Sub topic : Remote Login

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Objectives

On completion of this period, you would be able to understand about

• TELNET

• Secure Shell ( SSH )

• Virtual network computing

• Rlogin

• Remote Desktop Protocol (RDP)

Remote login-TELNET Protocol

• Remote login permits a user to connect to an account on a remote machine

• TELNET is the standard Internet application protocol for remote login

• A client program running on the user's machine communicates using the TELNET protocol with a server program running on the remote machine

• TELNET provides the encoding rules to link a user's keyboard/display on a client ("user") system with a command interpreter on a remote server system

• A subset of the TELNET protocol is also incorporated within other application protocols, e.g., FTP and SMTP

TELNET Application

• The user (say Ram) has an account on both the local and remote machines

• For example, Ram on apt.ac.in types telnet exam.dteap.ac.in at his user prompt

• Here, apt.ac.in is the client and exam.dteap.ac.in is the server

• The TELNET client would perform a methotrexate() call to determine the IP address of exam.dteap.ac.in

• Then the client would create a socket to communicate with the Telnet server

TELNET Application contd…

• The server prompts Ram for a login identifier - the name of the user's account on the remote server followed by a password

• The TELNET users interact with the remote machine in the same way they would interact with their local machine

• The client relays Ram's keystrokes to the remote server, and the remote server displays them on its pseudo terminal (which is actually the display screen on the client machine)

TELNET Program

• The TELNET client program performs two important functions - interacting with the user terminal on the local host and,

- exchanging messages with the TELNAT server• The client connects to port 23 on the remote machine,

which is the port number reserved for Telnet servers• The TCP connection persists for the duration of the login

session• Since Telnet is designed to work over two hosts

on different platforms, the protocol assumes that the two hosts run a Network Virtual Terminal (NVT)

TELNET Program contd…

• The TCP connection is set up across these two NVT terminals. The NVT is a very simple character device with a keyboard and a printer

• Data typed by the user on the keyboard is translated by the client software into NVT format and sent via its NVT terminal to the server. Data received in NVT format from the server is translated by the client into the local machine format and output to the printer

• The NVT terminals on the two hosts exchange data in the 7-bit U.S. variant of the ASCII format

TELNET- Data Flow• The figure 1 and Figure 2 describes the path of data in a

TELNET remote terminal session as it travels from the user's keyboard to the remote operating system

• User types the commands or enters data through terminal key board

• The TELNET client translates characters (data or commands) that come from the local terminal into Network Virtual Terminal (NVT) characters with the help a universal interface called the Network Virtual Terminal

• The NVT defines how data and commands are sent across the Internet

• TELNET Client sends the data/commands over the TCP connection to the TELNET Server

• The TELNET server translates data and commands from NVT form into the form acceptable by the remote computer

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Remote Login-TELNET Data Flow

Fig .1

Fig .2

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1111

• Disadvantages

– Inefficiency

– Each keystroke passes through

– User keyboard OS Client program OS Across

internet

– Then, Server’s OS Application Program

– Output comes back over same path

– Each keystroke requires several context switches

– Expensive; practical due to slow typing speeds

Disadvantages of TELNET

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Diagram Illustrating Local Login

Fig .3

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Some of the TELNET Interface Commands

Table - 1

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TELNET - Accommodating Heterogeneity

• Computers and Operating Systems are different

– Ending lines of text

Some require termination by CR

Others require LF

Other require two characters: CR-LF

– Key to interrupt running program

Most interactive systems provide a way

Specific keystroke is different (Control-C or ESC)

• TELNET takes care of heterogeneity with the help of

NVT

Network Virtual Terminal (NVT)

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• Defines how data and command sequences are sent

Fig 4: Use of NVT format by TELNET

NVT Definition Fairly Straightforward• Communication involves 8 bits

• Use USASCII 7-bit code for data• Command sequences have high order bit set• 95 printable letters, digits, punctuation marks• 33 control codes

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Remote Login- Secure Shell (SSH)

• Popular alternative to TELNET

• Uses TCP to connect remotely, like TELNET

• Two significant enhancements over TELNET

– Provides secure communications

– Can perform additional, independent data transfers

over same connection used for remote login

– Uses public key cryptography

– Really a general purpose secure connection

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Remote Login- Rlogin (BSD UNIX)

• BSD UNIX operating systems include remote login service

called rlogin

• Not a general purpose protocol like TELNET

• Protocol understands computing environments

• Exports part of user’s environment to remote machine

• Supports trusted hosts

• Administrator can choose set of machines over which login

names & file access protections are shared

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Remote Login- Virtual Network Computing (VNC)

• Provides remote desktop capability

• See exact copy of desktop on another computer

• Use keyboard, mouse to interact with remote machine

• Runs across multiple platforms

• Linux, Windows, etc

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Remote Login - Remote Desktop Protocol (RDP)

• Defined by Microsoft for their OS

• Similar to other remote desktop systems

• See exact copy of remote desktop

• Can be used across software platforms

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Summary

In this class, you have learnt about• Application level protocols can be built on TCP/IP services &

client-server model• TELNET

• TCP/IP internet standard• Widely available remote access system

• Secure Shell• Authenticated, confidential remote login• Multiple applications can share SSH connection

• Other remote access systems• rlogin, VNC, and RDP

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Quiz

• Authenticated, confidential remote login is TELNET/SSH• A:SSH

• What is popular alternative to TELNET• A:SSH

• What is meant by rlogin? • A: BSD UNIX operating systems include remote service called

rlogin

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Frequently Asked Questions

1. What are the advantages and disadvantages of TELNET

2. What are the functions of virtual network computing

3. Explain about Remote login