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Lesson 9 Zone files and Resource Records
Objective Match each Resource Record abbreviation with a description of its purpose.

Zone Files and Resource Records

Now that we have an overview of how DNS operates, we will focus on the actual form of the data in the DNS database.
As mentioned earlier, the data is stored in files, called zone files, which reside on the hosts running authoritative name servers for the zone.
In a DNS zone file, various Resource Record (RR) types are represented by abbreviations, each serving a specific purpose in the domain name resolution process. Here is a list of common RR abbreviations and their corresponding purposes:
  1. A (Address Record): Maps a domain name to an IPv4 address. It is used to translate a human-readable domain name into its corresponding IP address, enabling clients to access resources on the internet.
  2. AAAA (IPv6 Address Record): Similar to the A record, the AAAA record maps a domain name to an IPv6 address, facilitating access to resources on the internet using the newer IPv6 protocol.
  3. CNAME (Canonical Name Record): Defines an alias for another domain name. CNAME records are used when multiple domain names need to resolve to the same IP address, allowing one domain to point to another domain that has an A or AAAA record.
  4. MX (Mail Exchange Record): Specifies the mail server responsible for handling email for a particular domain. MX records include a preference value to indicate the priority of each mail server, with lower values having higher priority.
  5. NS (Name Server Record): Indicates the authoritative name servers responsible for the DNS zone. NS records are essential for delegating DNS queries to the appropriate name servers within the DNS hierarchy.
  6. PTR (Pointer Record): Used for reverse DNS lookups, PTR records map an IP address back to a domain name. They are typically used in conjunction with a special reverse lookup zone, such as for IPv4 or for IPv6.
  7. SOA (Start of Authority Record): The SOA record defines essential information about a DNS zone, including the primary name server, the email address of the responsible administrator, and various timing parameters for the zone, such as refresh and retry intervals.
  8. SRV (Service Record): Specifies the location and other details of services running on a domain, such as SIP, LDAP, or XMPP. SRV records include information about the service's hostname, port number, priority, and weight.
  9. TXT (Text Record): Holds arbitrary human-readable text or machine-readable data, such as SPF records for email authentication or DKIM records for email signing. TXT records are often used for verification purposes and to store metadata associated with a domain.

By understanding the purpose of each RR abbreviation, you can effectively create, manage, and interpret DNS zone files to configure and troublshoot dom ain name resolution settings in your DNS infrastructure.
The individualrecords in the zone files are called Resource Records (RRs) and come in various types, as shown in the table below:
Resource Record Type Abbreviation Purpose
State of Authority SOA Sets basic parameters for a zone, and establishes responsible parties
Address A Specifies a name-to-address mapping
Pointer PTR Specifies an address-to-name mapping
Host InformationHINFODescribes the architecture and operating system of a host in the database
Canonical NameCNAMEDefines an alias, or alternative name, for a host
Mail ExchangeMXDefines a mail exchange host
Name ServerNSDeclares a name server for a zone

We will examine each of these Resource Record types later in this module. We will discuss MX records later in this course.

View the following example to examine these zone files.

View Example: zone files

Zone file for
$ORIGIN  com.
corporation  IN SOA
( 19980416 1800 300 604800 1800 )
             IN      NS
             IN      NS

morgan       IN      A
             IN      HINFO    "PPC"  "MacOS"
vanderbilt   IN      A
             IN      HINFO    "Sparc"  "Solaris"
www          IN      CNAME
rockerfeller IN      A
             IN      HINFO    "Intel"  "Linux"

Zone file for
12  IN     SOA
                    ( 19980302 1800 300 604800 1800 )
    IN     NS
    IN     NS
$ORIGIN  12.143.196.IN-ADDR.ARPA.
200    IN     PTR
201    IN     PTR
202    IN     PTR

Hypothetical Zone Files

Let us look at hypothetical zone files for an organization called Corporation, Inc. Corporation, Inc. administers two DNS zones.
The first is the corporation .com zone. The second is the reverse lookup zone derived from Corporation's Class C network address 196.143.12.

Resource Record Abbreviations

The following paragraph discusses resource record abbreviations with their definitions.

Resource Record Types

This page shows matching terms for Resource Record abbreviations to their corresponding Resource record descriptions:
  1. NS (Name Server): Declares a name server for a zone
  2. SOA (State of Authority): Sets basic parameters for a zone
  3. MX (Mail Exchange): Defines a mail exchange host
  4. A (Address): Specifies a name-to-address mapping
  5. CNAME (Canonical Name): Defines an alias for a host
  6. PTR (Pointer): Specifies an address-to-name mapping
  7. HINFO (Host Information): Describes the architecture and operating system of a host in the database

System Information Record (HINFO)

Allows definition of the Hardware type and Operating System (OS) in use at a host. For security reasons these records are rarely used on public servers. If a space exists in the field it must be enclosed in quotes. Single space between Hardware and OS parameters.

owner-name ttl  class   rr      Hardware        OS
                IN      HINFO   PC-Intel-700mhz "Redhat Linux 7.1"

A HINFO-record specifies the host/server's type of CPU and operating system. This information can be used by application protocols such as FTP, which use special procedures when communicating with computers of a known CPU and operating system type. Standard CPU and operating system types are defined in RFC1700. To create a new HINFO-record, right-click a zone in the left list in the DNS Records window, and select "Other new record" from the pop-up menu. This record type is defined in RFC1035.