CONCEPT
Telnet Negotiations
DESCRIPTION
The telnet protocol is used to control textbased connections
between a client (the 'telnet' program or a mud client) and a
server (the game driver). Most of the options offered by the
protocol are optional and need to be negotiated between the
client and the server. Consequently, and due to their
specialized nature, mud clients don't have to support the full
telnet option feature set.
For the server to find out if a client supports the telnet
protocol at all, one good approach is to issue a simple,
commonly used telnet command to the client. If the client reaction
conforms to the protocol (or sends telnet commands itself), the
mud can continue to negotiate further options. If the client
does not react, the mud can safely refrain from further
negotiations.
The following list is a more or less comprehensive overview of
the telnet related RFCs (available for example on
http://www.faqs.org/rfcs):
RFC Title rel. Code
495 TELNET Protocol Specification
513 Comments on the new TELNET specifications
559 Comments on the new TELNET Protocol and its Implem
595 Some Thoughts in Defense of the TELNET Go-Ahead
596 Second Thoughts on Telnet Go-Ahead
652 Telnet Output Carriage-Return Disposition Option NAOCRD 10
653 Telnet Output Horizontal Tabstops Option NAOHTS 11
654 Telnet Output Horizontal Tab Disposition Option NAOHTD 12
655 Telnet Output Formfeed Disposition Option NAOFFD 13
656 Telnet Output Vertical Tabstops Option NAOVTS 14
657 Telnet Output Vertical Tab Disposition Option NAOVTD 15
658 Telnet Output Linefeed Disposition NAOLFD 16
698 Telnet Extended Ascii Option X-ASCII 17
727 Telnet Logout Option LOGOUT 18
728 A Minor Pitfall in the Telnet Protocol
735 Revised TELNET Byte Macro Option BM 19
749 Telnet SUPDUP-OUTPUT Option SUPDUP 22
764 Telnet Protocol Specification
779 Telnet SEND-LOCATION Option SENDLOC 23
818 The Remote User Telnet Service
854 Telnet Protocol Specification
855 Telnet Option Specifications
856 Telnet Binary Transmission BINARY 0
857 Telnet Echo Option ECHO 1
858 Telnet Suppress Go Ahead Option SGA 3
859 Telnet Status Option STATUS 5
860 Telnet Timing Mark Option TM 6
861 Telnet Extended Options - List Option EXOPL 255
884 Telnet Terminal Type Option TTYPE 24
885 Telnet End of Record Option EOR 25
930 Telnet Terminal Type Option TTYPE 24
933 Output Marking Telnet Option OUTMRK 27
946 Telnet Terminal Location Number Option TTYLOC 28
1043 Telnet Data Entry Terminal Option DODIIS Implement DET 20
1053 Telnet X.3 PAD Option X.3-PAD 30
1073 Telnet Window Size Option NAWS 31
1079 Telnet Terminal Speed Option TSPEED 32
1080 Telnet Remote Flow Control Option FLOWCTRL 33
1091 Telnet Terminal-Type Option TTYPE 24
1096 Telnet X Display Location Option XDISPLOC 35
1116 Telnet Linemode Option LINEMODE 34
1143 The Q Method of Implementing TELNET Option Negotia
1184 Telnet Linemode Option LINEMODE 34
1372 Telnet Remote Flow Control Option FLOWCTRL 33
1408 Telnet Environment Option ENVIRON 36
1571 Telnet Environment Option Interoperability Issues
1572 Telnet Environment Option NEWENV 39
2066 Telnet Charset Option CHARSET 42
2217 Telnet Com Port Control Option COMPORT 44
2877 5250 Telnet Enhancements
All negotiations start with the special character IAC which is
defined in /usr/include/arpa/telnet.h (or in
src/driver/telnet.h for 3.2(.1)) and has the decimal value of
255. Negotiations are based on different telnetoptions (their
values are defined in telnet.h too). Before a negotiation can
start the client and the server have to agree that they
support the option. This works in the following way:
If a client wants to send something to the server it has to
send 'IAC WILL option' (For terminaltype negotation this would
be the 3 bytes 255,251,24; again, check telnet.h) to confirm
that it is able to do that. If the server is supporting that
option and wants to receive something it sends 'IAC DO option'
(255,253,option)
If one side is receiving an 'IAC WILL option' and has not yet
sent with DO or DONT it has to respond with either 'IAC DO
option' if it will support this negotiation or 'IAC DONT
option' if it won't.
