Saturday, December 20, 2014

DNS

DNS ( Domain Name System)


Main Function of DNS is for Name Resolution. If you are trying to browse any web site on internet, you just type it's name. It is very easy to remember server from its name like WWW.Google.Com rather than its IP like 17.54.32.2. But for your Computer to connect to WWW.Google.Com it requires its IP address and same thing is performed by DNS. You just type web address and DNS just connect it resolving to its IP.


Hierarchical Namespace:-

DNS uses Hierarchical Namespace to locate computers. The Root Domain is represented by (.). The Root Domain is beginning of DNS Namespace and entire Namespace is located under Root. After Root at next stage it comes First-Level Domains (.Com, .Org). After that it comes Second-Level Domains,  which usually refers to Company Name that must register with Internet Authority. Under Second Level Domains are Sub Domains. It refers to department or division of that particular Company. These Sub Domains are are register and manage on DNS servers that contains about information of Second-Level Domains. Hierarchical Namespace is nothing but FQDN ( Fully Qualified Domain Name) WWW.Google.Com


Name Resolution Process:- 

DNS Hierarchical Namespace and distributed Database are used when Client tries to browse IP address of Internet resource. 


Resource Record:- 

The actual records that are stored in DNS zone files are called Resource Records. It contains actual information about Domain. The Most Common Resource Records are:-

# SOA Start Of Authority - It Identifies the Primary Name Server for Zone. Also sets parameters of Zone such as default settings for Zone transfers, expiration time on Zone information and Time To Live (TTL).

# Host A -  Identifies IP Address of particular Host Name. This is record that DNS server returns during Name resolution. 

# Mail Exchanger MX - This record is created if we are using Exchange Server in our Environment.

# Name Server NS -  Identifies all of Name Servers for Domain.

# Pointer (PTR) - Identifies Host Name mapped to specific IP Address. This records are stored in Reverse Lookup Zone.

# Canonical Name (CNAME) - Identifies alias for another host in Domain. This is used when more than one Host Name uses the same IP.

# Service Locator (SRV) - Identifies a Service that is available in Domain. AD makes extensive use of SRV to locate Domain Controllers.


Zones:-

Their are two main types of zones used in DNS. Forward Lookup Zone and Reverse Lookup Zone.


# Forward Lookup Zone - Forward Lookup Zone is used primarily to resolve Host Name to IP. The Host (A) records provide this information. This Zone also includes records like SOA and NS and may also include MX records, CNAME records, and SRV records. The Forward lookup zone is used when a client resolver queries the DNS server to locate IP Address of server on network.

# Reverse Lookup Zone - A Reverse Lookup Zone is used to resolve IP Address to Host Vice versa of Forward Lookup Zone. This Zone also have SOA and NS records but rest of records are PTR records. PTR record format is same as Host record format but it provides answer for Reverse Lookup Zone.


Primary Name Servers:-

Primary Name Server is only server with a writable copy of Zone Files. Zone on Primary Name Server is called Primary Zone. DNS administrator must have access to Primary Name Server whenever any changes need to make to ZOne information. After changed have made same information is getting replicated to Secondary Name Servers and this process is called Zone Transfer.


Secondary Name Servers:-

Secondary Name Server has read only copy of Zone files. Zone information on this server can be updated from through Zone transfer from Primary Name Server.


Stub Zones:-


Stub Zones are used for easy way of Name Resolution across multiple namespace. A Stub Zone is similar to Secondary Zone. Configuring Stub Zone you must specify IP of Primary name server for the Zone. 

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