Domain Name System (DNS) Part 2
This is the second part Domain Name System (DNS). Click here to see part 1.
DNS Server Hierarchy
Typically, there are three levels in the DNS server hierarchy
- Root DNS server: as of this writing, there are 13 root DNS servers, more here. These return IP addresses of top level domain DNS servers.
- Top level domain DNS server: these DNS servers are in charge of
.comor.edu… top level domains. They return IP address of authoritative DNS servers. - Authoritative DNS server:
Every organization with publicly accessible hosts (such as Web servers and mail servers) on the Internet must provide publicly acces- sible DNS records that map the names of those hosts to IP addresses. An organiza- tion’s authoritative DNS server houses these DNS records. ~ The Power of Habit
DNS Resource Record
Resource records (RR) are hostname-to-IP address mappings that are stored in the DNS system.
Each RR contains four fields: Name, Value, Type, TTL.
There are 04 types of records. The following table summarises each record type and what kind of mapping it provides.
| Record type | Mapping |
|---|---|
| A | hostname to IP address |
| CNAME | alias hostname to canonical name |
| NS | domain to hostname of authoritative DNS server |
| MS | alias hostname to canonical mail server’s name |
From The Power of Habit
If a DNS server is authoritative for a particular hostname, then the DNS server will contain a Type A record for the hostname.
If a server is not authoritative for a host- name, then the server will contain a Type NS record for the domain that includes the hostname; it will also contain a Type A record that provides the IP address of the DNS server in the Value field of the NS record.
Inserting Records into the DNS Database
In order to obtain a usable domain name, you need to register that domain at a registrar.
A registrar is a commercial entity that verifies the uniqueness of the domain name, enters the domain name into the DNS database (as discussed below), and collects a small fee from you for its services.
Registering a domain with a registra involving providing the name and IP address of your authoritative DNS servers. The registra will then make an NS record (domain -> authoritative DNS server’s hostname) and an A-type (DNS server’s hostname -> its IP address) in each of the corresponding top level domain DNS servers.