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What is Domain Name System (DNS)?

Domain Name Resolution

Name resolution systems provide the translation between alphanumeric names and numerical addresses, alleviating the need for users and administrators to memorize long strings of numbers.

There are two common methods for implementing name resolution:

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The two most common name resolution systems are Domain Name System (DNS) and Windows Internet Name Service (WINS). WINS was used in Microsoft networks to translate IP addresses to NetBIOS names, and is mostly deprecated.

[/threecol_one]DNS is heavily utilized on the Internet and on systems such as ActiveDirectory.

Domain Name System (DNS)
Domain Name System (DNS) translates between domain names and IP addresses, and is supported by nearly every operating system. All Internet based name resolution utilizes DNS.

DNS is organized as a hierarchy. Consider the following translation:
www.google.com = 209.85.225.104

The above domain name represents a Fully Qualified Domain Name (FQDN):
• .com represents a top level domain.
• .google represents a secondary level domain
• www represents a host computer in the .google.com domain.
Other top level domains include .org, .net, and .gov. Top level domains can also include country codes, such as .ca, .nl, and .de

Methods of configuring DNS

Recall that DNS name resolution can be implemented in the form of local HOSTS files, or a centralized name server(s). When employing HOSTS files, each translation must be statically configured on each device. In Windows 2000/XP operating systems, this file is located:

c:\windows\system32\drivers\etc\hosts

In UNIX/Linux operating systems, this file is generally located: /etc/hosts There are many disadvantages to using HOSTS files. The HOSTS file must be configured on every device. If a change occurs, every device’s HOSTS file must be updated.

Using one or more DNS servers provides several advantages over HOSTS files. All devices point to this centralized DNS server for name resolution, ensuring that changes only need to occur in one place.

If a particular DNS server does not contain the required DNS information, the request will can be forwarded to servers up the DNS hierarchy.

BIND (Berkeley Internet Name Domain) is the standard implementation of DNS. Microsoft, UNIX/Linux, and Novell all employ some version of BIND.

DNS servers assume one of three roles:

Both hosts and DNS servers will cache the result of DNS queries for a
period of time.

DNS Zone File Example
There are two types of zones in DNS:

The following is an example zone file for the fictional example.com domain:

Entries within a zone file are referred to as DNS records. There are a variety of DNS record types, including:

The number defined in the MX record is a priority. A lower priority is more
preferred.

DNS Process

DNS follows a strict process when performing a query. The process is as follows:

  1. The local DNS cache on the host is queried first.
  2. If there is no entry in the local cache, the local HOSTS file is queried next.
  3. If there is no entry in the local HOSTS, the query is forwarded to any configured DNS servers on the host. If no DNS servers are configured, the query will fail.
  4. If the configured DNS server is not authoritative for that domain, and does not have that DNS entry locally cached, the query will be forwarded up the DNS hierarchy. DNS servers can be configured with one or more forwarders. Organizations often point to their ISP’s DNS servers for DNS forwarding purposes.
  5. If no forwarders are available, the query is forwarded to the Root DNS server(s), which will likely have the entry cached.
  6. In the rare circumstance that the Root servers do not have a cached entry, the query will be forwarded back down the hierarchy to the authoritative DNS server for that domain.

Dynamic DNS allows DNS to be integrated with Dynamic Host Configuration Protocol (DHCP). When DHCP hands out an IP address lease,
it will automatically update the DNS entry for that host on the DNS server.

What is DHCP? How DHCP works?

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