3. Remote Login -- Telnet Protocol
3. REMOTE LOGIN -- TELNET PROTOCOL
3.1 INTRODUCTION
Telnet is the standard Internet application protocol for remote login. It provides the encoding rules to link a user's keyboard/display on a client ("user") system with a command interpreter on a remote server system. A subset of the Telnet protocol is also incorporated within other application protocols, e.g., FTP and SMTP.
Telnet uses a single TCP connection, and its normal data stream ("Network Virtual Terminal" or "NVT" mode) is 7-bit ASCII with escape sequences to embed control functions. Telnet also allows the negotiation of many optional modes and functions.
The primary Telnet specification is to be found in RFC-854 [TELNET:1], while the options are defined in many other RFCs; see Section 7 for references.
3.2 PROTOCOL WALK-THROUGH
3.2.1 Option Negotiation: RFC-854, pp. 2-3
Every Telnet implementation MUST include option negotiation and subnegotiation machinery [TELNET:2].
A host MUST carefully follow the rules of RFC-854 to avoid option-negotiation loops. A host MUST refuse (i.e, reply WONT/DONT to a DO/WILL) an unsupported option. Option negotiation SHOULD continue to function (even if all requests are refused) throughout the lifetime of a Telnet connection.
If all option negotiations fail, a Telnet implementation MUST default to, and support, an NVT.
DISCUSSION: Even though more sophisticated "terminals" and supporting option negotiations are becoming the norm, all implementations must be prepared to support an NVT for any user-server communication.
3.2.2 Telnet Go-Ahead Function: RFC-854, p. 5, and RFC-858
On a host that never sends the Telnet command Go Ahead (GA), the Telnet Server MUST attempt to negotiate the Suppress Go Ahead option (i.e., send "WILL Suppress Go Ahead"). A User or Server Telnet MUST always accept negotiation of the Suppress Go
Ahead option.
When it is driving a full-duplex terminal for which GA has no meaning, a User Telnet implementation MAY ignore GA commands.
DISCUSSION: Half-duplex ("locked-keyboard") line-at-a-time terminals for which the Go-Ahead mechanism was designed have largely disappeared from the scene. It turned out to be difficult to implement sending the Go-Ahead signal in many operating systems, even some systems that support native half-duplex terminals. The difficulty is typically that the Telnet server code does not have access to information about whether the user process is blocked awaiting input from the Telnet connection, i.e., it cannot reliably determine when to send a GA command. Therefore, most Telnet Server hosts do not send GA commands.
The effect of the rules in this section is to allow either end of a Telnet connection to veto the use of GA commands.
There is a class of half-duplex terminals that is still commercially important: "data entry terminals," which interact in a full-screen manner. However, supporting data entry terminals using the Telnet protocol does not require the Go Ahead signal; see Section 3.3.2.
3.2.3 Control Functions: RFC-854, pp. 7-8
The list of Telnet commands has been extended to include EOR (End-of-Record), with code 239 [TELNET:9].
Both User and Server Telnets MAY support the control functions EOR, EC, EL, and Break, and MUST support AO, AYT, DM, IP, NOP, SB, and SE.
A host MUST be able to receive and ignore any Telnet control functions that it does not support.
DISCUSSION: Note that a Server Telnet is required to support the Telnet IP (Interrupt Process) function, even if the server host has an equivalent in-stream function (e.g., Control-C in many systems). The Telnet IP function may be stronger than an in-stream interrupt command, because of the out- of-band effect of TCP urgent data.
The EOR control function may be used to delimit the
stream. An important application is data entry terminal support (see Section 3.3.2). There was concern that since EOR had not been defined in RFC-854, a host that was not prepared to correctly ignore unknown Telnet commands might crash if it received an EOR. To protect such hosts, the End-of-Record option [TELNET:9] was introduced; however, a properly implemented Telnet program will not require this protection.
3.2.4 Telnet "Synch" Signal: RFC-854, pp. 8-10
When it receives "urgent" TCP data, a User or Server Telnet MUST discard all data except Telnet commands until the DM (and end of urgent) is reached.
When it sends Telnet IP (Interrupt Process), a User Telnet SHOULD follow it by the Telnet "Synch" sequence, i.e., send as TCP urgent data the sequence "IAC IP IAC DM". The TCP urgent pointer points to the DM octet.
When it receives a Telnet IP command, a Server Telnet MAY send a Telnet "Synch" sequence back to the user, to flush the output stream. The choice ought to be consistent with the way the server operating system behaves when a local user interrupts a process.
