What is meant by DNS? What does it contain?

DNS or domain name system is a well known distributed system which is quite hierarchical in nature and is used for the following:

1. Computers
2. Services
3. Resources that are connected to some private network or internet and so on.

What does DNS contain?

- With the aid of DNS, the domain names with various participating entities contains various information. 

- A domain name system is also known as domain name service and has taken up the responsibility of resolving the queries for the above discussed domain names into the corresponding IP addresses. 

- The basic purpose of this whole process is spotting the location of devices and computer services on the World Wide Web.

- The domain name system had lately become quite an essential part of the functionality of the internet because of the world wide service it provides regarding the distributed key word based redirection. 

- To put it simply it acts as a phone book in disguise for the internet. 

- It serves as a phone book in the way that it translates the human friendly computer host names in to their corresponding IP addresses. 

For example,

The domain name: www. Abc. Com translates in to the following IP address (say): 192. 0 . 34 . 11 (IPv4) and 2630 : 0 : 2c0 : 201 : : 10 (IPv6) etc.

- Though DNS serves all the purposes of an ideal phone book in terms of the internet, it differs from the phone book in one respect which is that the DNS can be frequently updated and these updates in turn can be distributed but in phone book these tasks cannot be performed so.

- With the help of such a process the location of a particular service on a network can be easily changed without having any affect on the end users who keep on continuing with the same host name. 

-This advantage is further reaped by the users while they recite the meaningful e- mail addresses as well as the URLs (uniform resource locators) without even knowing the way via which the services are actually located by the computers. 

- With the help of domain name system, each and every domain in the network is assigned with an appropriate domain name and this domain name is mapped to corresponding IP addresses through the designation of the authoritative name servers for each and every domain. 

- These authoritative name servers hold the responsibility of their particular domains and also it helps in assigning the sub domains with their respective authoritative name servers.

- Such a mechanism has helped a lot in making the domain name system quite fault tolerant and distributed. 

- This mechanism in another way has eliminated the requirement of a single central register to be used continually for updating and consultation. 

- There is one more additional feature of the domain name system which is that the responsibility of the updating and maintenance of the master record of the domains is distributed among many domain name registrars.

- These domain name registrars are known for their competition for the domain owner’s and end user’s business. 

- The facility of moving the domains from one registrar to another has been very well provided in the domain name system.

- The technical functionality of the data base service as well as the DNS specification is also specified by the domain name system.

- This DNS protocol is a kind of detailed specification of the communication exchanges and data structures that are used in the domain name system which in turn forms a very important part of the whole internet protocol suite. 

Overview of Network Time Protocol (NTP)

Networked computers share resources such as files. These shared resources often have time-stamps associated with them so it is important that computers communicating over networks, including the Internet, are synchronized. The Network Time Protocol (NTP) is an Internet Standard Recommended Protocol for communicating the Coordinated Universal Time (UTC) from special servers called time servers and synchronising computer clocks on an IP network.

The NTP daemon can not only adjust its own computer's system time. Additionally, each daemon can be a client, server, or peer for other NTP daemons:
- As client it queries the reference time from one or more servers.
- As server it makes its own time available as reference time for other clients.
As peer it compares its system time to other peers until all the peers finally agree about the "true" time to synchchronize to.

Clock Strategy

NTP uses a hierarchical, semi-layered system of levels of clock sources, each level of this hierarchy is termed a stratum and assigned a layer number starting with 0 (zero) at the top. The stratum level defines its distance from the reference clock and exists to prevent cyclical dependencies in the hierarchy.

Importance of NTP

In a commercial environment, accurate time stamps are essential to everything from maintaining and troubleshooting equipment and forensic analysis of distributed attacks, to resolving disputes among parties contesting a commercially valuable time-sensitive transaction.
In a programming environment, time stamps are usually used to determine what bits of code need to be rebuilt as part of a dependency checking process as they relate to other bits of code and the time stamps on them, and without good time stamps your entire development process can be brought to a complete standstill.
So, time is inherently important to the function of routers and networks. It provides the only frame of reference between all devices on the network. This makes synchronized time extremely important and this is where Network Time Protocol comes into picture.

Supported Platforms

NTP's native operating system is UNIX. Today, however, NTP runs under many UNIX-like systems. NTP v4 has also been ported to Windows and can be used under Windows NT, Windows 2000, and newer Windows versions up to Windows Vista and Windows 7.
The standard NTP distribution can not be run under Windows 9x/ME because there are some kernel features missing which are required for precision time keeping.

FTP - File Transfer Protocol

- File Transfer Protocol (FTP), a standard Internet protocol, is the simplest way to exchange files between computers on the Internet.
- FTP is an application protocol that uses the Internets TCP/IP protocols.
- FTP is commonly used to transfer Web page files from their creator to the computer that acts as their server for everyone on the Internet.
- FTP is also commonly used to download programs and other files to your computer from other servers.
- Web browser can also make FTP requests to download programs you select from a Web page.
- FTP can also be used to update (delete, rename, move, and copy) files at a server.
- FTP can be run in active mode or passive mode, which control how the second connection is opened.
- In active mode the client sends the server, the IP address port number, that the client will use for the data connection, and the server opens the connection.
- Passive mode was devised for use where the client is behind a firewall and unable to accept incoming TCP connections.

The objectives of FTP are :
- to promote sharing of files (computer programs and/or data),
- to encourage indirect or implicit (via programs) use of remote computers,
- to shield a user from variations in file storage systems among hosts, and
- to transfer data reliably and efficiently.

