TELNET(1)

📦 version 0.17, in ubuntu jammy

okay bestie, buckle up — this is TELNET(1) from the General Commands Manual, and it's a long one so it comes in four parts. this is part one. hydrate first 🍓

✨ NAME ✨

telnet — user interface to the TELNET protocol. that's the whole vibe 🫶

🌈 SYNOPSIS 🌈

telnet [-468ELadr] [-S tos] [-b address] [-e escapechar] [-l user]
       [-n tracefile] [host [port]]

💖 DESCRIPTION 💖

okay so the telnet command is what you use for interactive communication with another host using the TELNET protocol. it starts you in command mode, where it prints a lil telnet prompt ("telnet> "). and if you invoke telnet with a host argument, it just performs an open command implicitly, no big deal, it's giving main character energy. see the description below 👇

🎀 Options 🎀

  • -4 — force IPv4 address resolution. we're being specific 💅
  • -6 — force IPv6 address resolution. also specific, also iconic.
  • -8 — request 8-bit operation. this causes an attempt to negotiate the TELNET BINARY option for both input and output. by default telnet is not 8-bit clean, so this is you asking for the upgrade ✨
  • -E — disables the escape character functionality; that is, it sets the escape character to ``no character''. obsessed, no notes.
  • -L — specifies an 8-bit data path on output. this causes the TELNET BINARY option to be negotiated on just output. only output, bestie.
  • -a — attempt automatic login. currently, this sends the user name via the USER variable of the NEW-ENVIRON option if the remote system supports it. the username is retrieved via getlogin(3).
  • -b address — use bind(2) on the local socket to bind it to a specific local address.
  • -d — sets the initial value of the debug toggle to TRUE. debug girlies rise.
  • -r — emulate rlogin(1). in this mode, the default escape character is a tilde. also, the interpretation of the escape character is changed: an escape character followed by a dot causes telnet to disconnect from the remote host. a ^Z instead of a dot suspends telnet, and a ^] (the default telnet escape character) generates a normal telnet prompt. these codes are accepted only at the beginning of a line. iykyk.
  • -S tos — sets the IP type-of-service (TOS) option for the telnet connection to the value tos.
  • -e escapechar — sets the escape character to escapechar. if no character is supplied, no escape character will be used. entering the escape character while connected causes telnet to drop to command mode. 🌀
  • -l user — specify user as the user to log in as on the remote system. this is accomplished by sending the specified name as the USER environment variable, so it requires that the remote system support the TELNET NEW-ENVIRON option. this option implies the -a option, and may also be used with the open command.
  • -n tracefile — opens tracefile for recording trace information. see the set tracefile command below.
  • host — specifies a host to contact over the network.
  • port — specifies a port number or service name to contact. if not specified, the telnet port (23) is used.

🦋 Protocol 🦋

once a connection has been opened, telnet will attempt to enable the TELNET LINEMODE option. if this fails, then telnet will revert to one of two input modes: either “character at a time” or “old line by line”, depending on what the remote system supports.

when LINEMODE is enabled, character processing is done on the local system, under the control of the remote system. when input editing or character echoing is to be disabled, the remote system will relay that information. the remote system will also relay changes to any special characters that happen on the remote system, so that they can take effect on the local system. teamwork fr fr 🤝

in “character at a time” mode, most text typed is immediately sent to the remote host for processing.

in “old line by line” mode, all text is echoed locally, and (normally) only completed lines are sent to the remote host. the “local echo character” (initially “^E”) may be used to turn off and on the local echo (this would mostly be used to enter passwords without the password being echoed — sneaky 💀).

if the LINEMODE option is enabled, or if the localchars toggle is TRUE (the default for “old line by line”; see below), the user's quit, intr, and flush characters are trapped locally, and sent as TELNET protocol sequences to the remote side. if LINEMODE has ever been enabled, then the user's susp and eof are also sent as TELNET protocol sequences, and quit is sent as a TELNET ABORT instead of BREAK. there are options (see toggle autoflush and toggle autosynch below) which cause this action to flush subsequent output to the terminal (until the remote host acknowledges the TELNET sequence) and flush previous terminal input (in the case of quit and intr). 🌈

👑 Commands 👑

the following telnet commands are available. unique prefixes are understood as abbreviations, so you can be lazy about typing. we love that 💅

