DNS [Domain Name Service]
What is DNS?
Domain Name System (DNS) is a database system that translates a computer's fully qualified domain name into an IP address. Networked computers use IP addresses to locate and connect to each other, but IP addresses can be difficult for people to remember. For example, on the web, it's much easier to remember the domain name www.amazon.com than it is to remember its corresponding IP address (207.171.166.48). DNS allows you to connect to another networked computer or remote service by using its user-friendly domain name rather than its numerical IP address. Conversely, Reverse DNS (rDNS) translates an IP address into a domain name. Each organization that maintains a computer network will have at least one server handling DNS queries. This server, called a name server, will hold a list of all the IP addresses within its network, plus a cache of IP addresses for recently accessed computers outside the network. Each computer on each network needs to know the location of only one name server. When your computer requests an IP address, one of three things happens, depending on whether or not the requested IP address is within your local network:

• If the requested IP address is registered locally (i.e., it's within your organization's network), you'll receive a response directly from one of the local name servers listed in your workstation configuration. In this case, there usually is little or no wait for a response.
• If the requested IP address is not registered locally (i.e., outside your organization's network), but someone within your organization has recently requested the same IP address, then the local name server will retrieve the IP address from its cache. Again, there should be little or no wait for a response.
• If the requested IP address is not registered locally, and you are the first person to request information about this system in a certain period of time (ranging from 12 hours to one week), then the local name server will perform a search on behalf of your workstation. This search may involve querying two or more other name servers at potentially very remote locations. These queries can take anywhere from a second or two up to a minute (depending on how well connected you are to the remote network and how many intermediate name servers must be contacted). Sometimes, due to the lightweight protocol used for DNS, you may not receive a response. In these cases, your workstation or client software may continue to repeat the query until a response is received, or you may receive an error message.

When you use an application such as telnet to connect to another computer, you most likely type in the domain name rather than the IP address of that computer. The telnet application takes the domain name and uses one of the above methods to retrieve its corresponding IP address from the name server. A good analogy is to think of DNS as an electronic telephone book for a computer network. If you know the name of the computer in question, the name server will look up its IP address. Within most Internet applications, you will not see the IP address of the computer to which you're connecting. If you want to find the IP address of a particular computer, refer to the appropriate Knowledge Base document listed below:

DHCP
What is DHCP?
Dynamic Host Configuration Protocol (DHCP) is a network protocol that automatically assigns TCP/IP information to client machines. Each DHCP client connects to the centrally located DHCP server, which returns that client's network configuration (including the IP address, gateway, and DNS servers). DHCP uses a server that contains a list of available IP addresses. Instead of configuring an IP address in the workstation, you ask for an address when needed. Resolves issues of moving between subnets and reconfiguring workstations

Why Use DHCP?
DHCP is useful for automatic configuration of client network interfaces. When configuring the client system, the administrator chooses DHCP instead of specifying an IP address, netmask, gateway, or DNS servers. The client retrieves this information from the DHCP server. DHCP is also useful if an administrator wants to change the IP addresses of a large number of systems. Instead of reconfiguring all the systems, he can just edit one DHCP configuration file on the server for the new set of IP addresses. If the DNS servers for an organization changes, the changes are made on the DHCP server, not on the DHCP clients. When the administrator restarts the network or reboots the clients, the changes will go into effect.

Samba
What Is Samba?
Samba implements the CIFS network protocol. By supporting this protocol, Samba enables computers running Unix-based operating systems to communicate with Microsoft Windows and other CIFS-enabled clients and servers. Some examples of common services offered by Samba are:

• Share one or more directory trees
• Provide a Distributed Filesystem (MS-DFS) namespace
• Centrally manage printers, print settings, and their associated drivers for access from Windows clients
• Assist clients with network browsing
• Authenticate clients logging onto a Windows domain
• Provide or assist with Windows Internet Name Service (WINS) name-server resolution

The Samba suite also includes client tools that allow users on a Unix system to access folders and printers that Windows systems and Samba servers offer on the network.

What Can Samba Do for Me?
As explained earlier, Samba can help Windows and Unix computers coexist in the same network.[*] However, there are some specific reasons why you might want to set up a Samba server on your network:

• You do not needor wish to pay fora full-fledged Windows server, yet you need the file and print functionality that one provides.
• You want to provide a common area for data or user directories to transition from a Windows server to a Unix one, or vice versa.
• You want to share printers among Windows and Unix workstations.
• You want to integrate Unix and Windows authentication, maintaining a single database of user accounts that works with both systems.
• You want to network Unix, Windows, Macintosh (OS X), and other systems using a single protocol.