When it receives a Telnet AO command, a Server Telnet MUST send a Telnet "Synch" sequence back to the user, to flush the output stream.
A User Telnet SHOULD have the capability of flushing output when it sends a Telnet IP; see also Section 3.4.5.
DISCUSSION: There are three possible ways for a User Telnet to flush the stream of server output data:
(1) Send AO after IP.
This will cause the server host to send a "flush- buffered-output" signal to its operating system. However, the AO may not take effect locally, i.e., stop terminal output at the User Telnet end, until the Server Telnet has received and processed the AO and has sent back a "Synch".
(2) Send DO TIMING-MARK [TELNET:7] after IP, and discard all output locally until a WILL/WONT TIMING-MARK is
received from the Server Telnet.
Since the DO TIMING-MARK will be processed after the IP at the server, the reply to it should be in the right place in the output data stream. However, the TIMING-MARK will not send a "flush buffered output" signal to the server operating system. Whether or not this is needed is dependent upon the server system.
(3) Do both.
The best method is not entirely clear, since it must accommodate a number of existing server hosts that do not follow the Telnet standards in various ways. The safest approach is probably to provide a user-controllable option to select (1), (2), or (3).
3.2.5 NVT Printer and Keyboard: RFC-854, p. 11
In NVT mode, a Telnet SHOULD NOT send characters with the high-order bit 1, and MUST NOT send it as a parity bit. Implementations that pass the high-order bit to applications SHOULD negotiate binary mode (see Section 3.2.6).
DISCUSSION: Implementors should be aware that a strict reading of RFC-854 allows a client or server expecting NVT ASCII to ignore characters with the high-order bit set. In general, binary mode is expected to be used for transmission of an extended (beyond 7-bit) character set with Telnet.
However, there exist applications that really need an 8- bit NVT mode, which is currently not defined, and these existing applications do set the high-order bit during part or all of the life of a Telnet connection. Note that binary mode is not the same as 8-bit NVT mode, since binary mode turns off end-of-line processing. For this reason, the requirements on the high-order bit are stated as SHOULD, not MUST.
RFC-854 defines a minimal set of properties of a "network virtual terminal" or NVT; this is not meant to preclude additional features in a real terminal. A Telnet connection is fully transparent to all 7-bit ASCII characters, including arbitrary ASCII control characters.
For example, a terminal might support full-screen commands coded as ASCII escape sequences; a Telnet implementation would pass these sequences as uninterpreted data. Thus, an NVT should not be conceived as a terminal type of a highly-restricted device.
3.2.6 Telnet Command Structure: RFC-854, p. 13
Since options may appear at any point in the data stream, a Telnet escape character (known as IAC, with the value 255) to be sent as data MUST be doubled.
3.2.7 Telnet Binary Option: RFC-856
When the Binary option has been successfully negotiated, arbitrary 8-bit characters are allowed. However, the data stream MUST still be scanned for IAC characters, any embedded Telnet commands MUST be obeyed, and data bytes equal to IAC MUST be doubled. Other character processing (e.g., replacing CR by CR NUL or by CR LF) MUST NOT be done. In particular, there is no end-of-line convention (see Section 3.3.1) in binary mode.
DISCUSSION: The Binary option is normally negotiated in both directions, to change the Telnet connection from NVT mode to "binary mode".
The sequence IAC EOR can be used to delimit blocks of data within a binary-mode Telnet stream.
3.2.8 Telnet Terminal-Type Option: RFC-1091
The Terminal-Type option MUST use the terminal type names officially defined in the Assigned Numbers RFC [INTRO:5], when they are available for the particular terminal. However, the receiver of a Terminal-Type option MUST accept any name.
DISCUSSION: RFC-1091 [TELNET:10] updates an earlier version of the Terminal-Type option defined in RFC-930. The earlier version allowed a server host capable of supporting multiple terminal types to learn the type of a particular client's terminal, assuming that each physical terminal had an intrinsic type. However, today a "terminal" is often really a terminal emulator program running in a PC, perhaps capable of emulating a range of terminal types. Therefore, RFC-1091 extends the specification to allow a
more general terminal-type negotiation between User and Server Telnets.
3.3 SPECIFIC ISSUES
3.3.1 Telnet End-of-Line Convention
The Telnet protocol defines the sequence CR LF to mean "end- of-line". For terminal input, this corresponds to a command- completion or "end-of-line" key being pressed on a user terminal; on an ASCII terminal, this is the CR key, but it may also be labelled "Return" or "Enter".