  • auth argument ... — the auth command controls the TELNET AUTHENTICATE protocol option. if telnet was compiled without authentication, the auth command will not be supported. valid arguments are as follows:
    • disable type — disable the specified type of authentication. to obtain a list of available types, use the auth disable ? command.
    • enable type — enable the specified type of authentication. to obtain a list of available types, use the auth enable ? command.
    • status — list the current status of the various types of authentication.
note: the current version of telnet does not support authentication. 😭 not me finding out like this.
  • close — close the connection to the remote host, if any, and return to command mode.
  • display argument ... — display all, or some, of the set and toggle values (see below).
  • encrypt argument ... — the encrypt command controls the TELNET ENCRYPT protocol option. if telnet was compiled without encryption, the encrypt command will not be supported.

valid arguments are as follows:

  • disable type [input|output] — disable the specified type of encryption. if you do not specify input or output, encryption of both is disabled. to obtain a list of available types, use ``encrypt disable ?''.
  • enable type [input|output] — enable the specified type of encryption. if you do not specify input or output, encryption of both is enabled. to obtain a list of available types, use ``encrypt enable ?''.
  • input — this is the same as ``encrypt start input''.
  • -input — this is the same as ``encrypt stop input''.
  • output — this is the same as ``encrypt start output''.
  • -output — this is the same as ``encrypt stop output''.
  • start [input|output] — attempt to begin encrypting. if you do not specify input or output, encryption of both input and output is started.
  • status — display the current status of the encryption module.
  • stop [input|output] — stop encrypting. if you do not specify input or output, encryption of both is stopped.
  • type type — sets the default type of encryption to be used with later encrypt start'' or encrypt stop'' commands.
note: the current version of telnet does not support encryption. 🙈 again!! the audacity.
  • environ arguments... — the environ command is used to propagate environment variables across the telnet link using the TELNET NEW-ENVIRON protocol option. all variables exported from the shell are defined, but only the DISPLAY and PRINTER variables are marked to be sent by default. the USER variable is marked to be sent if the -a or -l command-line options were used.

valid arguments for the environ command are:

  • define variable value — define the variable variable to have a value of value. any variables defined by this command are automatically marked for propagation (``exported''). the value may be enclosed in single or double quotes so that tabs and spaces may be included.
  • undefine variable — remove any existing definition of variable.
  • export variable — mark the specified variable for propagation to the remote host.
  • unexport variable — do not mark the specified variable for propagation to the remote host. the remote host may still ask explicitly for variables that are not exported.
  • list — list the current set of environment variables. those marked with a * will be propagated to the remote host. the remote host may still ask explicitly for the rest.
  • ? — prints out help information for the environ command.
  • logout — send the TELNET LOGOUT protocol option to the remote host. this command is similar to a close command. if the remote host does not support the LOGOUT option, nothing happens. but if it does, this command should cause it to close the connection. if the remote side also supports the concept of suspending a user's session for later reattachment, the logout command indicates that the session should be terminated immediately. dramatic exit, honestly 💅
  • mode type — type is one of several options, depending on the state of the session. telnet asks the remote host to go into the requested mode. if the remote host says it can, that mode takes effect. consent-based mode switching, we respect it ✨
    • character — disable the TELNET LINEMODE option, or, if the remote side does not understand the LINEMODE option, then enter “character at a time” mode.
    • line — enable the TELNET LINEMODE option, or, if the remote side does not understand the LINEMODE option, then attempt to enter “old-line-by-line” mode.

okay so back to the mode command — here are the lil modes you can flip, bestie 💅

  • isig (-isig) — try to enable (or disable) the TRAPSIG mode of the LINEMODE option. This requires that the LINEMODE option be enabled, fr.
  • edit (-edit) — try to enable (or disable) the EDIT mode of the LINEMODE option. This also requires LINEMODE to be enabled.
  • softtabs (-softtabs) — try to enable (or disable) the SOFT_TAB mode of the LINEMODE option. Needs LINEMODE enabled.
  • litecho (-litecho) — try to enable (or disable) the LIT_ECHO mode of the LINEMODE option. Also needs LINEMODE enabled.
  • ? — prints out help information for the mode command. ✨

open 🌐

open host [[-l] user][- port]

Open a connection to the named host. If you don't specify a port number, telnet will try to contact a telnet daemon at the standard port (23). The host specification may be a host name or IP address. You can use the -l option to specify a user name to be passed to the remote system, like the -l command-line option.

When connecting to ports other than the telnet port, telnet does not attempt telnet protocol negotiations. This makes it possible to connect to services that do not support the telnet protocol without making a mess. Protocol negotiation can be forced by placing a dash before the port number.

