Can I monitor a host without defining any services for it?

[Thomas Guyot-Sionnest]

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On 07/08/08 09:40 PM, Ian Masters wrote:
> Thomas
> 
> Thanks for your reply.
> 
>> You can use check_dummy for the service... In Nagios 3 with regularly
>> scheduled hosts checks it should work.
> 
> Well, at the moment I only have about 20 hosts being tested, and even
> though I have check_interval set to 1 in the host definition, looking at
> the scheduling queue, I can see that host checks are only occurring
> every 80 seconds. When I've added about 200 more hosts I'm wondering if
> this time lag will increase.
> 
> If that is the case, then I guess I will have to use check_ping after all.

I'm not sure what would cause that... AFAIK in Nagios 3 host checks end
up in the same queue as service checks, so the only explaination I could
fins would be:

1. Your interval_length is larger than the default of 60
2. check_result_reaper_frequency is too large (will delay the results,
and therefore delay the time at which nagios will re-schedule the checks)
3. Various commands (notifications, event handlers, performance data)
slowing down nagios (those are serialized just like host checks in
Nagios 2 - see below)

>> In Nagios 2 hosts check were normally on-demand so it wouldn't work if
>> your service always returns OK.
> 
> Interesting. I never used Nagios 2.

The logic in Nagios 2 is to check host only when a service fail, and it
does not support assync host checks (only service checks are assync).
The problem with it is that the host checks are serialized, so it adds
an artificial limit at which they can be performed (the time it takes to
detect a host down). This was an annoying bottleneck on huge setups.

> According to check_dummy -h:
> This plugin will simply return the state corresponding to the numeric
> value of the <state> argument with optional text
> 
> Am I right in thinking that this does not actually perform a test? It
> just returns a state (OK, Warning etc) depending on the argument you
> give to check_dummy. Is that correct?

Right. I mainly use it to make passive check with freshness checking
send an alert when the check becomes stale, and to reset volatile
services automatically.

> If that is the case, is there really any point in using it for my
> purposes. Obviously it prevents the Nagios warnings, but other than that
> is it really serving any purpose?

If it's only to avoid a warning, then you should just live with it. I
never tested myself so I assumed it wasn't working at all.

- --
Thomas
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