How to check Internet access and/or DNS?

[Guy B. Purcell]

On Jan 23, 2006, at 23:56, Marc Haber wrote:

> some services, most notably DNS, depend on the Internet to be
> available. I'd like to express this in Nagios configuration. I'd
> probably need to have a service "Internet" for that where my DNS
> service definitions could depend on to avoid DNS being reported down
> in case of an internet outage.
>
> Any idea how to do this elegantly? I don't find the idea of pinging my
> ISP's router at the national Internet Exchange Point very appealing.

Configure the "parents" field in the host definition for the host  
running the service you're checking across the Internet.  It's parent  
should be your boarder router.  You'll need to make a host def. for  
that router, as well as a service to check on it (use the "dummy"  
service for that).  With such a setup, there'll be no real periodic  
check of the router, but it will be ping'd if your across-the- 
Internet service fails, and you'll get just one notification (either  
router down or remote service unavailable).

> On a second thought, how do I check DNS? I mean, even with the
> Internet down, my local DNS servers probably can correctly serve DNS
> records for my local network which they're authoritative for. So, does
> it make sense to check the DNS servers once for an external host (like
> www.nagios.org) and once for an internal host, resulting in two
> services being checked on the DNS server?
>
> What do you do regarding DNS server checks?

Well, you check the DNS with the check_dns plug-in, of course ;^)   
Seriously, though, it depends on your infrastructure.  Generally  
speaking, I suggest that you use the plug-in to monitor all of _your_  
nameservers and just assume that the only reason for the DNS outside  
of your organization to be down is an outage of your network,  
absolving you from having to monitor the DNS at large.  You can  
monitor your Internet connection itself in many ways--paid service  
from the Internet, free from your home network, free from within your  
org. looking out (with lots of variation thereof).  Pick what suits  
you best.

-Guy




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