Need and services of DNS

Need of DNS

Hosts on the Internet are identified by IP addresses (e.g. 142.250.190.78) which are hard for people to remember, while humans prefer names such as www.example.com. The Domain Name System (DNS) is a distributed, hierarchical database and an application layer protocol (UDP/TCP port 53) that translates host names to IP addresses and back.

Why it is needed in an Internet or Intranet:

• Human-friendly names. Users remember names, not 32-bit/128-bit numbers.
• Flexibility. If a server moves to a new IP address, only the DNS record changes; users keep using the same name.
• Scalability. One central hosts file (HOSTS.TXT) cannot scale to millions of hosts. DNS spreads the data over many servers.
• Required by other services. Web, e-mail (MX records), FTP and proxy all depend on name resolution. In an intranet, names such as intranet.company.local or mail.company.local make internal services easy to reach.
• Load distribution and redundancy. One name can map to several IPs.

Hierarchy

              . (root)
        +-------+-------+
       com     org     np
        |               |
     google          edu.np
        |               |
       www            ioe

Root servers -> TLD servers (.com, .np) -> authoritative servers of each organisation. Local (recursive) DNS servers of an ISP or company resolve queries for clients and cache answers.

Services provided by DNS

1. Host name to IP address translation (forward lookup, A/AAAA records) and IP to name (reverse lookup, PTR records).
2. Host aliasing. A canonical name can have many aliases (CNAME), e.g. www.example.com -> web1.example.com.
3. Mail server aliasing. MX records identify the mail server of a domain, so addresses like user@example.com work.
4. Load distribution. A busy site may have several IPs for one name; DNS rotates the order of the list (round robin).
5. Name server records (NS) delegate a zone to its authoritative servers.
6. Caching. Servers cache answers for a TTL, reducing delay and traffic.
7. Other records: TXT (SPF, verification), SRV (service location), SOA (zone parameters).

Resolution can be recursive (server does the whole job for the client) or iterative (server returns a referral to the next server).