2.1 Introduction

With 27 different tiles in an English Scrabble set ("A" through "Z" plus the blanks) and 7 tiles in a rack, you can draw billions of different combinations. And with over 100,000 words in the Official Word List, you can assemble a lot of words from almost any of those combinations.

In DNS, there are fewer than 300 possible types of resource records, and of those, only a handful could be called common. Still, you can do a remarkable variety of interesting things with those records.

All resource records, when written in plain text (as they'd appear in a zone data file), share the following format:

[owner] [TTL] [class] <type> <RDATA>

The fields in square brackets ("[" and "]") are optional, while the fields between angle brackets ("<" and ">") are mandatory. Section 2.2 explains what happens when you leave out one or more of those fields.

The RDATA field often consists of multiple subfields. The number of subfields required depends upon the type of record. For example, SOA records take seven RDATA subfields, while A and NS records need just one.

A zone data file contains the resource records attached to all of the domain names in a zone. A zone's primary master name server loads the zone data file, and the zone's slaves transfer the zone data from the primary master.