4 A zone file

A zone file describes specific data of a DNS zone. The following is an example of a zone file:

• line 1: this line defines the name space.
• line 2: this line defines the time-to-live value. This value specifies how long a DNS server should cache the DNS records. The long this time, the longer it takes for any IP address change to propagate.
• line 3: this is the first line that define properties of this zone. IN means “internet”, and SOA means the “start of authority”. This example means that the name server ns.somens.net is the primary authoritative nameserver for this zone. The next item, root.drtak.org. indicates the email address of the account that is in charge of this zone. The last open parenthesis is necessary, as it indicates the beginning of other properties of this zone.
• line 4: this line defines a serial number for the zone file. This needs to be a unique number. In other words, when the file is changed, this number must change. It is easiest to use a date+time method to ensure the number is unique.
• line 5: this line define how often a slave (also known as secondary) DNS server should refresh itself. A slave DNS server is used to help offline DNS queries from the primary DNS server. This parameter specifies that a slave DNS server should synchronize with a master server.
• line 6: in case a slave DNS server cannot synchronize with the master DNS server, this entry defines how often the slave DNS server should retry the synchronization.
• line 7: if a slave DNS server fails to contact a master DNS server for this period, the slave DNS server should cease to respond to DNS queries.
• line 8: this specifies the amount of time to cache negative responses. This means that it specifies how long to keep a negative response (a miss) without relaying the request to another DNS server.
• line 10: this specifies that ns.drtak.org (the drtak.org part is implied if the name does not end with a period) is a nameserver for the domain drtak.org. An NS record is used to define name servers of a zone.
• line 11: this specifies that ns1.somens.net is another nameserver for this domain, this is a “backup” in case the first name server fails to respond.
• line 12: this specifies the mail.drtak.org is the primary email server. The number 10 is the least of all the MX records, as such, this entry is the highest priority mail server. An MX record defines the mail server of a zone so that SMTP servers can use this information to relay messages. Note that the email server can be defined by IP address, and it does not need to be related to the name of the zone itself.
• line 13: this specifies that mail1.drtak.org is a backup email server.
• line 14: this specifies that the name drtak.org maps to the IP address of 10.0.2.20. An “A” record defines how a name should be mapped to an IP address.
• lines 15 to 17: this is necessary to define where to find the name server and email servers.
• line 18: this defines an alias. To be more specific, www.drtak.org (remember that drtak.org is implied) is the same as drtak.org.