View source for Telnet

== What is Telnet ==

The Telnet protocol provides a way for both sides of a network connection (client and server) to express interest and support for certain options. Usually, there is no evidence of this conversation on the screen; it is not typically shown. Many common Internet protocols take the Telnet protocol as a starting point.

An important character in the telnet protocol is IAC ([[chr()|chr(255)]]). For either side to transmit y-umlaut, it must appear twice (IAC IAC). On the receiving side, any pair of IACs is understood to mean y-umlaut.

[[PennMUSH]] and [[TinyMUX]] both support the basic Telnet protocol including the commands: Are You There (AYT) and NOP (No Operation). NOP is somewhat useful in keeping NAT/Firewalls happy (see [[KEEPALIVE]]).

The Negotiation About Screen Size (NAWS) option along with [[width()]] and [[height()]] softcode functions expose the client window's height and width.

== Server Differences ==

[[TinyMUX]] version 2.4 added support for AYT, SGA, EOR, and NAWS. Version 2.7 added support for Environment (ENV), Character Sets (CHARSET), and Terminal Type (TTYPE).

[[PennMUSH]] also supports LINEMODE and ECHO.  While [[PennMUSH]] does not and will not support [[@program]], it has [[@prompt]] as a way for softcode to generate SGA/EOR.

== Prompts ==

MUD clients have a non-standard concept called ''prompts'' which are outside of the telnet RFCs. A ''prompt'' is a partial line of text (that is, it does not end in CRLF) followed by either IAC GA or IAC EOR. Prompts maintain their left-bottom position as new text from the server arrives. New text is placed above the prompt.

Many MUSH clients have a separate input box, so the cursor is never positioned next to a ''prompt'' anyway, however MUSHClient does have a 'Keep Commands On Prompt Line' option which will position entered text on these partial ''prompt'' lines. It does recognize these partial lines, but it does not appear to go so far as to display new text from the server above the last prompt.

This use of IAC GA is not consistent with the defined use of Go-Ahead (GA) command. The Go-Ahead command was original designed to manage a half-duplex channels. Go-Ahead acts as a sort of token that must be held before one side or the other can transmit. Of course, TCP/IP connections are always full-duplex, telnetd tries to negotiate the Surpress Go-Ahead (SGA) option, and despite whether that negotiation is successful, teletnd never sends a IAC GA itself.

We have not been able to coax SimpleMU to display a partial-line. It doesn't implement this non-standard use of IAC GA which one can argue is Good (tm). It also doesn't display any part of the line until CRLF is received which one can argue is Bad (tm). We haven't tried terminating with an IAC EOR, yet.

TinyFugue 4 doesn't support prompts. TinyFugue 5 apparently does.

Many other modern clients have standardized on the behavior of interpreting the IAC GA sequence as a newline, rather than trying to position the prompt in a static position.

The only ''prompt'' in TinyMUX occurs when [[@program]] is used.

Surpress Go-Ahead (SGA) and End Of Record (EOR) are negotiated in both directions except that either EOR must be disabled or SGA must be disabled so that ''prompts'' can be presented properly.

== Related RFCs ==

The full range of telnet RFC's is quite large and beyond our scope.

: [http://www.faqs.org/rfcs/rfc698.html Telnet extended ASCII option]
: [http://www.faqs.org/rfcs/rfc726.html Remote Controlled Transmission and Echoing Telnet option]
: [http://www.faqs.org/rfcs/rfc727.html Telnet logout option]
: [http://www.faqs.org/rfcs/rfc735.html Revised Telnet byte macro option]
: [http://www.faqs.org/rfcs/rfc736.html Telnet SUPDUP option]
: [http://www.faqs.org/rfcs/rfc749.html Telnet SUPDUP-Output option]
: [http://www.faqs.org/rfcs/rfc779.html Telnet send-location option]
: [http://www.faqs.org/rfcs/rfc854.html Telnet Protocol Specification]
: [http://www.faqs.org/rfcs/rfc855.html Telnet Option Specification]
: [http://www.faqs.org/rfcs/rfc856.html Telnet Binary Transmission]
: [http://www.faqs.org/rfcs/rfc857.html Telnet Echo Option]
: [http://www.faqs.org/rfcs/rfc858.html Telnet Suppress Go Ahead Option]
: [http://www.faqs.org/rfcs/rfc859.html Telnet Status Option]
: [http://www.faqs.org/rfcs/rfc860.html Telnet Timing Mark Option]
: [http://www.faqs.org/rfcs/rfc861.html Telnet Extended Options: List Option]
: [http://www.faqs.org/rfcs/rfc885.html Telnet end of record option]
: [http://www.faqs.org/rfcs/rfc927.html TACACS user identification Telnet option]
: [http://www.faqs.org/rfcs/rfc933.html Output marking Telnet option]
: [http://www.faqs.org/rfcs/rfc946.html Telnet terminal location number option]
: [http://www.faqs.org/rfcs/rfc1041.html Telnet 3270 regime option]
: [http://www.faqs.org/rfcs/rfc1043.html Telnet Data Entry Terminal option: DODIIS  implementation]
: [http://www.faqs.org/rfcs/rfc1053.html Telnet X.3 PAD option]
: [http://www.faqs.org/rfcs/rfc1073.html Telnet window size option]
: [http://www.faqs.org/rfcs/rfc1079.html Telnet terminal speed option]
: [http://www.faqs.org/rfcs/rfc1091.html Telnet terminal-type option]
: [http://www.faqs.org/rfcs/rfc1096.html Telnet X display location option]
: [http://www.faqs.org/rfcs/rfc1143.html The Q Method of Implementing TELNET Option Negotiation]
: [http://www.faqs.org/rfcs/rfc1184.html Telnet Linemode Option]
: [http://www.faqs.org/rfcs/rfc1372.html Telnet Remote Flow Control Option]
: [http://www.faqs.org/rfcs/rfc1411.html Telnet Authentication: Kerberos Version 4]
: [http://www.faqs.org/rfcs/rfc1412.html Telnet Authentication: SPX]
: [http://www.faqs.org/rfcs/rfc1572.html Telnet Environment Option]
: [http://www.faqs.org/rfcs/rfc2066.html TELNET CHARSET Option]
: [http://www.faqs.org/rfcs/rfc2217.html Telnet Com Port Control Option]
: [http://www.faqs.org/rfcs/rfc2941.html Telnet Authentication Option]
: [http://www.faqs.org/rfcs/rfc2942.html Telnet Authentication: Kerberos Version 5]
: [http://www.faqs.org/rfcs/rfc2943.html TELNET Authentication Using DSA]
: [http://www.faqs.org/rfcs/rfc2944.html Telnet Authentication: SRP]
: [http://www.faqs.org/rfcs/rfc2946.html Telnet Data Encryption Option]
: [http://www.faqs.org/rfcs/rfc2947.html Telnet Encryption: DES3 64 bit Cipher Feedback]
: [http://www.faqs.org/rfcs/rfc2948.html Telnet Encryption: DES3 64 bit Output Feedback]
: [http://www.faqs.org/rfcs/rfc2949.html Telnet Encryption: CAST-128 64 bit Output Feedback]
: [http://www.faqs.org/rfcs/rfc2950.html Telnet Encryption: CAST-128 64 bit Cipher Feedback]