The Domain Name System (DNS) is a distributed database, arranged hierarchically, containing records for domain names. The DNS system's main aim is to match a domain name to an IP Address. In order to fulfill this role, the DNS Server contains Resource Records (Records) in a Zone File, which contains the domain name and IP address mappings for computers contained within that Zone. All Resource Records have a Time To Live TTL (TTL), specifying the number of seconds other DNS servers and applications are allowed to cache the Record.

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Address (IPv4 A) Record (Anchor: defa): These are used to translate domain names into IP addresses.

AAAA (IPv6) Record (Anchor: defaaaa): The IPv6 Address Record is a much larger address space than that of a IPv4 Address Record. Addresses in IPv6 Address Records are 128 bits long while those in IPv4 Address Records are 32 bits long.

Note :
When you host your domain name with External Web Hosting company, you will be provided with either an IPv4 Address or an IPv6 Address. This needs to be set as an A Record or an AAAA Record respectively, for that particular domain name.

Mail Exchanger (MX) Record (Anchor: defmx): An MX Record identifies the email server(s) responsible for a domain name. When sending an email to user@xyz.com, your email server must first looks up the MX Record for xyz.com to see which email server actually handles email for xyz.com (this could be mail.xyz.com or someone else's email server like mail.isp.com). Then it looks up the A Record for the email server to connect to its IP address.

An MX Record has a Preference number, indicating the order in which the email server should be used. Email servers will attempt to deliver email to the server with the lowest preference number first, and if unsuccessful continue with the next lowest and so on.

Canonical Name (CNAME) Record (Anchor: defcname): CNAME Records are domain name aliases. Often computers on the Internet have multiple functions such as Web Server, FTP Server, Chat Server, etc.. To mask this, CNAME Records can be used, to give a single computer multiple names (aliases).

Sometimes companies register multiple domain names for their brand-names but still wish to maintain a single website. In such cases, a CNAME Record maybe used to forward traffic to their actual website.

Example:
www.abc.in could be CNAME to www.abc.com.

The most popular use of the CNAME Record, is to provide access to a Web Server using both the standard www.yourdomainname.com and yourdomainname.com (without the www). This is usually done by adding a CNAME Record for the www name pointing to the short name [while creating an A Record for the shorter name (without www)].

CNAME Records can also be used when a computer or service needs to be renamed, to temporarily allow access through both the old and new name.

Name Server (NS) Record (Anchor: defns): NS Records identify the DNS servers responsible (authoritative) for a Zone. A Zone should contain one NS Record for each of its own DNS servers (primary and secondary). This mostly is used for Zone Transfer purposes (notify). These NS Records have the same name as the Zone in which they are located.

The most important function of the NS Record is Delegation. Delegation implies that part of a domain is delegated to other DNS servers.

You can also delegate sub-domains of your own domain name (such as subdomain.yourdomainname.com) to other DNS servers. An NS Record identifies the name of a DNS server, not the IP Address. Because of this, it is important that an A Record for the referenced DNS server exists, otherwise there may not be any way to find that DNS server and communicate with it.

If an NS Record delegates a sub-domain (subdomain.yourdomainname.com)to a DNS Server with a name in that sub-domain (ns1.subdomain.yourdomainname.com), an A Record for that server (ns1.subdomain.yourdomainname.com) must exist in the Parent Zone (yourdomainname.com). This A Record is referred to as a Glue Record, because it doesn't really belong in the Parent Zone, but is necessary to locate the DNS Server for the delegated sub-domain.

Text (TXT) Records (Anchor: deftxt): TXT Records provide the ability to associate some text with a domain or a sub-domain. This text is meant to strictly provide information and has no functionality as such. A TXT Record can store upto 255 characters of free form text. This Record is generally used to convey information about the zone. Multiple TXT Records are permitted but their order is not necessarily retained.

Example:
You may add a TXT Record for yourdomainname.com with the value as This is my email server. Here, if anybody was checking the TXT Records of yourdomainname.com, would notice the above text appearing in the TXT Record.

TXT Record can be used to implement the following:

Sender Policy Framework (SPF): Sender Policy Framework is an extension to the Simple Mail Transfer Protocol (SMTP). SPF allows software to identify and reject forged addresses in the SMTP Mail From (Return-Path). SPF allows the owner of a domain to specify their mail sending policy, e.g. which mail servers they use to send mail from their domain name. 1. The technology requires two sides to work in tandem:

The domain owner publishes this information in an TXT Record in the domain's DNS zone, and when someone else's email server receives a message claiming to come from that domain, then

the receiving server can check whether the message complies with the domain's stated policy. If, for example, the message comes from an unknown server, it can be considered a fake.

DomainKeys: DomainKeys is an email authentication system (developed at Yahoo!) designed to verify the authenticity of the email sender and the message integrity (i.e., the message was not altered during transit). The DomainKeys specification has adopted aspects of Identified Internet Mail to create an enhanced protocol called DomainKeys Identified Mail (DKIM). 2

Service (SRV) Record (Anchor: defsrv): An SRV or Service Record is a category of data in the DNS specifying information on available services. When looking up for a service, you must first lookup the SRV Record for the service to see which server actually handles it. Then it looks up the Address Record for the server to connect to its IP Address.

The SRV Record has a priority field similar to an MX Record's priority value. Clients always use the SRV Record with the lowest priority value first, and only fall back to other SRV Records if the connection with this Record's host fails. If a service has multiple SRV records with the same priority value, clients use the weight field to determine which host to use. The weight value is relevant only in relation to other weight values for the service, and only among SRV Records with the same priority value.

Newer Internet Protocols such as SIP (Session Initiation Protocol) and XMPP (Extensible Messaging and Presence Protocol) often require SRV support from clients.

Start of Authority (SOA) Record (Anchor: defsoa): Each Zone contains a single SOA Record, which holds the following values for the Zone:

Name of Primary DNS Server: The domain name of the Primary DNS server for the Zone. The Zone should contain a matching NS Record.

Mailbox of the Responsible Person: The email address of the person responsible for maintenance of the Zone.

Serial Number: Used by the Secondary DNS servers to check if the Zone has changed. If the Serial Number is higher than what the Secondary server has, a Zone Transfer will be initiated. This number is automatically increased by our DNS servers when changes to the Zone or its Records are made.

Refresh Interval: How often the Secondary DNS servers should check if changes are made to the zone.

Retry Interval: How often the Secondary DNS server should retry checking, if changes are made if the first refresh fails.

Expire Interval: How long the Zone will be valid after a refresh. Secondary servers will discard the Zone if no refresh could be made within this interval.

Minimum (Default) TTL: Used as the default TTL for new Records created within the zone. Also used by other DNS servers to cache negative responses (such as Record does not exist, etc.).

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