Linux Network Administrators Guide

6.3.3. The DNS Database Files

Master files included with named , like named.hosts , always have a domain associated with them, which is called the origin . This is the domain name specified with the cache and primary options. Within a master file, you are allowed to specify domain and host names relative to this domain. A name given in a configuration file is considered absolute if it ends in a single dot, otherwise it is considered relative to the origin. The origin by itself may be referred to using (@ ).

The data contained in a master file is split up in resource records  (RRs). RRs are the smallest units of information available through DNS. Each resource record has a type. A records, for instance, map a hostname to an IP address, and a CNAME record associates an alias for a host with its official hostname. To see an example, look at Example 6-11 , which shows the named.hosts master file for the Virtual Brewery.

Resource record representations in master files share a common format:

[domain] [ttl] [class] type rdata

Fields are separated by spaces or tabs. An entry may be continued across several lines if an opening brace occurs before the first newline and the last field is followed by a closing brace. Anything between a semicolon and a newline is ignored. A description of the format terms follows:

• domain
   

This term is the domain name to which the entry applies. If no domain name is given, the RR is assumed to apply to the domain of the previous RR.



• ttl
   

In order to force resolvers to discard information after a certain time, each RR is associated a time to live (ttl  ). The ttl field specifies the time in seconds that the information is valid after it has been retrieved from the server. It is a decimal number with at most eight digits.

If no ttl value is given, the field value defaults to that of the minimum field of the preceding SOA record.



• class
   

This is an address class, like IN for IP addresses or HS for objects in the Hesiod class. For TCP/IP networking, you have to specify IN.

If no class field is given, the class of the preceding RR is assumed.



• type
   

This describes the type of the RR. The most common types are A, SOA, PTR, and NS. The following sections describe the various types of RRs.



• rdata
   

This holds the data associated with the RR. The format of this field depends on the type of RR. In the following discussion, it will be described for each RR separately.

The following is partial list of RRs to be used in DNS master files. There are a couple more of them that we will not explain; they are experimental and of little use, generally.

• SOA
   

This RR describes a zone of authority (SOA means "Start of Authority"). It signals that the records following the SOA RR contain authoritative information for the domain. Every master file included by a primary statement must contain an SOA record for this zone. The resource data contains the following fields:

• origin
   

This field is the canonical hostname of the primary name server for this domain. It is usually given as an absolute name.



• contact
   

This field is the email address of the person responsible for maintaining the domain, with the "@ " sign replaced by a dot. For instance, if the responsible person at the Virtual Brewery were janet , this field would contain janet.vbrew.com .



• serial
   

This field is the version number of the zone file, expressed as a single decimal number. Whenever data is changed in the zone file, this number should be incremented. A common convention is to use a number that reflects the date of the last update, with a version number appended to it to cover the case

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