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 next chapter discusses a topic that will probably be very interesting to you system administrators and postmasters: how DNS impacts 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 of advanced mail routing. When mailers had only the
HOSTS.TXT file (and its derivatives,
/etc/hosts in the Unix world and
%SYSTEM ROOT%\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, ...