DNS records

DNS records

A DNS server is configured with a "zone file" for each domain that contains "resource records." There are several types of records, and the most common are described below. See DNS.

Forward DNS and Reverse DNS (A and PTR)
The Address (A) record associates a domain name with an IP address, which is the primary purpose of the DNS system. The Pointer (PTR) record provides data for reverse DNS, which is used for logging the domain name and verification purposes. Also called "inverse DNS," the PTR record is an option. See reverse DNS.

Aliasing Names (CNAME)
The Canonical Name (CNAME) record is used to create aliases that point to other names. It is commonly used to map WWW, FTP and MAIL subdomains to a domain name; for example, a CNAME record can associate the subdomain FTP.COMPUTERLANGUAGE.COM with COMPUTERLANGUAGE.COM.

DNS Name Servers (NS)
The Name Server (NS) record identifies the authoritative DNS servers for a domain. A second name server is required for redundancy, and two NS records must be in the zone file (one for the primary; one for the secondary). The secondary server queries the primary server for changes.

Mail Servers (MX)
The Mail Exchange (MX) record identifies the server to which e-mail is directed. It also contains a priority field so that mail can be directed to multiple servers in a prescribed order.

Text Record (TXT)
A TXT record can be used for any kind of documentation. It is also used to provide information to the SPF e-mail authentication system. See SPF.

First Record in File (SOA)
Start of Authority (SOA) is the first record in the zone file. It contains the name of the primary DNS server, which must correspond to an NS record in the file, the administrator's e-mail address and the length of time records can be cached before going back to the authoritative DNS server.

The SOA also includes data for the secondary DNS server such as the date of last update (the "Serial Number") and time intervals for checking the domain.

 SUMMARY OF COMMON ZONE FILE RECORDS

 Record Name (purpose)
  Associate This to That

 SOA record (1st record)

 TXT record (documentation)

 CNAME record (aliases)
  associate an alias to a domain

 DNAME record (aliases)
  associate an alias to a subdomain

 MX record (mail server)
  associate mail to a mail server

 NS record (authoritative server)
  associate a domain to a DNS server

 A record (forward DNS - IPv4)
  associate subdomain to 32-bit IP

 AAAA record (forward DNS - IPv6)
  (1st IPv6 record)
  associate subdomain to 128-bit IP

 A6 record (forward DNS - IPv6)
  (future IPv6 record)
  associate subdomain to 128-bit IP

 PTR record (reverse DNS)
  associate IP address to a subdomain