A Primary DNS Server is the authoritative source of all information for a domain. This DNS server has all the answers for any host or computer in the domain.
The terms primary and master are often used interchangeably, though each refers to a different characteristics of the DNS server and are slightly different.
Primary servers are the servers listed as the primary authoritative server for the domain at the domain name registrar. The registrar assumes that the primary server will be configured to be authoritative for the domains which it is registered to serve. A master server is only required to have a master copy of the DNS zone file for the domain from which other slave servers may transfer the file.
Througout the rest of the DNS tutorials we will use the word primary.
The primary DNS server usually has been set up and configured manually by the DNS administrator. The primary server for a domain contains a database file that is stored in it's file system. This database file contains a Start Of Authority (SOA) record. The SOA resource record must contain the name of the server in order for it to be considered the Primary server. All secondary servers will regularly perform zone transfers from the primary server to provide redundancy and load balancing.
Any domain for which a DNS server is to be primary for must have a record similar to the following in the database file:
domain.com. IN SOA ns1.domain.com. admin.domain.com. (
1 ; Serial number
10800 ; Refresh after 3 hours
3600 ; Retry after 1 hour
604800 ; Expire after 1 week
86400 ; Minumum Time To Live of 1 day )
domain.com. IN NS ns1.domain.com.
domain.com. IN NS ns2.domain.com.
Note that the primary nameserver does not need to actually be part of the domain it is supporting. In addition, since a nameserver can have multiple DNS database files (called zone files), it can support multiple domains. Remember that a DNS server that is primary master for a domain is primary only for that domain because it is the authoritative server.