DNS down and false alerts...

[Martin Melin]

I don't know if I'm misreading the OP, but if the plugins start timing out
on only the boxes whose primary DNS is being rebooted, would adding a
caching DNS server to the Nagios box really make a difference?

I think the root cause to these timeouts is that the Nagios plugin timeout
is happening before the connection to the primary DNS on the target machine
has a chance to time out and then connect to the secondary DNS.

The correct course of action to resolve this would be to either make sure
that the DNS connection on the target machines fail quicker, or that
Nagios/the plugin waits longer for a result from the check. The DNS failover
is working as designed here but you're not giving it enough time to kick in.

On Tue, Jun 9, 2009 at 5:37 PM, Russell Adams <RLAdams at adamsinfoserv.com>wrote:

> Really the best choice is to using caching DNS on the Nagios
> server. I'd recommend dnsmasq, it just does caching locally without
> needing to do big zone transfers. It has low overhead and simple
> configuration as a result.
>
> Enjoy.
>
> On Tue, Jun 09, 2009 at 11:19:20AM -0400, Andrew Davis wrote:
> > I've observed an interesting issue with Nagios. Our environment is a mix
> > of UNIX, Linux, Apple, and Windows. The core of the network is Active
> > Directory including two AD servers that are both our primary, internal
> > DNS servers. All non-Windows systems have a resolv.conf that looks like:
> >
> >    *nameserver 10.1.1.13
> >    nameserver 10.1.1.14
> >    domain int.our.domain
> >    search int.our.domain*
> >
> > About half of the servers have the nameserver entries inverted (ie: .14
> > first, .13 second).
> >
> > The issue is that anytime one of the nameservers is rebooted (at least
> > once a month if staying current on patches thanks to Black Tuesdays),
> > whichever hosts have that nameserver listed first in its resolv.conf
> > start throwing the following errors:
> >
> >    *CRITICAL - Plugin timed out while executing system call.*
> >
> > This occurs for multiple tests for each host. Obviously, there's a name
> > resolution correlation here. If the nameserver with .13 is rebooted, all
> > hosts (about half of them) that list this IP first in their resolve.conf
> > then timeout for multiple tests. If the .14 server is rebooted, all the
> > other hosts timeout. Interestingly, none of the Windows clients issue
> > errors... only UNIX, Linux, and Mac's... only those with an
> > /etc/resolv.conf. The end result is a host of "false positives", but
> > more importantly it looks bad on availability reports and causes
> > phones/pagers to go ballistic with unneeded emails.
> >
> > I'm trying to find a solution and I can't find one that I like:
> >
> > Solution 1) is to cluster the DNS servers. We have lots of clusters
> > here. This isn't good, though, as you don't normally cluster DNS
> > servers... they're meant to be redundant for a reason... one fails and
> > it uses the next one.
> >
> > Solution 2) is to setup a service/host dependency. My thought would be
> > either a host dependency that says if either .13 or .14 are down, then
> > don't alert for any other host that uses them. Or a service to host
> > dependency... if the DNS service is down, then don't alert on any of
> > these dependent hosts. Honestly, I'm not sure if you can mix host and
> > service dependencies like this... plus... if the DNS server is actually
> > down, then the DNS service is down, so better to use a host dependency.
> > The problem is that now we're not alerting on any dependent hosts which
> > themselves could have a legitimate issue we want to know about. Plus,
> > what happens if the DNS server actually dies and take a few hours/days
> > to rebuild/restore? At this point, the dependent hosts aren't watched
> > for a very long time.
> >
> > Solution 3) is to setup a UNIX/Linux DNS server that slaves all zones
> > from the AD servers and have all UNIX/Linux/Apple clients query from
> > this server. This would work except that A) I need two of them to keep
> > redundancy and B) I've now added an extra layer of complication to
> > resolve an application (Nagios)... not exactly good practice.
> >
> > Solution 4) is to set the timeout value of a host querying a DNS server.
> > Perhaps adjust the client to timeout on the first listed nameserver
> > after only 10 seconds, then try the next one? Since most Nagios tests
> > have a minimum timeout value of 30 seconds, if the first DNS query timed
> > out after 10 seconds, it would go to the next one with, hopefully,
> > enough time to respond. The downside is having to adjust every single
> > server.
> >
> > Has anyone else seen this? Anyone else using Windows AD servers to
> > provide DNS for *nix servers?
> >
> > --
> >
> >
> >  A. Davis
> >  Email:     nccomp at gmail.com
> >
> >  "There is no limit to what a man can accomplish
> >   if he doesn't care who gets the credit." - Ronald Reagan
> >
>
> >
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>
> ------------------------------------------------------------------
> Russell Adams                            RLAdams at AdamsInfoServ.com
>
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>
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>
> ------------------------------------------------------------------------------
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> Check out the new simplified licensing option that enables unlimited
> royalty-free distribution of the report engine for externally facing
> server and web deployment.
> http://p.sf.net/sfu/businessobjects
> _______________________________________________
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