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Domain Network Services (DNS)

By vikas jagtap

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Domain Network Services (DNS)

7.1 Domain Names

7.2 Authoritative Hosts

7.3 Delegating Authority

7.4 Resource Record

7.5 SOA records

7.6 DNS protocol

7.7 DHCP & Scope Resolution

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Computers are designed to work with numbers, while humans are more comfortable working with words.

A mechanism that would permit users to refer to the network’s computers by names, rather than by address called as ‘Domain Name System’

IP address (32-bit) is Internet Protocol Address to identify machines in a network. Although such addresses provide a convenient, compact representation for specifying the source and destination in packets sent across an internet, user prefer to assign machines pronounceable, easily remembered names called as ‘Domain Name.’

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The DNS name space is based on a domains, which exist in a hierarchical structure much like the directory tree in a file system.

A domain is the equivalent of a directory, in that it can contain either subdomains (subdirectories) or hosts (files), forming a structure called DNS tree.

Root

com net org

wwwwww www www www

Fig. – The Domain Name System (Tree structure)

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The DNS name space function in the same way : administrators are assigned domain names and are then responsible for specifying host names to systems within that domain.

The result is that every computer on the Internet is uniquely identifiable by a DNS, name that consists of host name plus the names of all its parent domains, stretching up to the root of the DNS tree, separated by periods.

Each of the names between the periods can be up to 63 characters long, with a total length of 255 characters for a complete DNS name.

Domain and host names are not case sensitive, and can take any value except the null value.

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The naming scheme used in the Internet is called the Domain Name System (DNS).

Each computer name consists of a sequence of alpha-numeric segments separated by periods. For example, a computer in the computer science department at Pune University has the domain name : -

comp1.cs.pune.edu

The left-most segment if a name(comp1) is the name of an individual computer. Other segment in a domain name identify the group that owns the name. For example, the segment pune gives the name of the university.

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The domain name system does not specify an exact number of segments in each name nor does it specify what those segments represent.

The domain name system does specify values for the most significant segment, which is called a top-level domain (TLD).

Following table lists the top-level domains of the DNS :

Domain Name Assigned to

com Commercial Organization

edu Educational Institution

gov US government

org Non-commercial organization

mil US military

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Top – level domains are controlled by the Internet Corporation for Assigned Names and Numbers (ICANN). ICANN designates one or more domain registrars to administer a given top-level domain and approve specific names.

Top-level domains are important because each organization that choose to participate in the domain name system must apply for a name under one of the existing top level domains. Most corporations choose to register under the com domain.

To obtain a domain, an organization must register with an approved register. A unique domain suffix is assigned to each organization.

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The partitioning of a namespace must be defined in a way that support efficient name mapping and guarantee autonomous control of name assignment.

The namespace is partitioned at the top level, and authority for names is subdivisions is passed to designed agents. For example, one might choose to partition the namespace based on site name and to delegate to each site responsibility for maintaining names within its partition.

The topmost level of the hierarchy divides the namespace and delegates authority for each division.

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As an example, consider a namespace with names of the form :

local.site

Where site is the site name authorized by the central authority, local is the part of a name controlled by the site, and the period (“.”) is a delimiter used to separate them.

When the topmost authority approves adding a new site, X, it adds X to the list of valid sites and delegates to site X authority for all names that end in “.X”

Keep subdividing the namespace until each subdivision is small enough to be manageable. Subdividing the namespace introduces another partition of the name.

e.g.- local.group.site

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Thus, Domain name is something like a postal address, in which the top-level domain is the equivalent of the state, the second –level domain is the city, and the host name is the street address.

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Domain Name begins with WWW –

Although a domain name denotes a computer, many organizations assign domain names that reflect the service the computer provides.

For example – If Foobar corporation provides the FTP service, it can choose a computer to run the service, and assign the domain name:

ftp.foobar.com

Similarly a compute that runs a Web server, can be assigned the name:

www.foobar.com

The computer’s domain name does not need to contain www.

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DNS servers are basically database servers that store information about the hosts and subdomain for which they are responsible in resource records (RPs). When you run your own DNS server, you create a resource record for each host name that you want to be accessible by the rest of the network.

There are several different types of resource records used by DNS servers, the most important of which are as follows:

SOA (Start of Authority) –

Indicates that the server is the best authoritative source for data concerning the zone. Each zone must have an SOA record, and only one SOA record can be in a zone.

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NS (Name Server) –

Identifies a DNS server functioning as an authority for the zone. Each DNS server in the zone must be represented by an NS record.

A (Address) –

Provides a name-to-address mapping that supplies an IP address for a specific DNS name. This record type performs the primary function of the DNS, converting names to addresses.

PTR –

Provides an address-to-name mapping. This is the functional opposite of an A record, used for reverse lookups only.

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Dynamic Host Configuration Protocol (DHCP) is designed to handle automated address assignment.

DHCP allows a computer to obtain an IP address quickly and dynamically. To use DHCP’s dynamic address allocation mechanism, a manager must configure a DHCP server by supplying a set of IP addresses.]

Whenever a new computer connects to the network, the new computer contacts the server & request an address. The server chooses one of the address the manager specified, and allocates that address to the computer.

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DHCP allows 3 types of address assignment –

1) Manual Configuration –

In this configuration a manager can configure a specific address for a specific computer.

2) Automatic Configuration –

In this configuration a manager allows a DHCP server to assign a permanent address when a computer first attaches to the network.

3) Dynamic Configuration –

In this configuration a server “loans” an address to a computer for a limited time.

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DHCP uses the identify of the client to decide how to proceed. When a client contacts a DHCP server, the client sends an identifier, usually the client’s hardware address.

The server uses the client’s identifier and the network to which the client has connected to determine how to assign the client & IP address.

Thus, a manager has complete control over how address are assigned.

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When you type a URL containing a DNS name (such as www.microsoft.com ) into the browser’s address field and press the ENTER key, if you look quickly at the status bar in the lower-left corner, you’ll see a message that says “Finding Site: www.microsoft.com.” In a few seconds, you’ll then see a message that says “Connecting to,” followed by an IP address. It is during this interval between the Finding Site message and the Connecting to message that the DNS name resolution process occurs.

From the client’s perspective, the procedure that occurs during these few seconds consists of the application sending a query message to its destination DNS server that contains the name

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If you connect to the Internet, you use a DNS server each time you enter a server name or URL into a Web browser or other application to resolve the name of the system you specified into an IP address.

When a standalone computer connects to an Internet service provider (ISP), the ISP’s server usually supplies the addresses of the DNS servers that the system will use.

TCP/IP communications are based solely on IP addresses. Before one system can communicate with another, it must know its IP address. Often, the user supplies a friendly name (such as DNS name) for a desired server to a client application. The application must then resolve that server name into an IP address before it can transmit a message to it. If the name resolution mechanism fails to function, no communication with the server is possible.