Telnet terminal type option
Let Telnet clients tell servers what kind of terminal they are
Solomon, M. and E. Wimmers, "Telnet terminal type option", RFC 884, DOI 10.17487/RFC0884, December 1983, <https://www.rfc-editor.org/info/rfc884>.
@misc{rfc884,
series = {Request for Comments},
number = 884,
howpublished = {RFC 884},
publisher = {RFC Editor},
doi = {10.17487/RFC0884},
url = {https://www.rfc-editor.org/info/rfc884},
author = {M. Solomon and E. Wimmers},
title = {{Telnet terminal type option}},
pagetotal = 5,
year = 1983,
month = dec,
} RFC text
Contents
Network Working Group Marvin Solomon
Request for Comments: 884 Edward Wimmers
University of Wisconsin - Madison
December 1983
TELNET TERMINAL TYPE OPTION
This RFC specifies a standard for the ARPA Internet community. Hosts on
the ARPA Internet that exchange terminal type information within the
Telnet protocol are expected to adopt and implement this standard.
1. Command Name and Code
TERMINAL-TYPE 24
2. Command Meanings
IAC WILL TERMINAL-TYPE
Sender is willing to send terminal type information in a
subsequent sub-negotiation
IAC DO TERMINAL-TYPE
Sender is willing to receive terminal type information in a
subsequent sub-negotiation
IAC DON'T TERMINAL-TYPE
Sender refuses to accept terminal type information
IAC WON'T TERMINAL-TYPE
Sender refuses to send terminal type information
IAC SB TERMINAL-TYPE SEND IAC SE
Sender requests receiver to transmit his (the receiver's) terminal
type. The code for SEND is 1. (See below.)
IAC SB TERMINAL-TYPE IS ... IAC SE
Sender is stating the name of his terminal type. The code for IS
is 0. (See below.)
3. Default
DON'T TERMINAL-TYPE
WON'T TERMINAL-TYPE
Terminal type information will not be exchanged.
4. Motivation for the Option
This option allows a telnet server to determine the type of terminal
connected to a user telnet program. The transmission of such
information does not immediately imply any change of processing.
However, the information may be passed to a process, which may alter
the data it sends to suit the particular characteristics of the
terminal. For example, some operating systems have a terminal driver
that accepts a code indicating the type of terminal being driven.
Using the TERMINAL TYPE and BINARY options, a telnet server program
on such a system could arrange to have terminals driven as if they
were directly connected, including such special functions as cursor
addressing, multiple colors, etc., not included in the Network
Virtual Terminal specification. This option fits into the normal
structure of TELNET options by deferring the actual transfer of
status information to the SB command.
5. Description of the Option
WILL and DO are used only to obtain and grant permission for future
discussion. The actual exchange of status information occurs within
option subcommands (IAC SB TERMINAL-TYPE...).
Once the two hosts have exchanged a WILL and a DO, the sender of the
WILL TERMINAL-TYPE is free to transmit type information, spontan-
eously or in response to a request from the sender of the DO. At
worst, this may lead to transmitting the information twice. Only the
sender of the DO may send requests (IAC SB TERMINAL-TYPE SEND IAC SE)
and only the sender of the WILL may transmit actual type information
(within an IAC SB TERMINAL-TYPE IS ... IAC SE command).
The terminal type information is an NVT ASCII string. Within this
string, upper and lower case are considered equivalent. A few
terminal type names useful in the context of IBM systems are listed
below. It is anticipated that additional names will be added in the
future. The complete list of valid terminal types will be found in
the latest "Assigned Numbers" RFC.
The following is an example of use of the option:
Host1: IAC DO TERMINAL-TYPE
Host2: IAC WILL TERMINAL-TYPE
(Host2 is now free to send status information at any time.
Solicitations from Host1 are NOT necessary. This should not
produce any dangerous race conditions. At worst, two IS's will
be sent.)
Host1 (perhaps): IAC SB TERMINAL-TYPE SEND IAC SE
Host2:
IAC SB TERMINAL-TYPE IS IBM-3278-2 IAC SE
6. Implementation Suggestions
The "terminal type" information may be any NVT ASCII string meaning-
ful to both ends of the negotiation. The list of suggestions below
is intended to minimize confusion caused by alternative "spellings"
of the terminal type. For example, confusion would arise if one
party were to call a terminal "IBM3278-2" while the other called it
"IBM-3278/2". There is no negative acknowledgement for a terminal
type that is not understood, but certain other options (such as
switching to BINARY mode) may be refused if a valid terminal type
name has not been specified. In some cases, a particular terminal
may be known by more than one name, for example a specific type and a
more generic type. In such cases, the sender of the TERMINAL-TYPE IS
command should reply to successive TERMINAL-TYPE SEND commands with
the various names, from most to least specific. In this way, a
telnet server that does not understand the first response can prompt
for alternatives. However, it should cease sending TERMINAL-TYPE
SEND commands after receiving the same response two consecutive
times. Similarly, a sender should indicate it has sent all available
names by repeating the last one sent.
