telnet
TRANSCRIPT
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TCP/IP Protocol Suite 1
Upon completion you will be able to:
Remote Login:Remote Login:TelnetTelnet
• Understand how TELNET works• Understand the role of NVT in a TELNET communication• Understand TELNET option and suboption negotiation• Know how control characters are used• Know the TELNET modes of operation
Objectives
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TCP/IP Protocol Suite 2
TELNET is a general-purpose client-server application program.
Note:Note:
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TCP/IP Protocol Suite 3
CONCEPT
TELNET enables the establishment of a connection to a remote system TELNET enables the establishment of a connection to a remote system in such a way that the local terminal appears to be a terminal at the in such a way that the local terminal appears to be a terminal at the remote system.remote system.
The topics discussed in this section include:The topics discussed in this section include:
Time-Sharing Environment Time-Sharing Environment Login Login
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TCP/IP Protocol Suite 4
Figure : Local login
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TCP/IP Protocol Suite 5
Figure: Remote login
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TCP/IP Protocol Suite 6
NETWORK VIRTUAL TERMINAL (NVT)
Via a universal interface called the Network Virtual Terminal (NVT) Via a universal interface called the Network Virtual Terminal (NVT) character set, the TELNET client translates characters (data or character set, the TELNET client translates characters (data or commands) that come from the local terminal into NVT form and commands) that come from the local terminal into NVT form and delivers them to the network. The TELNET server translates data and delivers them to the network. The TELNET server translates data and commands from NVT form into the form acceptable by the remote commands from NVT form into the form acceptable by the remote computer. computer.
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TCP/IP Protocol Suite 7
Figure: Concept of NVT
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TCP/IP Protocol Suite 8
NVT CHARACTER SET
NVT uses two sets of characters, one for data and one for control. Both NVT uses two sets of characters, one for data and one for control. Both are 8-bit bytes. are 8-bit bytes.
The topics discussed in this section include:The topics discussed in this section include:
Data Characters Data Characters Control Characters Control Characters
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TCP/IP Protocol Suite 9
Figure: Format of data characters
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TCP/IP Protocol Suite 10
Figure: Format of control characters
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TCP/IP Protocol Suite 11
Table: Table: Some NVT control charactersSome NVT control characters
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TCP/IP Protocol Suite 12
EMBEDDING
The same connection is used by TELNET for sending both data and The same connection is used by TELNET for sending both data and control characters. TELNET accomplishes this by embedding the control control characters. TELNET accomplishes this by embedding the control characters in the data stream. characters in the data stream.
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TCP/IP Protocol Suite 13
Figure: An example of embedding
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TCP/IP Protocol Suite 14
OPTIONSTELNET lets the client and server negotiate options before or during the TELNET lets the client and server negotiate options before or during the use of the service. Options are extra features available to a user with a use of the service. Options are extra features available to a user with a more sophisticated terminal. more sophisticated terminal.
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TCP/IP Protocol Suite 15
Table Table OptionsOptions
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TCP/IP Protocol Suite 16
OPTION NEGOTIATION
To use any of the options first requires option negotiation between the To use any of the options first requires option negotiation between the client and the server. Four control characters are used for this purpose.client and the server. Four control characters are used for this purpose.
The topics discussed in this section include:The topics discussed in this section include:
Enabling an Option Enabling an Option Disabling an Option Disabling an Option Symmetry Symmetry
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TCP/IP Protocol Suite 17
Table Table NVT character set for option negotiationNVT character set for option negotiation
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TCP/IP Protocol Suite 18
Figure: Offer to enable an option
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TCP/IP Protocol Suite 19
Figure : Request to enable an option
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TCP/IP Protocol Suite 20
Figure : Offer to disable an option
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TCP/IP Protocol Suite 21
Figure : Request to disable an option
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TCP/IP Protocol Suite 22
In this example, the client wants the server to echo each character sent to the server. In other words, when a character is typed at the user keyboard terminal, it goes to the server and is sent back to the screen of the user before being processed. The echo option is enabled by the server because it is the server that sends the characters back to the user terminal. Therefore, the client should request from the server the enabling of the option using DO. The request consists of three characters: IAC, DO, and ECHO. The server accepts the request and enables the option. It informs the client by sending the three character approval: IAC, WILL, and ECHO.
