TCP / IP For Dummies
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Each DNS database zone file consists of one or more resource records, including the Canonical Name (CNAME), Pointer (PTR) and Mail Exchange (MX) records. Creating and updating the resource records that comprise the DNS zone files is one of the primary tasks of a DNS administrator.

CNAME records

A Canonical Name (CNAME) record creates an alias for a fully qualified domain name. When a user attempts to access a domain name that is actually an alias, the DNS system substitutes the real domain name — known as the Canonical Name — for the alias. The owner field in the CNAME record provides the name of the alias that you want to create. Then, the RDATA field provides the Canonical Name — that is, the real name of the host.

For example, consider these resource records:    IN  A  IN  CNAME

Here, the host name of an FTP server at is The CNAME record allows users to access this host as if they prefer.

PTR records

A Pointer (PTR) record is the opposite of an address record: It provides the fully qualified domain name for a given address. The owner field should specify the reverse lookup domain name, and the RDATA field specifies the fully qualified domain name. For example, the following record maps the address to IN  PTR

PTR records don’t usually appear in normal domain zones. Instead, they appear in special reverse lookup zones.

MX records

Mail Exchange (MX) records identify the mail server for a domain. The owner field provides the domain name that users address mail to. The RDATA section of the record has two fields. The first is a priority number used to determine which mail servers to use when several are available. The second is the fully qualified domain name of the mail server itself.

For example, consider the following MX records:    IN  MX  0    IN  MX  10

In this example, the domain has two mail servers, named and The priority numbers for these servers are 0 and 10. Because it has a lower priority number, mail will be delivered to first. The server will be used only if isn’t available.

The server name specified in the RDATA section should be an actual host name, not an alias created by a CNAME record. Although some mail servers can handle MX records that point to CNAMEs, not all can. As a result, you shouldn’t specify an alias in an MX record.

Be sure to create a reverse lookup record (PTR, described in the next section) for your mail servers. Some mail servers won’t accept mail from a server that doesn’t have valid reverse lookup entries.

About This Article

This article is from the book:

About the book authors:

Candace Leiden consults on systems and database performance and instructional design for international courseware. Marshall Wilensky is a retired consultant and network manager for multiprotocol networks.

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