DNS down and false alerts...

[Andrew Davis]

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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