When a Server Telnet receives the Telnet end-of-line sequence CR LF as input from a remote terminal, the effect MUST be the same as if the user had pressed the "end-of-line" key on a local terminal. On server hosts that use ASCII, in particular, receipt of the Telnet sequence CR LF must cause the same effect as a local user pressing the CR key on a local terminal. Thus, CR LF and CR NUL MUST have the same effect on an ASCII server host when received as input over a Telnet connection.
A User Telnet MUST be able to send any of the forms: CR LF, CR NUL, and LF. A User Telnet on an ASCII host SHOULD have a user-controllable mode to send either CR LF or CR NUL when the user presses the "end-of-line" key, and CR LF SHOULD be the default.
The Telnet end-of-line sequence CR LF MUST be used to send Telnet data that is not terminal-to-computer (e.g., for Server Telnet sending output, or the Telnet protocol incorporated another application protocol).
DISCUSSION: To allow interoperability between arbitrary Telnet clients and servers, the Telnet protocol defined a standard representation for a line terminator. Since the ASCII character set includes no explicit end-of-line character, systems have chosen various representations, e.g., CR, LF, and the sequence CR LF. The Telnet protocol chose the CR LF sequence as the standard for network transmission.
Unfortunately, the Telnet protocol specification in RFC- 854 [TELNET:1] has turned out to be somewhat ambiguous on what character(s) should be sent from client to server for the "end-of-line" key. The result has been a massive and continuing interoperability headache, made worse by various faulty implementations of both User and Server
Telnets.
Although the Telnet protocol is based on a perfectly symmetric model, in a remote login session the role of the user at a terminal differs from the role of the server host. For example, RFC-854 defines the meaning of CR, LF, and CR LF as output from the server, but does not specify what the User Telnet should send when the user presses the "end-of-line" key on the terminal; this turns out to be the point at issue.
When a user presses the "end-of-line" key, some User Telnet implementations send CR LF, while others send CR NUL (based on a different interpretation of the same sentence in RFC-854). These will be equivalent for a correctly-implemented ASCII server host, as discussed above. For other servers, a mode in the User Telnet is needed.
The existence of User Telnets that send only CR NUL when CR is pressed creates a dilemma for non-ASCII hosts: they can either treat CR NUL as equivalent to CR LF in input, thus precluding the possibility of entering a "bare" CR, or else lose complete interworking.
Suppose a user on host A uses Telnet to log into a server host B, and then execute B's User Telnet program to log into server host C. It is desirable for the Server/User Telnet combination on B to be as transparent as possible, i.e., to appear as if A were connected directly to C. In particular, correct implementation will make B transparent to Telnet end-of-line sequences, except that CR LF may be translated to CR NUL or vice versa.
IMPLEMENTATION: To understand Telnet end-of-line issues, one must have at least a general model of the relationship of Telnet to the local operating system. The Server Telnet process is typically coupled into the terminal driver software of the operating system as a pseudo-terminal. A Telnet end-of- line sequence received by the Server Telnet must have the same effect as pressing the end-of-line key on a real locally-connected terminal.
Operating systems that support interactive character-at- a-time applications (e.g., editors) typically have two internal modes for their terminal I/O: a formatted mode, in which local conventions for end-of-line and other
formatting rules have been applied to the data stream, and a "raw" mode, in which the application has direct access to every character as it was entered. A Server Telnet must be implemented in such a way that these modes have the same effect for remote as for local terminals. For example, suppose a CR LF or CR NUL is received by the Server Telnet on an ASCII host. In raw mode, a CR character is passed to the application; in formatted mode, the local system's end-of-line convention is used.
3.3.2 Data Entry Terminals
DISCUSSION: In addition to the line-oriented and character-oriented ASCII terminals for which Telnet was designed, there are several families of video display terminals that are sometimes known as "data entry terminals" or DETs. The IBM 3270 family is a well-known example.
Two Internet protocols have been designed to support generic DETs: SUPDUP [TELNET:16, TELNET:17], and the DET option [TELNET:18, TELNET:19]. The DET option drives a data entry terminal over a Telnet connection using (sub-) negotiation. SUPDUP is a completely separate terminal protocol, which can be entered from Telnet by negotiation. Although both SUPDUP and the DET option have been used successfully in particular environments, neither has gained general acceptance or wide implementation.
