[c-nsp] Strange corrupt DNS Cache in IOS

[Sascha E. Pollok]

Hello networking fellows!

We are trying to find the cause of a corrupt local DNS cache of a Cisco 
1803 running 15.1(4)M8 (also appeared on 12.4something - 15.1 ist just a 
desperate attempt of solving).

The router acts as a local DNS resolver for locally connected clients 
using "ip dns server".

Every now and then it seems to break locally cached IPv4 A-RRs like this:

Router#show hosts
test.fqdn.fqdn           None  (temp, OK)  0   IP    0.0.0.5  <---

This seems to happen for hosts that also have an AAAA RR. To us it looks 
like it mixes AAAA and A records as the IPv6 address for this host is 
[...]::5. This happens with other hosts too.

The host is sometimes first seen correctly with an "IP" and "IPv6" entry 
in the cache but then changes to the broken "IP" RR while sometimes even 
keeping the correct IPv6 entry. It never happens to the IPv6 address.

Debugging "debugging domain" and "debugging domain replies" didnt give a 
clue.

Thanks for any hints!
Sascha