[Nagiosplug-devel] check_dns issue

James Mernin jmernin at tssg.org
Mon Aug 11 22:31:59 CEST 2008

My Configuration

I would like to use the check_dns plugin (that is included in
nagios-plugins-1.4.12) to verify both forward and reverse DNS lookups for
one of our domains. I am reliably informed that since we don't actually own
the IP space where our servers reside, we have only been given partial
control over the DNS setup. As a result, we get somewhat unusual responses
from a straightforward (reverse) nslookup on some of our IP addresses.


Let me elaborate: 


# nslookup XXX.XXX.XXX.XXX

Server:         AAA.BBB.CCC.DDD


Non-authoritative answer:

XXX.XXX.X.XXX.in-addr.arpa    canonical name =

XXX.XXX-XXX.XXX.X.XXX.in-addr.arpa   name = host.mydomain.com.


Authoritative answers can be found from:






The Problem

When I use the check_dns plugin to test this reverse lookup, it seems to
return a list of 2 comma-separated matches for the hostname, and this makes
it very messy to write a clean nagios command based around check_dns.
Looking at the source (check_dns.c) I see that on line 135, it is matching
any line of output from the nslookup response that contains the string "name
= " and appending this to a list of host names associated with this IP


Proposed Solution

I have worked around this problem by modifying check_dns.c as follows:


$ diff check_dns.old check_dns.c


< if ((temp_buffer = strstr (chld_out.line[i], "name = ")))


> if ((temp_buffer = strstr (chld_out.line[i], "name = ")) && (NULL ==
strstr (chld_out.line[i], "canonical name = ")))


This change tells the plugin to ignore any nslookup output containing the
string "canonical name = " but still matches those containing "name = ".
And, for me, it does what I need.



So, my question is this. Is the above change valid and should it be included
in the next release, or perhaps a command-line option added to cater for
such situations.


All comments welcome.


Many thanks,


James Mernin
Senior Software Engineer,

Telecommunications Software & Systems Group,

Waterford Institute of Technology,

Waterford, Ireland.


+353 (0)51 845694 | mailto:jmernin at tssg.org | http://www.tssg.org
<http://www.tssg.org/>  | http://www.wit.ie <http://www.wit.ie/> 


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