NTMAIL - DNS Primer
Domain Name Server (DNS) is the name given to a program that converts the ASCII
address (e.g. net-shopper.co.uk) into its dotted-decimal equivalent
(e.g. 194.70.57.66). The DNS Database holds may different mappings from
address to dotted-decimal - the most common is CNAME and is the one that
most people think of as "DNS". CNAME addresses are the ones you use for
WWW and ftp. In general you will find one dotted-decimal address for a
given host name. For example, the CNAME record for www.net-shopper.co.uk is:
194.70.57.66 www.net-shopper.co.uk
There is a second type of mapping called Mail eXchange (MX) records which
are totally devoted to mail. A MX record for a particular address will
have several dotted-decimal address and priority. For example, to send email
to Internet Shopper Ltd, you need to use the address "net-shopper.co.uk". This
has the following MX records:
194.70.57.66 mailgate.net-shopper.co.uk 20
158.152.1.72 post.demon.co.uk 40
158.152.1.73 punt.demon.co.uk 40
158.152.1.65 gate.demon.co.uk 50
146.169.43.5 frigate.doc.ic.ac.uk 80
This tells mail software where to send the mail - and where to send it if any
machine is not available. So if the mail server is being rebooted at net-shopper,
a mail server will send mail to post.demon.co.uk or punt.demon.co.uk (since these
are the next entries in the MX record).
Many destinations do not have an MX records - in this case, mail software will
attempt to use the CNAME instead. In neither of these machines (or set of machines
are available) the mail should be held until it is possible to send it on.
The following points should be noted:
- It is wise to ask your provider to act as a secondary mail server for you
incase your machine becomes unavailable - for example, the disks could become
full, the link could break etc.
- The MX records and CNAME record for the same address could be different (thus
mail is sent to one machine and everyone else to a second machine).
- If MX records exist, they MUST be correct - otherwise mail will not be
delivered to your domain.
With NT Mail you can use the "MAIL" program to display your CNAME and MX records.
Entering "MAIL -mhostname" will display the MX records for the given name (if it
exists) and "MAIL -chostname" will display the CNAME records for the given host. If
you have not defined you DNS server, both NTMAIL and the MAIL utility will fail to
resolve the address. The MAIL utility uses exactly the same code to resolve MX
and CNAME addresses as in NTMAIL itself.
webmaster@net-shopper.co.uk (12th May 1995)