Using Round-Robin DNS

  

Figure 1: Servers with Configured for Round-Robin DNS

The DNS for each mail server that is to handle users' mail, should be configured such that a DNS lookup for the primary mail exchanger for the domain points to all of these machines. In addition a unique name for each machine should be set up in DNS to enable the machine to be addressed directly. The DNS records for two servers would be;

;Multiple addresses (hosts) for mail
mail    IN  A  10.0.0.100
mail    IN  A  10.0.0.101
;Real server names for administrative access
barney  IN  A  10.0.0.100
alf     IN  A  10.0.0.101

Corresponding records for the reverse lookup of the IP addresses to these names are also required.

100     IN  PTR  mail.bigisp.com
101     IN  PTR  mail.bigisp.com

Recent DNS servers including BIND 8.x and BIND 4.9 will deterministically issue the different IP addresses assigned to mail.bigisp.com in a round-robin fashion [LA98]. Hence, hosts that access mail.bigisp.com have an equal chance of accessing barney and alf. If the connection is for a user whose mail is hosted on the machine that is accessed then the connection is dealt with locally, otherwise the connection is forwarded to the server where the users' mail is hosted.