DNS (Domain Name System)

Note

Integrating a DNS Server Running Berkeley Internet Name Domain (BIND)

If you want to use a BIND DNS server to support Active Directory. It must be running version 8.12 or latter of the BIND software. Previous versions of BIND do not support the requirement for active directory.

How many root DNS servers are available in the world?

13 Root Servers are there.

What is DNS?

DNS stands for Domain Name System; this service is responsible for providing the hostname to IP address and IP address to hostname. DNS also interact with the AD for locate the servers that are providing the particular service to the clients.

What is the difference between DNS/WINS?

WINS and DNS are both name resolution Services.

DNS maps Hostnames to IP Address

Wins maps NetBIOS names to IP Address.

Hostnames can upto 255 characters in length by definition, while a NetBIOS name

Is a 16 byte address, the first 15 characters of your hostname plus an additional Character to identify a service?

DNS is the primary name resolution method in an Active Directory domain (2000 and

2003). Clients are configured with the address of a DNS server in their TCP/IP properties (manually or through DHCP). The clients then register their host names and IP addresses with this server, and domain controllers also register a list of the

Services that they offer (service records).Clients can then query DNS for a list of available domain controllers, or they can query DNS for the name or IP address of a PC to facilitate making a connection to it. You cannot install Active Directory without having DNS. DNS is that important...

WINS was the primary name resolution method before the introduction of Active Directory and Windows 2000. So networks that include older clients, find that they may still need to setup a WINS server on their network.

Resolving Host Names:-

On a computer running windows 2000 the table is named hosts, and it is located in the %System Root%\system32\drivers\etc folder.

What is Start of Authority (SOA)?

The SOA resource record identifies which name server is the authoritative source of information for data within this domain.
Name Server (NS):

The NS resource record identifies the name server that is the authority for the particular zone or domain.

Host (A):

The A resource records is the fundamental data unit of the DNS. This resource record has a single address field that contains the IP address associated with the system identified in the Name Field. Host resource records provide the name to IP mappings that DNS name servers use to perform name resolution.

Alias (CNAME)

The canonical name (CNAME) resource record is used to specify an alias or alternative name for the system specified in the name field. The resource record contains a single CNAME field that holds another name in the standard DNS naming format. You create CNAME resource records to use more than one name to point to a single IP address. For example you can host a file Transfer Protocol (FTP) server such as ftp.test.com and a web server such as a www.test.com on the same computer by creating an A record in the test.com domain for the host name www and a CNAME record equating the host name FTP with the A record for www.

Host Information (HINFO)

The HINFO resource record contains two fields called CPU and OS which contains values identifying the processor type and operating system used by the listed host. You can use this record type as a low-cast resource tracking tool.

Mail Exchange (MX):

A secondary but crucial function of the DNS is the direction of e-mail message to the appropriate mail server. The resource records in general use for e-mail transmission in the MX record.

The resource records contains two fields

Called Preference

The Preference field contains an integer value that indicates the relative priority of the resource records compared to other of the same type and class in the same domain the lower value higher priority. 

Exchange

The Exchange field contains the name of a computer that is capable of acting as an

E-mail server for the domain specified in the name field.

MX Record is the Main Exchanger Record. This is a service record which specifies where the mails for a particular domain are to be delivered. There can be more than one MX record for a given Domain. This is to have one or more backup servers to which the mails can be delivered in the event the default server has failed for some reason. The MX records have a field for Priority number. This is mainly to specify which is the first server to which a Domains emails should be delivered and which falls next in the queue in the event the 1st server is down and thereon. The server which has the lowest priority number will be the default server to which the mail is delivered. The next lower priority server will be the 1st backup and the next the 2nd backup server and so on. CNAME Syntax: Itsyourip.com. IN MX 0 mail.pcsupport.x-host.uni.cc Where, IN indicates Internet MX indicates the Mail Exchanger Record The above indicates that mails for itsyourip.com are being delivered to the server mail.pcsupport.x-host.uni.cc. Wilcards can be used in the MX records to allow mails for all the hosts to be delivered to a particular host. For example, *. Itsyourip.com. IN MX 0 mail.pcsupport.x-host.uni.cc indicates that all emails for all the hosts (or sub domains) of itsyourip.com will be delivered to mail.pcsupport.x-host.uni.cc

Pointer (PTR)

The PTR resource record is the functional opposite of the A record providing an IP to Name mapping for the system identified in the name field using the in-addr-arpa domain name.

The PTR resource record contains a single PTR Name field which contains the FQDN of the system identified by the IP address in the name field.

When you create the appropriate reverse lookup zone on your DNS server you can create PTR resource records automatically with you’re A records.

Service (SRV)

The service resource (SRV) record enables clients to locate servers that are providing a particular service.

Name Caching

The other mechanism that speeds up the DNS name resolution process is name caching. Most DNS server implementation a cache of information they receive from other DNS servers.

Recursive vs. Iterative Queries

Contrasting Iterative and Recursive Queries:

         To help explain the difference between these methods, let’s take a slide trip to a real world case. Suppose you are trying to find phone number of your old friend carol, with whom you have not spoken in years, you call your friend Joe; he does not have carol’s number, but he gives you John’s number, suggesting you call him. So you dial up john; he does not have the information but he knows the number of carol’s best friend, Debbie, and gives that to you. You call Debbie and she gives you carol’s information. This is an example of an Iterative process. In contrast, suppose you called Joe and Joe said,” I’ don’t know, but I think I know how to find out”. He called John and Debbie and called you back with phone number. That would be like Recursive Resolution.

Key Concept:- 

   The two method of Name resolution in DNS are Iterative resolution and recursive resolution in Iterative resolution. If a client sends a request to a name server that does not have the information the client needs, the server returns a pointer to a different name server and the client sends a new request to that server.

    In recursive resolution, if a client sends as request to a server that does not have the requested information, that server takes on the responsibility for sending requests to other servers to find the necessary records the returns them to the client. A server doing this takes on the role client for its requests to other servers.

Recursive query

Ask server to get answer for you

E.g., request 1 and response 8

Iterative query

Ask server who to ask next

E.g., all other request-response pairs