After establishing a connection, any commands associated with the remote host in /etc/telnetrc and the user's .telnetrc file are executed, in that order.

The format of the telnetrc files is as follows: Lines beginning with a #, and blank lines, are ignored. The rest of the file should consist of hostnames and sequences of telnet commands to use with that host. Commands should be one per line, indented by whitespace; lines beginning without whitespace are interpreted as hostnames. Lines beginning with the special hostname ‘DEFAULT’ will apply to all hosts. Hostnames including ‘DEFAULT’ may be followed immediately by a colon and a port number or string. If a port is specified it must match exactly with what is specified on the command line. If no port was specified on the command line, then the value ‘telnet’ is used. Upon connecting to a particular host, the commands associated with that host are executed.

quit 👋

Close any open session and exit telnet. An end of file condition on input, when in command mode, will trigger this operation as well. Bye bestie!

send 📡

send arguments

Send one or more special telnet protocol character sequences to the remote host. The following are the codes which may be specified (more than one may be used in one command):

  • abort — Sends the TELNET ABORT (Abort Processes) sequence.
  • ao — Sends the TELNET AO (Abort Output) sequence, which should cause the remote system to flush all output from the remote system to the user's terminal.
  • ayt — Sends the TELNET AYT (Are You There?) sequence, to which the remote system may or may not choose to respond.
  • brk — Sends the TELNET BRK (Break) sequence, which may have significance to the remote system.
  • ec — Sends the TELNET EC (Erase Character) sequence, which should cause the remote system to erase the last character entered.
  • el — Sends the TELNET EL (Erase Line) sequence, which should cause the remote system to erase the line currently being entered.
  • eof — Sends the TELNET EOF (End Of File) sequence.
  • eor — Sends the TELNET EOR (End of Record) sequence.
  • escape — Sends the current telnet escape character.
  • ga — Sends the TELNET GA (Go Ahead) sequence, which likely has no significance to the remote system.
  • getstatus — If the remote side supports the TELNET STATUS command, getstatus will send the subnegotiation to request that the server send its current option status.
  • ip — Sends the TELNET IP (Interrupt Process) sequence, which should cause the remote system to abort the currently running process.
  • nop — Sends the TELNET NOP (No Operation) sequence.
  • susp — Sends the TELNET SUSP (Suspend Process) sequence.
  • synch — Sends the TELNET SYNCH sequence. This sequence causes the remote system to discard all previously typed (but not yet read) input. This sequence is sent as TCP urgent data (and may not work if the remote system is a 4.2BSD system -- if it doesn't work, a lower case “r” may be echoed on the terminal).
  • do cmd, dont cmd, will cmd, wont cmd — Sends the TELNET DO cmd sequence. cmd can be either a decimal number between 0 and 255, or a symbolic name for a specific TELNET command. cmd can also be either help or ? to print out help information, including a list of known symbolic names.
  • ? — Prints out help information for the send command. 🫶

set / unset 🎛️

set argument value

unset argument value

The set command will set any one of a number of telnet variables to a specific value or to TRUE. The special value off turns off the function associated with the variable. This is equivalent to using the unset command. The unset command will disable or set to FALSE any of the specified variables. The values of variables may be interrogated with the display command. The variables which may be set or unset, but not toggled, are listed here. In addition, any of the variables for the toggle command may be explicitly set or unset.

