Telnet ssh
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Telnet/SSH. Tim Jansen, Mike Stanislawski. TELNET is short for Terminal Network Enables the establishment of a connection to a remote system, so that the local terminal appears to be the terminal at the remote location. Usually establishes its connection on port 23. Local log-in.

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Telnet ssh


Tim Jansen, Mike Stanislawski

  • TELNET is short for Terminal Network

  • Enables the establishment of a connection to a remote system, so that the local terminal appears to be the terminal at the remote location.

  • Usually establishes its connection on port 23

Local log in
Local log-in

  • Log-in is performed with a username and a password.

  • Keystrokes accepted at the terminal driver and passed to the operating system.

  • Operating system interprets the keystrokes and performs an action

Remote log in
Remote Log-in

  • Terminal Driver accepts keystrokes and passes them to the TELNET client

  • Client transforms characters to network virtual terminal (NVT) character and delivers them to TCP/IP protocol stack

  • Passed online to the server’s TCP/IP stack and then to the TELNET server which deciphers the NVT characters

  • The decoded characters are passed to a pseudoterminal driver because the Operating System is only designed to receive characters from a terminal driver.


  • Uses two sets of characters one for data and the other for control

  • Data characters are represented with the same 7 lowest-order bits as ASCII and the highest-order bit is 0.

  • Control characters highest-order bit is 1


  • The same connection is used for sending both data and control characters.

  • Control characters are embedded in the data stream.

  • A control character is recognized by a preceding character of 11111111, this is the IAC (Interpret as control) character.


  • Options are extra features available to a user with a more sophisticated terminal.

  • Simple terminals use default features.

  • Option negotiation entails using four control characters, WILL, WONT, DO, DONT

  • WILL is sent as a question “Will I enable the option?

  • DO is sent as a request “Do enable the request”

  • WONT is sent to say “I won’t use this option anymore”

  • DONT is sent to say “Don’t use it anymore”


  • Default Mode – used if no other modes are invoked.

  • Character Mode – each character is sent to the server, but the server may delay sending back the information. Creates overhead because 3 TCP segments are sent per character.

  • Line Mode – line editing is done by the client, and then sent as a whole line to the server.


  • TELNET is unencrypted, making it very easy to intercept.

  • Most implementations have no authentication to ensure communication is carried out.

Telnet today

  • TELNET has in the past few years been overtaken by remote login

  • It is still used in such ways as debugging network services.


  • SSH is short for Secure Shell

  • Unlike TELNET, SSH uses a form of encryption

  • The encryption is public-key

Uses of ssh
Uses of SSH

  • For remote login

  • Executing single lines on a remote host

  • Copying files from a local server to a remote host

  • As a secure alternative to FTP

Transport layer
Transport Layer

  • Handles initial key exchange

  • Sets up encryption and compression

  • Also arranges for key re-exchange after an hour or 1GB is transferred.

User authentication layer
User Authentication Layer

  • Handles client authentication

  • Password authentication

  • Public key authentication

  • And others

Connection layer
Connection Layer

  • Defines channel requests

  • Multiple channels can be hosted at once

  • Direct-TCP/IP handles client-to-server forwarded connections

  • Forwarded-TCP/IP handles server-to-client forwarded connections


  • Defends against

    • IP spoofing, where a remote host sends out packets which pretend to come from another, trusted host. Ssh even protects against a spoofer on the local network, who can pretend he is your router to the outside.

    • IP source routing, where a host can pretend that an IP packet comes from another, trusted host. DNS spoofing

    • DNS spoofing, where an attacker forges name server records

Security cont
Security cont.

  • Also protects against

    • Interception of cleartext passwords and other data by intermediate hosts

    • Manipulation of data by people in control of intermediate hosts

  • In short, SSH trusts nothing

  • SSH is secure assuming the option of encryption “none” is not chosen