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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 canand most organizations on
the Internet douse 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.
