Another approach to multiplexing incoming mail is to have two layers of servers, a front line of relays that accept connections from foreign mail servers and a back line of mail servers that actually house users' mail. The front line servers or relay hosts should be set up so that their IP addresses are A records for mail.bigisp.com. In addition an A record should assign a unique host name to each relay host so that specific servers can be accessed for administration purposes. The back end servers that hold users' mail should only be accessed by the relay hosts and can be given any host name as long as it is unique. In this example barney and alf are relay servers, while ingrid and fritz are the back end servers where mail is stored. The forward DNS records for this setup are as follows;
; Mail addresses to be accessed by foreign hosts mail IN A 22.214.171.124 mail IN A 126.96.36.199 ; Names for relay hosts for administrative access barney IN A 188.8.131.52 alf IN A 184.108.40.206 ; Back end servers where mail is stored ingrid IN A 220.127.116.11 fritz IN A 18.104.22.168
The reverse DNS records should include mail.bigisp.com and not the administrative names for the relay hosts. A reverse lookup on an address given for mail.bigisp.com should match the forward delegation. It is important to keep a consistent view of mail.bigisp.com as some mail servers are very pedantic about matching DNS records for mail servers in an attempt to filter out unsolicited email. The resulting reverse DNS records for 0.8.61.in-addr.arpa. are;
100 IN PTR mail.bigisp.com 101 IN PTR ingrid.bigisp.com 102 IN PTR mail.bigisp.com 103 IN PTR fritz.bigisp.com
As for single layer multiplexing layer 4 switching can be employed instead of round-robin DNS to distribute clients to the front-end servers. This greatly simplifies the DNS configuration and gives a more fine-grained resolution on client distribution to front end servers.