OpenLDAP
What is LDAP?
LDAP stands for Lightweight Directory Access Protocol. As the name suggests, it is a lightweight protocol for accessing directory services, specifically X.500-based directory services. LDAP runs over TCP/IP or other connection oriented transfer services. The nitty-gritty details of LDAP are defined in RFC2251 "The Lightweight Directory Access Protocol (v3)."

How does LDAP work?
LDAP directory service is based on a client-server model. One or more LDAP servers contain the data making up the directory information tree (DIT). The client connects to servers and asks it a question. The server responds with an answer and/or with a pointer to where the client can get additional information (typically, another LDAP server). No matter which LDAP server a client connects to, it sees the same view of the directory; a name presented to one LDAP server references the same entry it would at another LDAP server. This is an important feature of a global directory service, like LDAP.

Network File System (NFS)
What is Network File System?
Network File System easily allows to share data between several computers. For example, a user logged on a network won't need to log on a specific computer: via NFS, he will access his home directory (we say exported) on the machine he is working at. The first file servers were developed in the 1970s, and in 1985 Sun Microsystems created the file system called “Network File System” (NFS) which became the first widely used network file system.

How It Works?
NFS consists of at least two main parts: a server and one or more clients. The client remotely accesses the data that is stored on the server machine. In order for this to function properly a few processes have to be configured and running.
The server has to be running the following daemons:

Daemon Description
nfsd The NFS daemon which services requests from the NFS clients. Mountd The NFS mount daemon which carries out the requests that nfsd(8) passes on to it. Rpcbind This daemon allows NFS clients to discover which port the NFS server is using.

File Transfer Protocol (FTP)
What is FTP?
FTP, a standard protocol, is the simplest way to exchange files between computers on the Internet. Like the Hypertext Transfer Protocol (HTTP), which transfers displayable Web pages and related files, and the Simple Mail Transfer Protocol (SMTP), which transfers email, FTP is an application protocol that uses the Internet's TCP/IP protocols. FTP is commonly used to transfer Web page files from their creator to the computer that serves the pages to everyone on the Internet. It's also commonly used to download programs and other files to your computer from other servers. As a user, you can use FTP with a simple command line interface (for example, from the Windows MS-DOS Prompt window) or with a commercial program that offers a graphical user interface. Your Web browser can also make FTP requests to download programs you select from a Web page. Using FTP, you can also update (delete, rename, move, and copy) files at a server. You need to log on to an FTP server. FTP is usually provided as part of a suite of programs that come with TCP/IP.

Simple Mail Transfer Protocol (SMTP)
SMTP is a short for Simple Mail Transfer Protocol and it is used to transfer e-mail messages between computers. It is a text based protocol and in this, message text is specified along with the recipients of the message. Simple Mail Transfer Protocol is a 'push' protocol and it cannot be used to 'pull' the messages from the server. A procedure of queries and responses is used to send the message between the client and the server. An end user's e-mail client or the relaying server's Mail Transport Agents can act as an SMTP client which is used to initiate a TCP connection to the port 25 of the server. SMTP is used to send the message from the mail client to the mail server and an e-mail client using the POP or IMAP is used to retrieve the message from the server.

HTTP
HyperText Transfer Protocol, the underlying protocol used by the World Wide Web. HTTP defines how messages are formatted and transmitted, and what actions Web servers and browsers should take in response to various commands. For example, when you enter a URL in your browser, this actually sends an HTTP command to the Web server directing it to fetch and transmit the requested Web page. The other main standard that controls how the World Wide Web works is HTML, which covers how Web pages are formatted and displayed.

HTTP is called a stateless protocol because each command is executed independently, without any knowledge of the commands that came before it. This is the main reason that it is difficult to implement Web sites that react intelligently to user input. This shortcoming of HTTP is being addressed in a number of new technologies, including ActiveX,Java,JavaScript and cookies.

Using these servers we can configure our own organization network, for example we have explained 500-User organization network.

The 500 User Office



The above figure has contained 500-user organizatoin Network. This organization network have been configured by all the linux servers, [DNS, Samba, LDAP, DHCP, NFS, Firewall, Printer] Which we have explained adove in this documents. The PDC [Primary Domain Controller ] Server connecting internet through the firewall. This 500-user large network has divided in three sections, these sections have a different network IP ranges address. The 300 users Property Insurance gruop network has configured 172.16.0.0/22 range network. The 50 user Accounting network has configued 172.16.4.0/22 range netowrk. The 150 user Financial Services network has configured 192.16.8.0/22 range network.