User's Manual

Chapter 2 DNS Service 33
Common Network Administration Tasks That Use
DNS Service
The following sections illustrate some common network administration tasks that
require DNS service.
Setting Up MX Records
If you plan to provide mail service on your network, you must set up DNS so that
incoming mail is sent to the appropriate mail host on your network. When you set up
mail service, you define a series of hosts, known as mail exchangers or MX hosts, with
different priorities. The host with the highest priority gets the mail first. If that host is
unavailable, the host with the next highest priority gets the mail, and so on.
For example, let’s say the mail server’s host name is “reliable” in the “example.com”
domain. Without an MX record, the users’ mail addresses would include the name of
your mail server computer, like this:
[e-mail]
If you want to change the mail server or redirect mail, you must notify potential
senders of a new address for your users. Or, you can create an MX record for each
domain that you want handled by your mail server and direct the mail to the correct
computer.
When you set up an MX record, you should include a list of all possible computers that
can receive mail for a domain. That way, if the server is busy or down, mail is sent to
another computer. Each computer on the list is assigned a priority number. The one
with the lowest number is tried first. If that computer isn’t available, the computer with
the next lowest number is tried, and so on. When a computer is available, it holds the
mail and sends it to the main mail server when the main server becomes available, and
then the server delivers the mail. A sample list might look like this:
example.com
10 reliable.example.com
20 our-backup.example.com
30 last-resort.example.com
MX records are used for outgoing mail, too. When your mail server sends mail, it looks
at the MX records to see whether the destination is local or somewhere else on the
Internet. Then the same process happens in reverse. If the main server at the
destination is not available, your mail server tries every available computer on that
destination’s MX record list, until it finds one that will accept the mail.
Note: If you don’t enter the MX information into your DNS server correctly, mail won’t
work.
LL2351.Book Page 33 Monday, September 8, 2003 2:47 PM