Example 1
See Next Slide
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TCP/IP Protocol Suite 23
Figure: Example 1: Echo option
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TCP/IP Protocol Suite 24
SUBOPTION NEGOTIATION
Some options require additional information. The client and server Some options require additional information. The client and server agree through a process called suboption negotiation.agree through a process called suboption negotiation.
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TCP/IP Protocol Suite 25
Table: Table: NVT character set for suboption negotiationNVT character set for suboption negotiation
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TCP/IP Protocol Suite 26
Figure : Example of suboption negotiation
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TCP/IP Protocol Suite 27
CONTROLLING THE SERVER
Control characters can be used to control the remote server. Control characters can be used to control the remote server.
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TCP/IP Protocol Suite 28
Table: Table: Characters used to control the applicationCharacters used to control the application program running on remote server program running on remote server
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TCP/IP Protocol Suite 29
Figure: Example of interrupting an application program
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TCP/IP Protocol Suite 30
OUT-OF-BAND SIGNALING
To make control characters effective in special situations, TELNET uses To make control characters effective in special situations, TELNET uses out-of-band signaling, a technique in which the control characters are out-of-band signaling, a technique in which the control characters are preceded by IAC and are sent out of order to the remote process.preceded by IAC and are sent out of order to the remote process.
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TCP/IP Protocol Suite 31
Figure : Out-of-band signaling
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TCP/IP Protocol Suite 32
ESCAPE CHARACTER
When a user wants characters interpreted by the client instead of the When a user wants characters interpreted by the client instead of the server, he can use an escape character, normally Ctrl+] (shown as ^]).server, he can use an escape character, normally Ctrl+] (shown as ^]).
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TCP/IP Protocol Suite 33
Figure: Two different interruptions
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TCP/IP Protocol Suite 34
MODE OF OPERATION
Most TELNET implementations operate in one of three modes: default Most TELNET implementations operate in one of three modes: default mode, character mode, or line mode. mode, character mode, or line mode.
The topics discussed in this section include:The topics discussed in this section include:
Default ModeDefault ModeCharacter Mode Character Mode Line Mode Line Mode
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TCP/IP Protocol Suite 35
In this example, we use the default mode to show the concept and its deficiencies even though it is almost obsolete today. The client and the server negotiate the terminal type and terminal speed and then the server checks the login and password of the user.
Example 2
See Next Slide
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TCP/IP Protocol Suite 36
Figure: Example 2
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TCP/IP Protocol Suite 37
In this example, we show how the client switches to the character mode. This requires that the client request the server to enable the SUPPRESS GO AHEAD and ECHO options.
Example 3
See Next Slide
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TCP/IP Protocol Suite 38
Figure : Example 3
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TCP/IP Protocol Suite 39
USER INTERFACE
The operating system (UNIX, for example) defines an interface to The operating system (UNIX, for example) defines an interface to TELNET with user-friendly commands. The interface is responsible for TELNET with user-friendly commands. The interface is responsible for translating the user-friendly commands to the TELNET commands.translating the user-friendly commands to the TELNET commands.
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TCP/IP Protocol Suite 40
Table: Table: Examples of interface commandsExamples of interface commands
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TCP/IP Protocol Suite 41
SECURITY ISSUE
TELNET suffers from security problems. TELNET requires a login TELNET suffers from security problems. TELNET requires a login name and password (when exchanging text). A microcomputer name and password (when exchanging text). A microcomputer connected to a broadcast LAN can easily eavesdrop using snooper connected to a broadcast LAN can easily eavesdrop using snooper software to capture a login name and the corresponding password (even software to capture a login name and the corresponding password (even if it is encrypted).if it is encrypted).
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TCP/IP Protocol Suite 42
University Questions:
1. Write notes on:a) TELNET protocol.b) SSH. (10 marks) (Dec 2007)
2. What are the basic services provided by TELNET? (4 marks) (Nov 2010)3. What are the different options available in TELNET? (10 marks) (May
2010)