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:

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)