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Double Layer SMTP Multiplexing

Figure 2: Network Setup for Double Layer SMTP Multiplexing

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 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
mail   IN  A
; Names for relay hosts for administrative access
barney IN  A
alf    IN  A
; Back end servers where mail is stored
ingrid IN  A
fritz  IN  A

The reverse DNS records should include and not the administrative names for the relay hosts. A reverse lookup on an address given for should match the forward delegation. It is important to keep a consistent view of 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 are;

100  IN  PTR
101  IN  PTR
102  IN  PTR
103  IN  PTR

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.

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