If one side is receiving an 'IAC DO option' and has not yet
sent a WILL or WONT it has to reply with either 'IAC WILL
option' if it supports the option or 'IAC WONT option' if not.
A small example: Lets assume we want to negotiate
terminaltype. (TELOPT_TTYPE with value 24). client is the
telnet executable on the playerside, the server is the
gamedriver.
client server
IAC WILL TTYPE
IAC DO TTYPE
Or:
IAC DO TTYPE
IAC WILL TTYPE
After this we are ready to transfer the terminaltype from the
client to the server as explained below.
Now we are ready to start the real negotiations. I explain the
3 options I have currently implemented.
First TerminalType aka TTYPE aka 24 aka TELOPT_TTYPE assuming
the client and the server have exchanged WILL/DO.
The server is now free to send 'IAC SB TELOPT_TTYPE
TELQUAL_SEND IAC SE' which will be replied with 'IAC SB
TELOPT_TTYPE TELQUAL_IS terminaltype IAC SE' where
terminaltype is a non-zero terminated string (it's terminated
by the IAC) (For values look up telnet.h) AND switch the
client's terminalemulation to 'terminaltype'. terminaltype is
case-insensitive. terminal-type may be UNKNOWN. The server may
repeat the SEND request and the client will respond with the
next preferred terminaltype. If this is the same as the
previous received, it marks the end of the list of
terminaltypes. The next SEND request will start the
terminaltypes from the beginning.
Example: (we have exchanged WILL/DO already)
client server
IAC SB TTYPE SEND IAC SE
IAC SB TTYPE IS VT200 IAC SE
IAC SB TTYPE SEND IAC SE
IAC SB TTYPE IS VT100 IAC SE
IAC SB TTYPE SEND IAC SE
IAC SB TTYPE IS VT52 IAC SE
IAC SB TTYPE SEND IAC SE
IAC SB TTYPE IS VT52 IAC SE
/* this marks that we have all terminaltypes. We decide to use the
* vt200 mode so we have to skip to VT200
*/
IAC SB TTYPE SEND IAC SE
IAC SB TTYPE IS VT200 IAC SE
Next important option is NAWS (31) or WindowSizeNegotiation.
This one is a bit easier than terminaltype. After having
received a IAC DO NAWS from the server, the client will reply
with IAC WILL NAWS and immediately after that send IAC SB NAWS
columns_high columns_low lines_high lines_low IAC SE where
xx_low refers to the lowbyte of xx and xx_high refers to the
highbyte of xx. This will be automagically resent at every
windowresize (when the client gets a SIGWINCH for example) or
at your request with 'IAC SB NAWS SEND IAC SE'.
Example: (WILL/DO exchanged)
client server
IAC SB NAWS 0 80 0 24 IAC SE /* the standard vt100 windowsize */
/* no reply */
And, a bit less important but most complex, the LINEMODE (34)
option. It was implemented it due to the fact, that
some weird DOS telnets would not work otherwise. Implemented
are only the absolute basic feature, which is the actual
switching the telnet to linemode. After exchanging WILL/DO the
server sends a modechange request to the client using IAC SB
LINEMODE LM_MODE MODE_EDIT IAC SE, which should turn on local
commandline-editing for the client. If a client supports
LINEMODE it HAS to support this modechange. The client will
reply with IAC SB LINEMODE LM_MODE MODE_EDIT|MODE_ACK IAC SE
(x|y is bitwise or). That's it for linemode. (You will perhaps
receive other IAC SB LINEMODEs with other LM_xxx ... you may
ignore them. (At least IRIX 5.x sends IAC SB LINEMODE LM_SLC
.... IAC SE which declares the local characterset.)).
Example: (WILL/DO negotiated)
client server
IAC SB LINEMODE LM_MODE
MODE_EDIT IAC SE
IAC SB LINEMODE LM_MODE
MODE_EDIT|MODE_ACK IAC SE
Note: The option is more interesting than it looks here. For
example it supports a mixed mode between linemode and
charactermode, flushing the input at certain characters (at
ESC or TAB for shell-like commandline completition). We suggest
reading RFC 1184.
You might be interested in TELOPT_XDISPLAYLOC and TELOPT_ENVIRON too.
Now, how to implement this using LDMud?
0. Patch src/driver/comm1.c, function init_telopts() to include
telopts_do[TELOPT_XXX] = reply_h_telnet_neg;
telopts_dont[TELOPT_XXX] = reply_h_telnet_neg;
telopts_will[TELOPT_XXX] = reply_h_telnet_neg;
telopts_wont[TELOPT_XXX] = reply_h_telnet_neg;
for every telnet negotiation you want to use.
