DNS down and false alerts...

[Russell Adams]

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

PGP Key ID:     0x1160DCB3           http://www.adamsinfoserv.com/

Fingerprint:    1723 D8CA 4280 1EC9 557F  66E8 1154 E018 1160 DCB3

------------------------------------------------------------------------------
Crystal Reports - New Free Runtime and 30 Day Trial
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
_______________________________________________
Nagios-users mailing list
Nagios-users at lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/nagios-users
::: Please include Nagios version, plugin version (-v) and OS when reporting any issue. 
::: Messages without supporting info will risk being sent to /dev/null