MX--Mail Exchanger

Example 28-16 shows the syntax for a mail-exchanger (MX) resource record.

Example 28-16 MX Record Format

name [optional TTL] class	MX preference-value mailer-exchanger

The mail-exchanger resource records are used to specify a machine that knows how to deliver mail to a domain or specific machines in a domain. There may be more than one MX resource record for a given name. In Example 28-17, Seismo.CSS.GOV. (note the fully qualified domain name) is a mail gateway that knows how to deliver mail to Munnari.OZ.AU. Other machines on the network cannot deliver mail directly to Munnari. Seismo and Munnari may have a private connection or use a different transport medium. The preference-value field indicates the order a mailer should follow when there is more than one way to deliver mail to a single machine. The value 0 (zero) indicates the highest preference. If there is more than one MX resource record for the same name, records may or may not have the same preference value.

You can use names with the wildcard asterisk (*) for mail routing with MX records. There are likely to be servers on the network that simply state that any mail to a domain is to be routed through a relay. In Example 28-17, all mail to hosts in domain foo.com is routed through RELAY.CS.NET. You do this by creating a wildcard resource record, which states that the mail exchanger for *.foo.com is RELAY.CS.NET. The asterisk will match any host or subdomain of foo.com, but it will not match foo.com itself.

Example 28-17 Sample MX Resource Record

;name	 	[TTL]	class		MX	 preference mailer-exchanger
Munnari.OZ.AU.		IN		MX	 0	Seismo.CSS.GOV.
foo.com.		IN		MX	 10	RELAY.CS.NET.
*.foo.com.		IN		MX	 20	RELAY.CS.NET.