Chapter 5. DNS and Electronic Mail

And here Alice began to get rather sleepy, and went on saying to herself, in a dreamy sort of way, "Do cats eat bats? Do cats eat bats?" and sometimes "Do bats eat cats?" for, you see, as she couldn't answer either question, it didn't much matter which way she put it.

I'll bet you're drowsy too, after that looong chapter. Thankfully, this chapter discusses a topic that will probably be very interesting to you system administrators and postmasters: how DNS affects electronic mail. And even if it isn't interesting to you, at least it's shorter than the last chapter.

One of the advantages of the Domain Name System over host tables is its support for advanced mail routing. When mailers had only the HOSTS.TXT file (and its derivatives, /etc/hosts in the Unix world and %SYSTEMROOT%\system32\drivers\etc\HOSTS under Windows) to work with, the best they could do was to attempt delivery to a host's IP address. If that failed, they could either defer delivery of the message and try again later or bounce the message back to the sender.

DNS offers a mechanism for specifying backup hosts for mail delivery. The mechanism also allows hosts to assume mail-handling responsibilities for other hosts. This lets diskless hosts that don't run mailers, for example, have mail addressed to them processed by their servers.

DNS, unlike host tables, allows arbitrary names to represent electronic mail destinations. You can—and most organizations on the Internet do—use the domain name of your main forward-mapping zone as an email destination. Or you can add domain names to your zone that are purely email destinations and don't represent any particular host. A single logical email destination may also represent several mail servers. With host tables, mail destinations were hosts, period.

Together, these features give administrators much more flexibility in configuring electronic mail on their networks.