Here are a few terminal types useful in the IBM environment:
IBM-3275-2
IBM-3276-2
IBM-3276-3
IBM-3276-4
IBM-3277-2
IBM-3278-2
IBM-3278-3
IBM-3278-4
IBM-3278-5
IBM-3279-2
IBM-3279-3
Here are a few terminal types useful in the TOPS20 environment:
ANN-ARBOR-AMBASSADOR
CONCEPT-100
DATAMEDIA-2500
DEC-LA30
DEC-VT100
DEC-VT52
EXECUPORT-4000
HAZELTINE-1500
HP-2621
HP-2640
HP-2645A
HP-2649
NETWORK-VIRTUAL-TERMINAL
TEKTRONIX-4025
TELERAY-1061
TELETYPE-33
TELETYPE-37
TELEVIDEO-950
TERMINET-300
TI-700
ZENITH-H19
Here are a few terminal types used in the Unix environment:
ADDS-CONSUL-980
ADDS-REGENT-200
ANDERSON-JACOBSON-832
ANN-ARBOR-AMBASSADOR
BITGRAPH
CDI-1203
COMPUCOLOR-II
CONCEPT-100
DATA-GENERAL-6053
DATAGRAPHIX-132A
DATAMEDIA-3045A
DATAPOINT-3360
DEC-DECWRITER-II
DEC-GT40
DEC-VT52
DELTA-DATA-5000
DIABLO-1620
EXECUPORT-4000
GENERAL-TERMINAL-100A
HAZELTINE-1500
HAZELTINE-2000
HP-2621
HP-2640A
HP-2645
HP-2649A
IBM-3101
INFOTON-100
LSI-ADM-3
MICROTERM-ACT-V
MICROTERM-MIME-2
NETWORK-VIRTUAL-TERMINAL
PERKIN-ELMER-1100
PLASMA-PANEL
SUPERBEE-III-M
TEKTRONIX-4014
TELERAY-3700
TELETYPE-33
TELETYPE-37
TELEVIDEO-912
TERMINET-300
TI-700
TI-733
TI-745
VISUAL-200
XEROX-1720
ZENITH-H19
ZENTEC-30
The type "UNKNOWN" should be used if the type of the terminal is
unknown or unlikely to be recognized by the other party.
The complete and up-to-date list will be maintained in the "Assigned
Numbers".
AI ELI5
On this page
§1Why Terminal Type Matters
RFC 884 specifies a Telnet option that allows clients to inform servers of their terminal type. Different terminals have different capabilities: some support cursor addressing, color, or special function keys, while others are simple line-oriented displays. Without knowing what terminal is connected, a Telnet server must assume minimal capabilities or send escape sequences that work only on specific terminals.
The option lets a server determine the terminal type and pass that information to its processes or terminal drivers, enabling features like cursor positioning, multiple colors, and special functions that would otherwise be unavailable over a generic Telnet connection. The option integrates smoothly with other Telnet options like BINARY mode.
When a Telnet connection starts, the terminal sends its type (like "VT100" or "IBM-3278") so the server knows what features it can use.
§2Exchange of Terminal Type Information
The TERMINAL-TYPE option (code 24) uses two negotiation commands: WILL and DO. After the two sides exchange WILL TERMINAL-TYPE and DO TERMINAL-TYPE, the client may send terminal type information either spontaneously or in response to the server's request.
The server uses IAC SB TERMINAL-TYPE SEND IAC SE to request the terminal type, and the client responds with IAC SB TERMINAL-TYPE IS [name] IAC SE where [name] is the terminal name as an NVT ASCII string. Case is not significant. The option allows the client to send multiple names in response to repeated SEND commands, listing from most specific to most generic, allowing the server to use the best match.
The server asks for the terminal type, and the client answers with a name like "VT100". If the server doesn't understand, it can ask again and the client sends a less specific name.
§3Terminal Type Names
RFC 884 provides suggested terminal type names for three environments. IBM terminals include IBM-3275 through IBM-3279 series models. TOPS-20 environment terminals include DEC models (VT52, VT100), HP models (2621, 2640, 2645), and others like ANN-ARBOR-AMBASSADOR and TEKTRONIX-4025. Unix environment terminals are more numerous: DEC DECWRITER and VT models, HP 26xx series, various data entry and graphics terminals like BITGRAPH, and ASCII terminals like ADM-3.
The RFC notes that the complete list of valid names is maintained in the Assigned Numbers RFC. The type "UNKNOWN" should be used if the terminal type is not known or unlikely to be recognized. The RFC anticipates that additional names will be added over time as new terminals are encountered.
There are hundreds of possible terminal type names organized by manufacturer: DEC, HP, IBM, Hazeltine, Tektronix, and others. If the server doesn't recognize one, the client can try a more generic name.