A different approach to DET interaction has been developed for supporting the IBM 3270 family through Telnet, although the same approach would be applicable to any DET. The idea is to enter a "native DET" mode, in which the native DET input/output stream is sent as binary data. The Telnet EOR command is used to delimit logical records (e.g., "screens") within this binary stream.
IMPLEMENTATION: The rules for entering and leaving native DET mode are as follows:
o The Server uses the Terminal-Type option [TELNET:10] to learn that the client is a DET.
o It is conventional, but not required, that both ends negotiate the EOR option [TELNET:9].
o Both ends negotiate the Binary option [TELNET:3] to
enter native DET mode.
o When either end negotiates out of binary mode, the other end does too, and the mode then reverts to normal NVT.
3.3.3 Option Requirements
Every Telnet implementation MUST support the Binary option [TELNET:3] and the Suppress Go Ahead option [TELNET:5], and SHOULD support the Echo [TELNET:4], Status [TELNET:6], End-of- Record [TELNET:9], and Extended Options List [TELNET:8] options.
A User or Server Telnet SHOULD support the Window Size Option [TELNET:12] if the local operating system provides the corresponding capability.
DISCUSSION: Note that the End-of-Record option only signifies that a Telnet can receive a Telnet EOR without crashing; therefore, every Telnet ought to be willing to accept negotiation of the End-of-Record option. See also the discussion in Section 3.2.3.
3.3.4 Option Initiation
When the Telnet protocol is used in a client/server situation, the server SHOULD initiate negotiation of the terminal interaction mode it expects.
DISCUSSION: The Telnet protocol was defined to be perfectly symmetrical, but its application is generally asymmetric. Remote login has been known to fail because NEITHER side initiated negotiation of the required non-default terminal modes. It is generally the server that determines the preferred mode, so the server needs to initiate the negotiation; since the negotiation is symmetric, the user can also initiate it.
A client (User Telnet) SHOULD provide a means for users to enable and disable the initiation of option negotiation.
DISCUSSION: A user sometimes needs to connect to an application service (e.g., FTP or SMTP) that uses Telnet for its
control stream but does not support Telnet options. User Telnet may be used for this purpose if initiation of option negotiation is disabled.
3.3.5 Telnet Linemode Option
DISCUSSION: An important new Telnet option, LINEMODE [TELNET:12], has been proposed. The LINEMODE option provides a standard way for a User Telnet and a Server Telnet to agree that the client rather than the server will perform terminal character processing. When the client has prepared a complete line of text, it will send it to the server in (usually) one TCP packet. This option will greatly decrease the packet cost of Telnet sessions and will also give much better user response over congested or long- delay networks.
The LINEMODE option allows dynamic switching between local and remote character processing. For example, the Telnet connection will automatically negotiate into single- character mode while a full screen editor is running, and then return to linemode when the editor is finished.
We expect that when this RFC is released, hosts should implement the client side of this option, and may implement the server side of this option. To properly implement the server side, the server needs to be able to tell the local system not to do any input character processing, but to remember its current terminal state and notify the Server Telnet process whenever the state changes. This will allow password echoing and full screen editors to be handled properly, for example.
3.4 TELNET/USER INTERFACE
3.4.1 Character Set Transparency
User Telnet implementations SHOULD be able to send or receive any 7-bit ASCII character. Where possible, any special character interpretations by the user host's operating system SHOULD be bypassed so that these characters can conveniently be sent and received on the connection.
Some character value MUST be reserved as "escape to command mode"; conventionally, doubling this character allows it to be entered as data. The specific character used SHOULD be user selectable.
On binary-mode connections, a User Telnet program MAY provide an escape mechanism for entering arbitrary 8-bit values, if the host operating system doesn't allow them to be entered directly from the keyboard.
IMPLEMENTATION: The transparency issues are less pressing on servers, but implementors should take care in dealing with issues like: masking off parity bits (sent by an older, non-conforming client) before they reach programs that expect only NVT ASCII, and properly handling programs that request 8-bit data streams.
3.4.2 Telnet Commands
A User Telnet program MUST provide a user the capability of entering any of the Telnet control functions IP, AO, or AYT, and SHOULD provide the capability of entering EC, EL, and Break.
3.4.3 TCP Connection Errors
A User Telnet program SHOULD report to the user any TCP errors that are reported by the transport layer (see "TCP/Application Layer Interface" section in [INTRO:1]).
