A zone file stores the DNS data of a specific domain. In a zone file, a resource record defines the characteristics and properties of an entity available within the domain. There are many types of resource records. The most important types are SOA, NS, MX, A or AAAA, and CNAME.
The SOA (Start of Authority) record
It defines the global characteristics of the domain and authorizes a name server for it. It is a compulsory record. Each zone file must include one SOA record. You cannot add two or more SOA records in a zone file. You can add only one SOA record in a zone file.
It uses the following syntax.
domain-name TTL class-type record-type name-server e-mail-address serial-number refresh-time retry-time expiry-time negative-cache-TTL
The following is an example SOA record. It authorizes ns1.example.com. name server for the domain.
; The SOA (Start of Authority) record
@ IN SOA ns1.example.com. hostmaster.example.com. (
2023013100 ; serial-number
12 h; refresh-time
15 m; retry-time
3 w; expiry-time
2 h; negative-cache-TTL
)
If you use a single @ character in the domain name field, it replaces it with the domain name stored in the $ORIGIN directive.
This tutorial is the third part of the tutorial series "DNS zone files Explained with Examples.". Each tutorial of this series explains a specific aspect of the zone file. Other tutorials of the series are the following.
DNS zone file Format, Configuration, and Directives
The TTL and ORIGIN directives in the zone file
The SOA Record Explained in the DNS zone file
The NS Record, Glue Record, and Lame Delegation
The MX record in a zone file explained
The NS (Name Server) record
It defines the authorized DNS servers for this domain. It is a compulsory record. There must be at least two name server records in a zone file. You can also add a reference to an external name server.
It uses the following syntax.
domain-name TTL class-type record-type authorized-name-server
The following are examples of the NS record.
; The NS records.
; Primary or main NS server. Available within the domain.
3w IN NS ns1.example.com.
; Secondary or backup NS server. Available outside the domain.
IN NS ns2.example.net.
The above example NS records configure two name servers. The ns1.example.com. is the primary name server and is available within the domain. The ns2.example.net. is the secondary or backup name server. It is available outside the domain.
The MX (Mail exchanger) record
It defines the mail servers for this domain. It is an optional record. If the domain does not provide e-mail services, you do not need to add it to the zone file. You can also add a reference to an external email server.
It uses the following syntax.
domain-name TTL class-type record-type preference email-server-name
The following are examples of the MX record.
; The MX records
; Primary or main NS server. Available within the domain.
3w IN MX 10 mail.example.com.
; Secondary or backup NS server. Available outside the domain.
IN MX 20 mail.example.net.
In the above example, MX records configure two mail servers. The mail.example.com mail server is internal and is available within the domain. The mail.example.net mail server is external. It is available outside the domain.
The A (Address) record
It defines the IPv4 address of a host or a service that exists in this domain and is required to be publicly visible. It is an optional record. It uses the following syntax.
name TTL class-type record-type IPv4 address
The following are example A records.
; The A records ns1 IN A 172.168.1.1 mail IN A 172.168.1.2 www IN A 172.168.1.3
The AAAA (Quad A) record
It defines the IPv6 address of a host or a service that exists in this domain and is required to be publicly visible. It is an optional record. It uses the following syntax.
name TTL class-type record-type IPv4 address
The following are examples of AAAA records.
ns1 IN AAAA 2002:db7:: mail IN AAAA 2002:db8:: www IN AAAA 2002:db9::
The CNAME record
It defines an alias name of a host or a service that exists within the domain and is required to be publicly visible by the alias name.
It uses the following syntax.
alias-name TTL class-type record-type host-or-service-name
The following are example CNAME records.
; The CNAME records ftp IN CNAME fhost www IN CNAME whost