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The TELNET Protocol: 

Netprog: TELNET 1 The TELNET Protocol Reference: RFC 854

TELNET vs. telnet: 

Netprog: TELNET 2 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. Many application protocols are built upon the TELNET protocol.

The TELNET Protocol: 

Netprog: TELNET 3 The TELNET Protocol TCP connection data and control over the same connection. Network Virtual Terminal negotiated options

Network Virtual Terminal: 

Netprog: TELNET 4 Network Virtual Terminal intermediate representation of a generic terminal. provides a standard language for communication of terminal control functions.

Network Virtual Terminal: 

Netprog: TELNET 5 Network Virtual Terminal NVT NVT Server Process TCP TCP

Negotiated Options: 

Netprog: TELNET 6 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).

Negotiated Options: 

Netprog: TELNET 7 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.

Option examples: 

Netprog: TELNET 8 Option examples Line mode vs. character mode echo modes character set (EBCDIC vs. ASCII)

Control Functions: 

Netprog: TELNET 9 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.

Control Functions: 

Netprog: TELNET 10 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.

More Control Functions: 

Netprog: TELNET 11 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.

Command Structure: 

Netprog: TELNET 12 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.

Looking for Commands: 

Netprog: TELNET 13 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.

Command Codes: 

Netprog: TELNET 14 Command Codes IP 243 AO 244 AYT 245 EC 246 EL 247 WILL 251 WON’T 252 DO 253 DON’T 254 IAC 255

Playing with TELNET: 

Netprog: TELNET 15 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).

Some TCP Servers you can play with: 

Netprog: TELNET 16 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

telnet hostname port: 

Netprog: TELNET 17 telnet hostname port > telnet 7 Trying Connected to ( Escape character is '^]'. Hi dave Hi dave stop it stop it ^] telnet> quit Connection closed.

telnet vs. TCP: 

Netprog: TELNET 18 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 .