Re: DNS Lookups

[web400@xxxxxxxxxxxxxxxx]

> 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.

An A record is an "address record".  Its the record that specifies the IP 
address that's associated with a domain name.

When that A record doesn't exist, (or when e-mail software tries to 
connect to port 25 on that address, and doesn't have an SMTP server 
running) then there has to be a different host that handles e-mail for the 
domain name.

This different host is called a "mail exchanger".  You look up the mail 
exchanger for a given domain name by looking up the MX records. There can 
be more than one MX record if there's more than one SMTP server that 
accepts mail for the domain.  (This can also provide redundancy in case 
there's a problem.)

It's very common for people to name their mail exchangers starting with 
the word "mail".  Why? Because it's a logical name to use.  Just as people 
often name their ftp servers "ftp.foo.com" and their web servers 
"www.foo.com".  It's just a logical thing to do.

However, it's certainly not required.  I can call my mail server 
mail.scottklement.com, or smtp.scottklement.com or mail1.scottklement.com 
or mail2.scottklement.com... all logical names...  or I can call it 
"stinky.scottklement.com" or some other name that's not so logical.  The 
fact that many people use "mail.xxx.com" doesn't mean that it's okay to 
assume that all mail servers will be called mail.whatever.com

Furthermore, I might not even be handling my own e-mail.  I might decide 
to have my ISP take care of scottklement.com for me.... so if my ISP is 
TDS Metrocom, the mail exchangers might be mail1.tds.net, mail2.tds.net... 
and they have nothing to do with my domain at all.

So the question is, how do you find out what the mail exchangers are 
called?   The answer is: you look up the MX records.