Do not overwrite the TELOPT_ECHO and TELOPT_SGA hooks.
Alternatively, set the driver hook H_NOECHO in master.c:
this diverts _all_ telnet data into the mudlib.
1. Add a new driver hook to master.c just below the others.
set_driver_hook(H_TELNET_NEG,"telnet_neg"),
2. Make a telnet.h for your mudlib... just change the arrays in
src/driver/telnet.h.
3. define a function
void telnet_neg(int cmd, int option, int * optargs)
in your interactive objects (login.c , shells, player.c or
whereever). And note, in ALL objects, through which a
player is handed through (in TAPPMud these are login.c and
player.c). [Ok, master.c is interactive for a very short
time too, but it won't accept input, will it?]
'cmd' will be TELCMD_xxxx (see telnet.h), 'option' one of
TELOPT_xxxx and 'optargs' will be an array of ints (bytes in
fact) when 'cmd' is SB.
Parse 'cmd'/'option' and reply with appropiate answers
using binary_message() (appropiate meaning sending the
right DO/DONT/WILL/WONT if not sent before and using the SB
return values).
3.1. Send IAC DO TTYPE IAC DO NAWS IAC DO LINEMODE at the
first time you can do it (before cat()ing /WELCOME perhaps).
3.2. Note all sent and received WILL/WONT/DO/DONT options for
conforming to the standard, avoiding endless loops and for
easy debugging :)
3.3. Pass those recevied/sent data and other data when the
interactive object is changed (from login.c to player.c or
at other bodychanges). Clear the data when the player goes
linkdead or quits. You won't need to save this data.
3.4. Lower_case() terminaltypes... ;)
3.5. Use reasonable defaultvalues if the client does not
support one of the options. (columns 80, lines 24 if not
NAWS, unknown or vt100 for no terminaltype)
The WILL/WONT/DO/DONT data is best saved in a mapping looking
like this:
([ "received": ([ option1: DO_DONT_OR_0;WILL_WONT_OR_0, ... ])
, "sent" : ([ option1: DO_DONT_OR_0;WILL_WONT_OR_0, ... ])
])
(Ok, it can be done better. But not without confusing *me*
more.)
Before sending anything check
TN["sent"][option,0_if_do_dont_or_1_if_will_wont]
so you don't enter endless loops, save network traffic and the
like.
The windowsize is best saved in the players environment
variables so that he can modify them later on. (Or in two
integers in the player object...). Use for these values is
clear I think.
The terminaltypes received using above mentioned method are
best stored in an array. The actual set terminaltype is best
stored in an environment variable where the player can modify
it. Upon modifying it the IAC SB TTYPE SEND IAC SE cycle
should be started to match the emulation to the entered new
terminaltype. You then may use data retrieved from
/etc/termcap (man 5 termcap) or /usr/lib/terminfo/*/* (SysVID,
man 5 terminfo) to implement terminalcontrol codes dependend
on the terminaltype. /etc/termcap may prove to be the easiest
way tough /usr/lib/terminfo/*/* is the newer (and better) SysV
way of doing it.
[Anyone got a description of the internal terminfo format for
me? -Marcus]
LINEMODE replies may be left alone if only using the mode
change to MODE_EDIT
Some statistics about what clients support telnet negotiations:
Tinyfugue and some other mudclients usually do not support
negotiations.
Except for TF, which supports the Telnet End-Of-Record option
as marker for the end of the prompt. So if you send IAC EOR
after every prompt, it will print the prompt always in the
input window. (Do not forget to negotiate that. First IAC WILL
TELOPT_EOR/wait for IAC DO TELOPT_EOR). Newer versions of
TF will support NAWS and there will be a patch for TTYPE
negotiation available soon.
All telnets able to do negotiations I've encountered support
the TTYPE option.
HP9.x,Irix5.x,Linux,EP/IX,CUTELNET/NCSATELNET (Novell) and
perhaps more support NAWS.
At least Irix5.x,Linux,CU/NCSATELNET support LINEMODE.
SUN does not support NAWS and LINEMODE neither in SunOS 4.1.3
nor in Solaris 2.3.
For getting RFCs you can for example use
ftp://ftp.uni-erlangen.de/pub/doc/rfc/
BUGS
Not all aspects of the options are mentioned to keep this doc
at a reasonable size. Refer to the RFCs to get more confused.
CREDITS
Provided by Marcus@TAPPMud (Marcus Meissner,
<msmeissn@cip.informatik.uni-erlangen.de>).
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