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Mail and dns

The problem here is a DNS problem with resolving the IP address of mail.yourdomain.com which is both a local machine and a machine on the Internet.

Under some restricted conditions that can work - a local DNS server can return the IP of an internal machine to local clients and let the rest of the world see the internet address.

It's when the local machines want both that it gets harder. A router could send specific ports to specific places but that gets complicated fast.

From: Mike Kenyon <mike@davidaustinroses.co.uk>
Newsgroups: comp.unix.sco.misc
Subject: Choice of DNS domain for dial-up LAN
Date: Thu, 27 Jul 2000 13:26:56 +0100
Message-ID: <39802A90.9C7994F@davidaustinroses.co.uk> 

Examples of DNS setups often quote the domain as "yourdomain.com" and then
show you how to set up all the fun that is DNS.

So lets run with this and see what happens:

Company Yourdomain is a small outfit with a dial-up link to an ISP. A third
party is hosting the website at www.yourdomain.com. The local system consists
of a LAN with an ISDN router and a SCO box on it, on which will reside the
mail and DNS. The local domain is set to yourdomain.com in the DNS, and PC's
use the SCO box for DNS and the ISDN router for a gateway. The SCO box's name
is sco.yourdomain.com. There is the possibility that the mail server may move
to a new machine so the sysadmin created a CNAME record so that
mail.yourdomain.com points to sco.yourdomain.com. The arbitary PC email
clients point to mail.yourdomain.com for both POP3/IMAP and SMTP.
Www.yourdomain.com is also in the DNS as an A record to point to the external
web server.

It works. Everybody's happy.

One day, the third party that hosts the web and does email forwarding decides
that Yourdomain can manage their own mail, and says that if they connect to
mail.yourdomain.com:12345, and login then they can do this.

DISASTER!

Yourdomain's sysadmin is faced with the following options:
a) Renamed the whole domain to something that wouldn't exist on the Internet
to avoid this problem ever happening again, which then causes problems if they
buy a leased-line at some stage in the future.
b) Rename the mail CNAME record which requires almost as much work as all the
PC mail clients will require tweaking. This runs the risk of the same
happening again.

Any other suggestions, folks?
-- 
Mike Kenyon <mike@davidaustinroses.co.uk>
I.T. Manager for David Austin Roses Ltd

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