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Telnet Telnet is generally considered the first
network application. (late 1960s) Uses a client/server model to define a
standard way in which to interact with a remote system.
The telnet protocol is the basis for many protocols FTP, HTTP, SMTP, NTTP just to name a few.
They don’t open a telnet connection, just use the protocol.
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TELNET vs. telnet
TELNET is a protocol that provides “a
general, bi-directional, eight-bit byte
oriented communications facility”.
telnet is a program that supports the
TELNET protocol over TCP.
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The TELNET Protocol
TCP connection data and control over the same connection. Network Virtual Terminal negotiated options
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Network Virtual Terminal
All communications uses a standard terminal called NVT The client takes input, translates into NVT sends
it to the server, which translates to the representation that the computer uses.
Special Telnet commands are interspersed with the data to perform the functions needed to for the operation of the protocol
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NVT ASCII Control Codes
Three mandatory codes 0, Null
No operation, no effect on output 10, Line Feed
Moves printer to the next line, keep same print position
13, Carriage Return Moves the printer to the left margin of the current line
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NVT ASCII Control Codes(2)
Optional Codes 7, Bell 8, Back Space
Move the print position one character to the left 9, Horizontal tab 11, Vertical Tab
devices negotiate to agree on tab stops 12, Form Feed
next page or clear the screen and put cursor at the top
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NVT ASCII Control Codes(3)
End of line marker Since different computers use different standards
for the end of line marker, CR+LF is defined as the end of line marker.
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Negotiated Options
All NVTs support a minimal set of capabilities.
Some terminals have more capabilities than the minimal set.
The 2 endpoints negotiate a set of mutually acceptable options (character set, echo mode, etc).
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Negotiated Options
The protocol for requesting optional features is well defined and includes rules for eliminating possible negotiation “loops”.
The set of options is not part of the TELNET protocol, so that new terminal features can be incorporated without changing the TELNET protocol.
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Control Functions
TELNET includes support for a series of control functions commonly supported by servers.
This provides a uniform mechanism for communication of (the supported) control functions.
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Control Functions
Interrupt Process (IP) suspend/abort process.
Abort Output (AO) process can complete, but send no more output
to user’s terminal. Are You There (AYT)
check to see if system is still running.
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More Control Functions
Erase Character (EC) delete last character sent typically used to edit keyboard input.
Erase Line (EL) delete all input in current line.
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Command Structure
All TELNET commands and data flow through the same TCP connection.
Commands start with a special character called the “Interpret as Command” escape character (IAC).
The IAC code is 255. If a 255 is sent as data - it must be followed
by another 255.
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Looking for Commands
Each receiver must look at each byte that arrives and look for IAC.
If IAC is found and the next byte is IAC - a single byte is presented to the application/terminal (a 255).
If IAC is followed by any other code - the TELNET layer interprets this as a command.
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commands
WILL option (to Enable an option) Response is either
DO option, indicate agreement and enable option DONT option, initiator must not use the option.
WONT option (to disable option) Response must be
DONT option, option is disabled.
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Command Codes
IP 243 AO 244 AYT 245 EC 246 EL 247
WILLWILL 251251 WON’TWON’T 252252 DODO 253253 DON’TDON’T 254254 IACIAC 255255
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Telnet Options
When a option needs more parameters Use SB (subnegotiation) and SE (end
subnegotiation )command
Example: negotiate the terminal type (IAC is interpret as command)
IAC WILL Terminal-type IAC DO Terminal-type IAC SB terminal-type SEND IAC SE IAC SB TERINAL-TYPE IS <type> IAC SE
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Playing with TELNET
You can use the telnet program to play with the TELNET protocol.
telnet is a generic TCP client. Sends whatever you type to the TCP socket. Prints whatever comes back through the TCP
socket. Useful for testing TCP servers (ASCII based
protocols).
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Some TCP Servers you can play with
Many Unix systems have these servers running (by default): echo port 7 discard port 9 daytime port 13 chargen port 19
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telnet hostname port> telnet rcs.rpi.edu 7Trying 128.113.113.33...Connected to cortez.sss.rpi.edu (128.113.113.33).
Escape character is '^]'.Hi daveHi davestop itstop it^]telnet> quitConnection closed.
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telnet vs. TCP
Not all TCP servers talk TELNET (most don't)
You can use the telnet program to play with these servers, but the fancy commands won't do anything. type ^], then "help" for a list of fancy TELNET stuff you
can do in telnet.