
> Rhis sounds like it depends only on your Nagios setup and your 
> network. For example, if you _only_ use IP adresses and not DNS in 
> your Nagios configuration, all your routers do _not_ need DNS, and all 
> your services do _not_ depend on DNS (think of verifying reverse 
> mapping for mail address verification, or SSL certificate checks, for 
> example, or nrpe or (x)inetd or TCP wrappers... lots of opportunities 
> to need DNS without being aware of it) having a DNS outage should not 
> affect service state.
>
> In other words: Since you experienced service failures, something 
> _must_ depend on either DNS or some outage not directly monitored 
> affected DNS as well as all other checks you run. Or it's all been a 
> great coincidence...

You are right. I have written out of only half reduced panic. Of course 
it need not be nagios service checks depending on dns but the services 
themselves. Since most checks are done via ssh and ssh communication can 
depend on mutual dns verification ...
Maybe I find the time to dig into it somewhat deeper.

Thanks for getting me back on the track!

Dirk


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