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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

Master File Directives

In addition to resource records, most master file implementations also support the use of directives. These are commands that specify certain important pieces of information to guide how the master file is to be interpreted. Three of the most common directives are:

• $ORIGIN: Specifies the domain name that is appended to unqualified specifications; this is the “base” used to convert PQDNs to FQDNs. For example, if the origin is “xyzindustries.com.”, then a PQDN such as “sales” will be interpreted as “sales.xyzindustries.com.”. Once defined, the origin can be referenced by just using “@” in place of a name, as we will see in the example at the end of this topic.
  
  
• $TTL: Specifies the default Time To Live value to be used for any resource records that do not specify a TTL value in the record itself. (This value was formerly specified by the Minimum field in the Start Of Authority record.)
  
  
• $INCLUDE: Allows one master file to include the contents of another. This is sometimes used to save the duplication of certain entries that are common between zones.

There are a few other syntax rules for DNS master files, some of which are intended to save further time or energy on the part of administrators:

• Multiple-Record Shorthand: If multiple consecutive records pertain to the same domain, the “<domain-name>” is specified for the first one, and can be then be left blank for the subsequent ones. The server will assume that any resource records without a “<domain-name>” indicated apply to the last “<domain-name>” it saw.
  
  
• Comments: A semicolon (“;”) marks a comment. Any text from the semicolon until the end of the line is ignored.
  
  
• Escape Character: A backslash (“\”) is used to “escape” the special meaning of a character. For example, a double-quote (") is used to delimit text strings; a literal double-quote character is indicated by a backslash-double-quote combination (\").
  
  
• White Space: Tabs and spaces are used as delimiters and blank lines are ignored. For readability, most smart administrators indent using tabs to make more clear which records belong with which names, and group records using blank lines and comments.
  
  
• Case: Like DNS domain names, master file entries are case-insensitive.

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