Remote Login TELNET, SSH

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Remote Login(TELNET, SSH)

• Chapter 24

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Remote Interactive Computing

• TCP can be used for interactive use of remote
  machines
• User establish remote login session
• Then executes commands
• Remote login may not be simple
• Most systems designed to expect direct login
• May have to modify the machines OS
• Building interactive client may be difficult, too
• Passing on Control-C commands

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TELNET Protocol

• TCP/IP suite includes TELNET
• Simple remote terminal protocol
• Allows user to log into computer over internet
• Establishes TCP connection
• Passes keystrokes to remote machine
• Carries output back to users screen
• Transparent
• Gives appearance that user keyboard and display
  is directly connected to the remote machine

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• TELNET is not sophisticated as some
• Is widely available
• ID remote machine by domain name or IP address
• Offers three basic services
• Defines network virtual terminal
• Clients only have to build to the standard
  interface
• Allows negotiation of some standard options
• Treats both ends of connection symmetrically
• Does not force keyboard input or screen output

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Figure 24.1
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• Server is more complex than shown
• Must handle multiple, concurrent connections
• TELNET server consists of
• Master server that listens for new requests
• Slave that handles one particular connection
• Pseudo terminal
• OS entry point
• Allows application to transfer characters to OS
  as if they came from a keyboard
• Cannot build TELNET server without it

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• Adv of TELNET server as application pgm
• Modification control of server easier
• Since code is not part of OS
• Disadv
• Inefficiency
• Each keystroke
• User keyboard ? OS ? Client program ?OS ?Across
  internet
• Then, Servers OS ?Application Program
• Output comes back over same path
• Each keystroke requires several context switches
• Expensive practical due to slow typing speeds

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Accommodating Heterogeneity

• Computers and OSs 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)

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• Use network virtual terminal
• Defines how data and command sequences are sent

Figure 24.2
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• 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

Figure 24.3
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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
• Versus just a secure remote login service

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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 users 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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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 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

• 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