Re: DNS Lookups

[Rich Duzenbury <rduz-midrange@xxxxxxxxxxxxxxxxxxx>]

On Thu, 2006-01-19 at 13:24 -0500, Jon Paris wrote:
> Thanks to Aaron and Scott - it seems that PHP has the routines I need (now
> that I understand _what_ I need!) - but they are not part of the version
> supported on Windows - which means I have to use the PEAR functions instead
> and that is too much work for today since I have yet to install/use PEAR.
> 
> I'll see how I get on with Aaron's Java code for now.
> 
> Given Scott's explanation, I don't quite understand why gethost... xxx.xxx
> fails, but gethost... mail.xxx.xxx succeeds - but perhaps I shouldn't worry.

It's fairly simple.  The mail host name is not hardcoded, it's up to
each domain to specify the hostname.   One cannot assume that the
mailhost is called mail.foo.com, it might be smtp.foo.com or some such.
So, DNS provides a mechanism to ask the question - what is the name of
the mail server?  The answer could be given, or perhaps there isn't one,
and no answer will be the result.

Same for the domain itself.  There is no hardcoded server name set to
answer at foo.com.  It has to be set up to do so, and not everyone does.
I do have my servers set to respond to www.westernmidrange.com and the
westernmidrange.com names, but I had to go to the trouble of doing so.

When I set up machines on, I think in terms of machines and services.  

Every machine is set up with a name and IP.  Mine happened to be named
after pets.  So, I've got tinker, tiger, sambo, pippen, sparky and so
on.  

Then, I create aliases that match services to those machines, as in
www.westernmidrange.com, ftp.westernmidrange.com,
ns1.westernmidrange.com, ns2.westernmidrange.com.

Finally, I create some DNS records that tell the world that ns1 and ns2
are the name servers and which machine(s) are the mail server.  

So, if you want to know about the mailserver for westernmidrange.com,
you ask the DNS at westernmidrange.com for the mx (mail exchanger)
records.  What you actually find is that mail for westernmidrange.com is
handled by three servers ms10.spamsoap.com, mx20.spamsoap.com, and
mx30.spamsoap.com.

HTH

Regards,
Rich