RE: Reverse DNS Lookup...

You're on the right track, but not quite right.

MX is a mail exchanger record.  In other words, it points to a host
that can receive mail for a given domain, and really is beside the
point here, as it only affects e-mail.

The "A" record, is "forward" DNS, in other words its used to convert
a domain name to an IP address.

The "PTR" record is "reverse" DNS, which is used when converting an
IP address into a name.

Also, when setting up "reverse" (IP to name) DNS, you have to lay out
the domain in a rather unusual way.   If your IP is '192.168.0.1' then
the reverse DNS would be set up as "1.0.168.192.in-addr.arpa"  note
that the numbers are in the reverse order from those of the IP address.

If you're setting up DNS for the first time, I strongly suggest reading
the book called "DNS and BIND" which is available from O'Reilly.  It's
designed for the UNIX name server (i.e. BIND, or Berkeley Internet Name
Daemon) but is the "bible" for ISPs running DNS.   The concepts that it
explains apply to every DNS server, be it UNIX, NT or OS/400.



On Wed, 5 Dec 2001, Jim Langston wrote:

> DNS entries for machines have an MX and an A record.
>
> The MX stands for Mail Exchange.  Not sure what the A
> records stands for.  One converts from the name to the
> IP address, once converts from the IP address to the
> name.
>
> If you can not do a reverse DNS lookup on specific
> hosts, I would suspect they are missing one of these
> records (I believe the A records is IP to Name).
>
> Try other IP addresses that you know have both records.
> Here's a good example.  In windows go to a DOS prompt.
> type:
> ping www.midrage.com
> that tells us the IP address is 63.167.147.106.
> then type:
> ping -a 63.167.147.106
> No name comes back.  Looks like David doesn't have an
> 'A' record for his DNS entry.
>
> If we do the same for www.microsoft.com:
> ping www.microsoft.com (207.46.197.101)
> ping -a 207.46.197.101 (microsoft.com)
>
> Microsoft does have an 'A' record associated with it's
> DNS entry.
>
> Sorry for showing this in PC commands, not positive what
> the AS/400 commands are (and my AS/400 isn't connected to
> the internet anyway).
>
> Regards,
>
> Jim Langston
>
> -----Original Message-----
> From: Scott Klement [mailto:klemscot@klements.com]
> Sent: Tuesday, December 04, 2001 3:53 PM
> To: rpg400-l@midrange.com
> Subject: RE: Reverse DNS Lookup...
>
> I can't do reverse lookup on your "www.ctlaltdel.org" either.  Not with
> the program, not with the UNIX "host" command, not with the NSLOOKUP
> command...   it does not appear to have reverse DNS associated with it.
>
> The 216.115.108.245 does work from my AS/400, however.
>
> The other example (the one that you originally posted) is one that I would
> never use in production :)   It may work some of the time, but it surely
> will also fail some of the time.   Really.  Use the one that I posted, or
> another one that's written correctly :)
>
>
> On Tue, 4 Dec 2001, Andrew Borts wrote:
>
> > OK - Cut & pasted your results. Compiled (it compiled on the first time
> > - stop showing off!) and I still get blanks returned.  I even tried
> > using the example that Peter gave of 216.115.108.245 to do my lookup
> > instead of my home server (www.ctlaltdel.org ) and STILL got blanks.
> >
> > I'm leaning toward setting up the DNS server on my AS/400 @ home, and
> > trying the code there...
> >
> >
> > Andrew Borts / Webmaster