DNS check fails on working DNS server

Problem

I'm running Nagios on Solaris 9 from the CSW (Blastwave) repository:

• nagios 2.10,REV=2007.11.19
• nagiosp 1.4.11,REV=2008.01.05

I've seen this in two circumstances:

• I had a working AD server; I re-ip'd it to another subnet. The check-dns worked correctly in the old location; in the new location, it claims the DNS server is not working.
• I had a Solaris system running a DNS server. I created two secondary interfaces (to catch DNS requests sent to the old location of the AD server, above). check-dns claims that the DNS server on the secondary interfaces do not work, while the DNS server on the main interface does work.

# /opt/csw/libexec/nagios-plugins/check_dns -H google.ca dns-ph-10
Domain google.ca was not found by the server

Solution

Check-dns is running nslookup. Nslookup fails when the DNS server has no reverse-PTR record.

# /opt/csw/libexec/nagios-plugins/check_dns -v -H google.ca dns-ph-10
/usr/sbin/nslookup google.ca dns-ph-10
*** Can't find server name for address 10.0.0.10: Non-existent host/domain
Domain google.ca was not found by the server

Add a reverse-PTR entry, and check_dns works (again).