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Lesson 11 Types of Resource Records
Objective Describe the format of SOA, A, PTR, HINFO, CNAME, and NS records.

State of Authority (SOA) records

An SOA record sets up basic parameters for a DNS zone of authority and marks the beginning of a zone. Every zone or subzone must have exactly one SOA record and the zone continues until another SOA record is encountered. The fields in an SOA record have the following meanings:
  1. KEY is the zone to which this SOA applies.
  2. VALUE has the following form:

Resource Records Types
Click the link below to view the meanings of the fields in an SOA record.

Name SOA record VALUE field format

  1. The FQDN of the host where this SOA record resides; in other words, the authoritative server for this zone
  2. The email address of the administrator for this zone, written with the usual @ sign replaced by a dot.
  3. A number identifying this version of the zone file. This field is used by secondary servers to decide if the zone file has changed, so it should be incremented every time the file is modified.
  4. How often (in seconds) the secondary servers should reload this zone file to check for changes.
  5. How long (in seconds) the secondary servers should wait after a failed zone file transfer before retrying.
  6. How long (in seconds) the secondary servers should wait between successful zone file transfers before discarding the data.
  7. Tells other servers how long, by default, to cache results from this server.

Address (A) records and Pointer (PTR) records

Address records specify a mapping from the host name to the IP address. These records are the heart of the DNS database. Address records appear in the forward lookup zone file (for corporation.com in our previous example), and PTR records appear in the reverse file (the in-addr.arpa zone file).

Host information (HINFO) records

Host information records give the operating system and architecture for the specified host. Many sites do not put this information in their DNS database because it provides extra information to hackers. However, it can be quite convenient on networks with many different kinds of machines. The format is:

KEY IN HINFO  Architecture   O/S

The valid names for architecture and operating system are the official names supplied by the system. You can get these names by using the uname command.

Canonical name (CNAME) records

Canonical name records define an alias. Looking at our previous example, the record:

www IN CNAME  vanderbilt.corporation.com.
declares that www.corporation.com (remember the $ORIGIN keyword) is an alias for the machine vanderbilt.corporation.com.

Name server (NS) records

A name server (NS) record declares a machine to be a name server for a specified zone. The KEY is the name of the zone, and the value is the FQDN for the name server. Usually, the NS records immediately follow the SOA record, with the first NS record pointing to the authoritative name server for the zone.
Zone files use Fully Qualified Domain Names, so be very careful to make sure that you use periods at the end of host names when they occur as VALUE fields in A records, and in the SOA record. Ninety-nine percent of the time, a problem with a zone file is due to a missing period.

Resource Records - Quiz

Click the Quiz link below to take a brief multiple-choice quiz on Resource Records.
Resource Records - Quiz