Anonymous FTP

Thousands of hosts on the Internet run ftp servers that permit guests to login. Such servers usually contain data and software of interest to the general public. They are often called anonymous ftp servers because the guest login name is anonymous. To login to an anonymous ftp server, enter the name anonymous when prompted for a username or userid. When prompted for a password, enter your full e-mail address, unless the on-screen instructions specify an alternative guest password.

RARP : Reverse Address Resolution Protocol

- RARP (Reverse Address Resolution Protocol) is a protocol by which a physical machine in a local area network can request to learn its IP address from a gateway server's Address Resolution Protocol (ARP) table or cache.
- A reverse address resolution protocol (RARP) is used for disk less computers to determine their IP address using the network. The RARP message format is very similar to the ARP format.
- When a new machine is set up, its RARP client program requests from the RARP server on the router to be sent its IP address.
- The RARP server will return the IP address to the machine which can store it for future use assuming that the entry has been put in the router table.
- RARP is available for Ethernet, Fiber Distributed-Data Interface, and Token Ring LANs.
- The 'operation' field in the RARP packet is used to differentiate between a RARP request and a RARP reply packet.
- Since a RARP request packet is a broadcast packet, it is received by all the hosts in the network. But only a RARP server processes a RARP request packet, all the other hosts discard the packet.
- The RARP reply packet is not broadcast, it is sent directly to the host, which sent the RARP request.

When a RARP server receives a RARP request packet, it performs the following steps:
- The MAC address in the request packet is looked up in the configuration file and
mapped to the corresponding IP address.
- If the mapping is not found, the packet is discarded.
- If the mapping is found, a RARP reply packet is generated with the MAC and IP
address. This packet is sent to the host, which originated the RARP request.

When a host receives a RARP reply packet, it gets its IP address from the packet and completes the booting process.

Serial Line Internet Protocol - SLIP protocol

The need for a data link layer protocol to let IP operate over serial links was identified very early on in the development of TCP/IP. To solve the problem they created a very simple protocol that would frame IP data grams for transmission across the serial line. This protocol is called the Serial Line Internet Protocol, or SLIP for short.
SLIP modifies a standard TCP/IP data gram by appending a special "SLIP END" character to it, which distinguishes data gram boundaries in the byte stream. SLIP requires a serial port configuration of 8 data bits, no parity, and either EIA hardware flow control, or CLOCAL mode (3-wire null-modem) UART operation settings.

- Serial Line Interface Protocol (SLIP) is a TCP/IP protocol used for
communication between two machines that are previously configured for communication with each other.
- The dial-up connection to the server is typically on a slower serial line rather than on the parallel or multiplex lines.
- SLIP does not provide error detection, being reliant on other high-layer protocols for this.
- A SLIP connection needs to have its IP address configuration set each time before it is established.
- The Serial Line Internet Protocol (SLIP) is a mostly obsolete encapsulation of the Internet Protocol designed to work over serial ports and modem connections.
- A version of SLIP with header compression is called CSLIP (Compressed SLIP).
- The Parallel Line Internet Protocol (PLIP) is very similar to SLIP, but works at higher speeds via a parallel port.
- SLIP is a STREAMS-based computer networking facility that provides for the transmission and reception of IP packets over serial lines.
- SLIP can be used to connect one host to another via a single, physical serial line connection between serial ports or over longer distances using a modem at each end of a telephone line.

Quick Tip: URLs - Uniform Resource Locator

URLs, or Uniform Resource Locators, are the method by which documents or data are addressed in the World Wide Web. The URL contains the following information:

- the protocol.
- the DNS name of the machine on which the page is located.
- the local name uniquely indicating the specific page.
- the location of the resource in the directory structure of the server.

To make a piece of text clickable, the page writer must provide two items of information : the clickable text to be displayed and the URL of the page to go to if the text is selected. Once the text is selected, the browser looks up the host name using DNS. Now armed with the host's IP address, the browser establish TCP connection to host. Over that connection, it sends the file name using the specified protocol.
The URL scheme is open to have protocols other than HTTP also. In short, URL's have been designed to not only allow users to navigate the Web, but to deal with FTP, news, Gopher, email, and telnet as well, making all the specialized user interface programs for those other services unnecessary, and thus integrating nearly all Internet access into a single program, the Web Browser.
The growing use of the Web has turned up an weakness in URL scheme. A URL points to one specific host. For pages that are heavily referenced, it is desirable to have multiple copies far apart, to reduce network traffic. The advent of systems such as Akami are meant to meet that need, distributing content over multiple servers on a global level.

WWW - The Server Side

For all the incoming connectionts from different clients, every website is associated with a server process listening to TCP port 80. The client sends a request after the connection is made and the server sends the reply and then the connection is released. The protocol that is responsible for requests and replies is called HTTP.

The steps that occur between the user clicking and a page being displayed are:
- The browser determines the URL.
- The browser asks DNS for IP address.
- DNS replies.
- Browser makes a TCP connection to the port.
- It then sendsthe GET command.
- The server sends the file.
- The TCP connection is released.
- The browser displays the text of the file.
- The browser fetchesand displays all images of the file.

Not all servers speak HTTP, Old servers use FTP, Gopher or other protocols. Given the number of different protocols, it was thought impractical to make browser understand different protocols. However, since there is a need to make information available (where the server talks in protocols other than HTTP), a solution was required. This solution is something called a proxy server. A proxy server takes a HTTP request from the browser and translates these requests into the FTP/Gopher/other protocols. The proxy server is a separate logical server.
A proxy server also serves to provide an important function called caching. Through caching, a proxy server keeps a local copy of the pages that pass through it. If a user requests for a page, if the page is present on the cache of the proxy server, it serves the page to the user. this way it serves to reduce load on final server.