reverse DNS resolution...

Eric F Crist ecrist at
Wed Oct 24 11:33:51 PDT 2007

Thanks to all for their help.  I was ignoring the forward DNS, and  
many things don't resolve reverse DNS unless there's a matching  
forward DNS.



Eric Crist

On Oct 23, 2007, at 12:00 PMOct 23, 2007, Oliver Fromme wrote:

> Eric F Crist wrote:
>> As I already stated, if I do a host 172.30.x.x, I get a the correct
>> reverse resolution.  dig works as well.  What isn't working is the
>> reverse resolution in certain command outputs, etc.
> Note that the DNS tools (host, nslookup, dig) use their
> own resolver code, not the one from FreeBSD's libc, like
> all other tools.  That might explain the difference.
> Make sure that you have configured /etc/nsswitch.conf
> and /etc/resolv.conf correctly.  Also note that /etc/hosts
> overrides DNS by default.
> You can use tcpdump to check if a reverse lookup request
> is sent to the DNS server when the failure occurs, and
> what the reply looks like.  E.g. let this command run in
> one terminal:
> # tcpdump -i tun0 -s 1500 -l -n -vvv udp port domain
> Add an -i option to specify the interface to listen on,
> if you have multiple interfaces (e.g. -i fxp0).
> Then run the command (w, irc client, whatever) in another
> terminal and watch the tcpdump output.  Oh by the way,
> I think the addresses in IRC are resolved by the servers,
> not by the clients, so you would have to run the tcpdump
> command on the IRC server (if it's an internal one to
> which you can login and have root access).
> Best regards
>    Oliver
> -- 
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Eric F Crist
Secure Computing Networks

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