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TELNETTELNET
1.TELNET
2.SSH
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TELNET
TELNET is an abbreviation for TErminaL NETwork.
It is the standard TCP/IP protocol for virtual terminal
service as proposed by ISO.
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 remote system.
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Topics to be DiscussedTopics to be Discussed
Concepts Time-Sharing Environment Network Virtual Terminal (NVT) Embedding Options and Suboption Negotiation Controlling the Server Out-of-Band Signaling Escape Character Modes of Operation User Interface Security Issue
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TELNET is a general-purpose client-server application program.
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Figure 1 Local login
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Figure 2 Remote login
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Figure 3 Concept of NVT
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Figure 4 Format of data and control characters
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Figure 5 An example of embedding
c a t f i e al IAC EC 1
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Figure 6 Offer to enable an option
WILL
Will I enable the option?
DO or DONT
Do (or don’t) enable the option
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Figure 7 Request to enable an option
DO
Do enable the option
I will (won’t) enable the option
WILL or WONT
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Figure 8 Offer to disable an option
WONT
I won’t use the option any more
DONT
Don’t use it
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Figure 9 Request to disable an option
DONT
Don’t use the option any more
WONT
I won’t
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Figure 10 shows an example of option negotiation.
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.
ExampleExample 1
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Figure 10 Example 1: Echo option
Do enable the echo option
IACDOECHO1
I will enable the echo option
IAC WILL ECHO 22
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Figure 11 Example of sub-option negotiation
I will enable the terminal option
IACWILLTerminal type1
Do enable terminal option
IAC DO Terminal type 22
Set the terminal type to “VT”
IACIAC SB‘V’‘T’ Terminal typeSE3
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Figure 12 Example of interrupting an application program
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Figure 13 Out-of-band signaling
IACIPDM Data
DiscardedKept
Urgent pointer
Data
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Figure 14 Two different interruptions
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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 (see Figuren15).
ExampleExample 2
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Figure 15 Example 2
WILL TERMINAL TYPE
DO TERMINAL TYPE
GOAHEAD
GOAHEAD
GOAHEAD
cp file1 file2
GOAHEAD
GOAHEAD
WILL TERMINAL SPEED
DONT TERMINAL SPEED
Login:
Password:
forouzan
XXXXX
1
2
3
4
5
6
7
8
9
10
11
12
13
14
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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 (see Figure
16).
ExampleExample 20.3
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Figure 16 Example 3
GOAHEAD
DO SUPPRESS GOAHEAD
WILL SUPPRESS GOAHEAD
DO ECHO
WILL ECHO
Login:
f
o
o
f
1
2
3
4
5
6
7
8
9
10
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2 SECURE SHELL (SSH)
Another popular remote login
application program is Secure Shell
(SSH). SSH, like TELNET, uses TCP as
the underlying transport protocol.
SSH is more secure and provides more
services than TELNET.
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Topics to be DiscussedTopics to be Discussed
Versions Components Port Forwarding Format of the SSH Packet
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Figure 17 Components of SSH
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Figure 18 Port forwarding
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Figure 19 SSH packet format