3.4.4 Non-Default Telnet Contact Port
A User Telnet program SHOULD allow the user to optionally specify a non-standard contact port number at the Server Telnet host.
3.4.5 Flushing Output
A User Telnet program SHOULD provide the user the ability to specify whether or not output should be flushed when an IP is sent; see Section 3.2.4.
For any output flushing scheme that causes the User Telnet to flush output locally until a Telnet signal is received from the Server, there SHOULD be a way for the user to manually restore normal output, in case the Server fails to send the expected signal.
3.5. TELNET REQUIREMENTS SUMMARY
| | | | |S| | | | | | |H| |F | | | | |O|M|o | | |S| |U|U|o | | |H| |L|S|t | |M|O| |D|T|n | |U|U|M| | |o | |S|L|A|N|N|t | |T|D|Y|O|O|t
| FEATURE | SECTION | T | T | e | |||
|---|---|---|---|---|---|---|---|
| Option Negotiation | 3.2.1 | x | |||||
| Avoid negotiation loops | 3.2.1 | x | |||||
| Refuse unsupported options | 3.2.1 | x | |||||
| Negotiation OK anytime on connection | 3.2.1 | x | |||||
| Default to NVT | 3.2.1 | x | |||||
| Send official name in Term-Type option | 3.2.8 | x | |||||
| Accept any name in Term-Type option | 3.2.8 | x | |||||
| Implement Binary, Suppress-GA options | 3.3.3 | x | |||||
| Echo, Status, EOL, Ext-Opt-List options | 3.3.3 | x | |||||
| Implement Window-Size option if appropriate | 3.3.3 | x | |||||
| Server initiate mode negotiations | 3.3.4 | x | |||||
| User can enable/disable init negotiations | 3.3.4 | x | |||||
| Go-Aheads | |||||||
| Non-GA server negotiate SUPPRESS-GA option | 3.2.2 | x | |||||
| User or Server accept SUPPRESS-GA option | 3.2.2 | x | |||||
| User Telnet ignore GA's | 3.2.2 | x | |||||
| Control Functions | |||||||
| Support SE NOP DM IP AO AYT SB | 3.2.3 | x | |||||
| Support EOR EC EL Break | 3.2.3 | x | |||||
| Ignore unsupported control functions | 3.2.3 | x | |||||
| User, Server discard urgent data up to DM | 3.2.4 | x | |||||
| User Telnet send "Synch" after IP, AO, AYT | 3.2.4 | x | |||||
| Server Telnet reply Synch to IP | 3.2.4 | x | |||||
| Server Telnet reply Synch to AO | 3.2.4 | x | |||||
| User Telnet can flush output when send IP | 3.2.4 | x | |||||
| Encoding | |||||||
| Send high-order bit in NVT mode | 3.2.5 | x | |||||
| Send high-order bit as parity bit | 3.2.5 | x | |||||
| Negot. BINARY if pass high-ord. bit to applic | 3.2.5 | x | |||||
| Always double IAC data byte | 3.2.6 | x |
Double IAC data byte in binary mode |3.2.7 |x| | | | | Obey Telnet cmds in binary mode |3.2.7 |x| | | | | End-of-line, CR NUL in binary mode |3.2.7 | | | | |x| | | | | | | | End-of-Line | | | | | | | EOL at Server same as local end-of-line |3.3.1 |x| | | | | ASCII Server accept CR LF or CR NUL for EOL |3.3.1 |x| | | | | User Telnet able to send CR LF, CR NUL, or LF |3.3.1 |x| | | | | ASCII user able to select CR LF/CR NUL |3.3.1 | |x| | | | User Telnet default mode is CR LF |3.3.1 | |x| | | | Non-interactive uses CR LF for EOL |3.3.1 |x| | | | | | | | | | | | User Telnet interface | | | | | | | Input & output all 7-bit characters |3.4.1 | |x| | | | Bypass local op sys interpretation |3.4.1 | |x| | | | Escape character |3.4.1 |x| | | | | User-settable escape character |3.4.1 | |x| | | | Escape to enter 8-bit values |3.4.1 | | |x| | | Can input IP, AO, AYT |3.4.2 |x| | | | | Can input EC, EL, Break |3.4.2 | |x| | | | Report TCP connection errors to user |3.4.3 | |x| | | | Optional non-default contact port |3.4.4 | |x| | | | Can spec: output flushed when IP sent |3.4.5 | |x| | | | Can manually restore output mode |3.4.5 | |x| | | | | | | | | | |