DNS

DNS (Domain Name System)

It is a hierarchical, distributed database that comprises the mappings of DNS domain names to various types of data, like IP addresses of the system. DNS enables the location of computers and services by user-friendly names, and it also helps to enable the discovery of other information stored in the database. DNS provides translation between the host name and IP address. DNS names are user-friendly, which means that they are easier to remember than IP addresses. The DNS names are presumed to remain more constant than IP addresses. An IP address for a server can change, but the server name remains the same in any system. The DNS allows users to connect to local servers using the same naming convention as the Internet. It uses UDP port no. 53.

Name Servers

1. Root Domains
   1. The root domain is at the top of the hierarchy and is represented as a period ( ).
   2. Organizations, including Network Solutions, Inc. manages the internet root domain.
   3. 13 Root Name Servers
2. Top-Level Domains
   1. There are two- or three-character name codes in Top-level domains.
   2. Top-level domains are presumed to be grouped in categories like organization type or geographic location.
3. Second Level Domains

Types of Name Servers

1. Top-level domain (TLD) servers: TLD is the one responsible for com, org, net,edu, etc, and all top-level country domains uk,fr, ca, jp.
   1. The Network solutions of the system maintain servers for com TLD
   2. Educational institutions use for edu TLD
2. Authoritative DNS servers: IP mappings for organization’s servers (e.g., Web and mail) from hostname, id is provided by the DNS servers as the organization's DNS servers
   1. Can be maintained by organization or service provider
3. Local DNS servers: It does not strictly belong to hierarchy
   1. It does not strictly belong to hierarchy
      Each ISP (residential ISP, company, university) has one.A query is sent to its local DNS server, whenever a host makes a DNS query. Here,Local DNS is also widely recognized as the “default name server” . Acts as a proxy, forwards query into the hierarchy.

DNS Services

DNS Services helps in hostname to IP address translation. It also performs host aliasing: Canonical(Real) and alias names. DNS services perform mail server aliasing. Load distribution: Replicated Web servers: it is a set of IP addresses for one canonical name of the system.

DNS Records

DNS: distributed database storing Resource Records (RR). The DNS record has following RR format.

1. Type=A

   1. name here representshostname used for the server
   2. valuerepresents the IP addressof the RR format

2. Type=NS

   1. name is domain (e.g. asdf.com)
   2. value used here is the IP address of all the authoritative name server that are used for this domain

3. Type=CNAME

   1. nameused here represents the name for some “canonical” name (the real name). For Example, www.ibm.com is really servereast.backup2.ibm.com
   2. value used here is a canonical name

4. Type=MX

   1. value is a name of mail server associated with the name

References:

1. A.S. Tanenbaum, “Computer Networks”, 3rd Edition, Prentice Hall India, 1997.
2. W. Stallings, “Data and Computer Communication”, Macmillan Press, 1989.
3. Kurose Ross, “Computer Networking: A top-down approach”, 2nd Edition, Pearson Education
4. Larry L. Peterson, Bruce S. Davie, “Computer Networks: A Systems Approach”, 3rd Edition, Morgan Kaufmann Publishers