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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  Name Systems and TCP/IP Name Registration and Name Resolution            9  TCP/IP Name Systems: Host Tables and Domain Name System (DNS)                 9  TCP/IP Domain Name System (DNS)                      9  DNS Name Servers and Name Resolution                           9  DNS Messaging and Message, Resource Record and Master File Formats

DNS Name Notation and Message Compression Technique 1 2 3 4 DNS Changes To Support IP Version 6

DNS Common Master File Record Format

Just as all resource records are stored internally using a common field format, they also use a common master file format. Each record normally appears on a separate line of the file. This format is as follows, with optional fields shown in square brackets:

<domain-name>  [<ttl>]  <class>  <type>  <rdata>

The fields are as follows:

• <domain-name>: A DNS domain name, which may be either a fully-qualified domain name (FQDN) or a partially-qualified name (PQDN). See below.
  
  
• <ttl>: A Time To Live value, in seconds, for the record. If omitted, the default TTL value for the zone is used. In fact, most resource records do not have a specified TTL, just using the default provided by the Start Of Authority record.
  
  
• <class>: The resource record class. For modern DNS this field is optional, and defaults to “IN” for “Internet”
  
  
• <type>: The resource record type, specified using a text code such as “A” or “NS”, not the numeric code.
  
  
• <rdata>: Resource record data, which is a set of space-separated entries that depends on the record type.

The “<rdata>” can be either a single piece of information or a set of entries, depending on the record type. In the case of longer record types, especially the Start Of Authority record, multiple entry “<rdata>” fields are spread over several lines and enclosed in parentheses; the parentheses make all the entries act as if they were on a single line. Note that if the “<ttl>” field is present, the order of it and the “<class>” field may be switched; this causes no problems because one is a number and the other text (“IN”).

Domain names may be mixed between FQDNs and PQDNs. Partially-qualified names are used to make master files faster to create and more readable, by cutting down on the common parts of names; they are sort of the “human equivalent” of DNS message compression. A FQDN is shown as a full domain name ending in a dot (“.”) to represent the DNS name tree root. A PQDN is given as just a partial name with no root, and is interpreted as a FQDN by the software reading the master file (see the $ORIGIN directive below for more.)

It is important to remember the trailing dot to mark FQDNs; if the origin is “xyzindustries.com” and in its zone file the name “bigisp.net” appears, the server will read this as “bigisp.net.xyzindustries.com”—probably not what you want. Also, e-mail addresses, such as the <r-name> field in the SOA record, have the “@” of the e-mail address converted to a dot, following the standard DNS convention.

DNS Name Notation and Message Compression Technique 1 2 3 4 DNS Changes To Support IP Version 6