DNS & BIND Cookbook

Chapter 4. Electronic Mail

Introduction

Remember when you were little, and it was a thrill to get mail, any mail at all? Postcards from relatives on vacation, greeting cards, the odd misdirected credit card offer -- it didn’t matter. I’ll bet that if you think back far enough, you can remember a time when it was exciting to get Internet email, too.

Now, of course, most Internet email is spam, but we can dream of a simpler time, before the Nigerian email scam and ordering Viagra on the Internet.

One of the linchpins of getting all of that email into your inbox is DNS. Mail transport agents all over the Internet look up MX records attached to your domain names to determine where to deliver your mail. I showed you the basic syntax of the MX record back in Recipe Section 2.5, but you also can craft those MX records to designate backup mail servers (Section 4.2), multiple, equivalent mail servers (Recipe Section 4.3), and more. This chapter will show you how.

Configuring a Backup Mail Server in DNS

Problem

You want to configure a backup mail server for a mail destination.

Solution

Add an MX record for the mail destination, listing the domain name of the backup mail server at a higher preference value (and therefore lower preference) than the main mail server. For example, say the MX record for the mail destination foo.example, currently looks like this:

foo.example.    IN    MX    10 mail.foo.example.

To add smtp.isp.net as a backup mail server for foo.example, add another MX record to foo.example ...