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5. Domain Name System

The Domain Name System is a distributed, hierarchical directory service.

Its official root is the authoritative name servers that are responsible for the DNS Root Zone. These are also generally known as Root Servers.

"hosts" file

Before a client uses a network service, it will ask its local "table". This file can be found in all popular operating systems. Its content can be output as follows:

DNS Cache

If a local DNS cache is available on the client (this depends on the operating system and its version), the client will not yet ask a network service in the second step. If a local DNS cache is in place, the storage time for successful queries unfortunately varies.

Normally, the client will subsequently ask a DNS cache in its own network or the ISP 's network (which can also return manipulated answers).
However, we will assume in the following that the client contacts the root servers directly.

DNS of the Internet

The client asks one of the root servers for the IP address of an internet domain (e.g. "webmail.123-reg.co.uk").

However, since the root servers only know which name servers are responsible for resolving the ".uk" (and ".co.uk") TLD s, the root server will refer the client to the relevant name server (in our example, this is "ns.webfusion.co.uk").

This delegation is controlled in what is called NS Resource Record.

The Nominet UK name servers themselves do not know the answer either.
They also refer the client to the name server responsible for the internet domain (e.g."123-reg") (in our example, this is "ns.webfusion.co.uk").

If further name servers are responsible for additional subdomains, this will go on until the last name server will finally know the answer.

Depending on the record the client requested (A for IPv4, AAAA for IPv6, CNAME for an alias or MX for the Email Server), the last name server will return the requested answer.

The SOA Resource Record contains the following information in the order shown below:

• the primary name server for the zone
• the name server Administrator's email address (Note that any dot must be replaced by the @ symbol.)
• serial number (recommended by RFC 1912: date of last modification with a suffixed two-digit counter)
• [...]
• the expire time (86400 seconds = 24 hours)

In the case of CNAME or MX Identifier Records, the result of the query can be yet another internet domain.

In the past, a domain's " www " subdomain regularly referred to a HTTPD (webserver). However, today, "www" is increasingly often omitted in an internet domain.

In a LINUX/BSD/Unix shell the answer can be output as follows (as long as it is possible to establish a direct contact with the root servers or the Resolver/Cache admits a corresponding query):

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