  • ayt — If telnet is in localchars mode, or LINEMODE is enabled, and the status character is typed, a TELNET AYT sequence is sent to the remote host. The initial value for the "Are You There" character is the terminal's status character.
  • echo — This is the value (initially “^E”) which, when in “line by line” mode, toggles between doing local echoing of entered characters (for normal processing), and suppressing echoing of entered characters (for entering, say, a password).
  • eof — If telnet is operating in LINEMODE or “old line by line” mode, entering this character as the first character on a line will cause this character to be sent to the remote system. The initial value of the eof character is taken to be the terminal's eof character.
  • erase — If telnet is in localchars mode (see toggle localchars below), and if telnet is operating in “character at a time” mode, then when this character is typed, a TELNET EC sequence (see send ec above) is sent to the remote system. The initial value for the erase character is taken to be the terminal's erase character.
  • escape — This is the telnet escape character (initially “^]”) which causes entry into telnet command mode (when connected to a remote system).
  • flushoutput — If telnet is in localchars mode (see toggle localchars below) and the flushoutput character is typed, a TELNET AO sequence (see send ao above) is sent to the remote host. The initial value for the flush character is taken to be the terminal's flush character.
  • forw1, forw2 — If TELNET is operating in LINEMODE, these are the characters that, when typed, cause partial lines to be forwarded to the remote system. The initial value for the forwarding characters are taken from the terminal's eol and eol2 characters.
  • interrupt — If telnet is in localchars mode (see toggle localchars below) and the interrupt character is typed, a TELNET IP sequence (see send ip above) is sent to the remote host. The initial value for the interrupt character is taken to be the terminal's intr character.
  • kill — If telnet is in localchars mode (see toggle localchars below), and if telnet is operating in “character at a time” mode, then when this character is typed, a TELNET EL sequence (see send el above) is sent to the remote system. The initial value for the kill character is taken to be the terminal's kill character.
  • lnext — If telnet is operating in LINEMODE or “old line by line“ mode, then this character is taken to be the terminal's lnext character. The initial value for the lnext character is taken to be the terminal's lnext character.
  • quit — If telnet is in localchars mode (see toggle localchars below) and the quit character is typed, a TELNET BRK sequence (see send brk above) is sent to the remote host. The initial value for the quit character is taken to be the terminal's quit character.
  • reprint — when telnet is running in LINEMODE or in "old line by line" mode, this char is treated as your terminal's reprint character. The initial value for the reprint character is taken to be the terminal's reprint character. 💫
    • rlogin — this is the rlogin mode escape character. Set it and boom, rlogin mode is on, exactly like using the r command-line option (q.v.). 👑
    • start — if the TELNET TOGGLE-FLOW-CONTROL option has been enabled, then this character is taken to be the terminal's start character. The initial value for the kill character is taken to be the terminal's start character. (yes bestie, it says kill character right there, not me, the page is just like that 🙈)
    • stop — if the TELNET TOGGLE-FLOW-CONTROL option has been enabled, then this character is taken to be the terminal's stop character. The initial value for the kill character is taken to be the terminal's stop character. (same little quirk, love that for us 💀)
    • susp — if telnet is in localchars mode, or LINEMODE is enabled, and you type the suspend character, a TELNET SUSP sequence gets sent to the remote host (see send susp above). The initial value for the suspend character is taken to be the terminal's suspend character. ✨
    • tracefile — the file where the output goes when netdata or option tracing is TRUE. Set it to - and all that tracing info gets written to standard output instead (which is the default, fyi). 📝
    • worderase — if telnet is operating in LINEMODE or "old line by line" mode, this character is taken to be the terminal's worderase character. The initial value for the worderase character is taken to be the terminal's worderase character. 🧹
    • ? — displays the legal set (unset) commands. iykyk, it's the cheat sheet. 💅
  • slc state — the slc command (short for Set Local Characters, obvi) sets or changes the state of the special characters whenever the TELNET LINEMODE option has been enabled. And what even is a special character? It's a character that gets mapped to TELNET command sequences (like ip or quit) or to line editing characters (like erase and kill). By default, the local special characters are exported. 🦋
    • check — verify the current settings for the current special characters. The remote side gets asked to send over all its current special character settings, and if anything disagrees with the local side, the local side switches to the remote value. vibe check, but make it protocol. 🌸
    • export — switch to the local defaults for the special characters. Those local default characters are the ones belonging to your local terminal at the moment telnet was started. 🏠
    • import — switch to the remote defaults for the special characters. Those remote default characters are the ones belonging to the remote system at the moment the TELNET connection was established. 🌍
    • ? — prints out help information for the slc command. 💖
  • status — shows the current status of telnet. That means the name of the remote host, if there is one, plus whatever mode you're currently in. main character check-in energy. 👑
  • toggle arguments ... — flips (between TRUE and FALSE) a bunch of flags that control how telnet reacts to events. You can also set these flags explicitly to TRUE or FALSE with the set and unset commands. Multiple flags at once? Absolutely, toggle away. And you can peek at the state of these flags with the display command. Here's the full roster of valid flags: ⭐
    • authdebug — turns on debugging for the authentication code. Heads up: this flag only exists if authentication support is enabled. 🔍
    • autoflush — if autoflush and localchars are both TRUE, then when the ao or quit characters get recognized (and transformed into TELNET sequences; see set above for the deets), telnet straight up refuses to display any data on your terminal until the remote system acknowledges — via a TELNET TIMING MARK option — that it has processed those TELNET sequences. The initial value for this toggle is TRUE if the terminal user had not done an "stty noflsh", otherwise FALSE (see stty(1)). 🚰
    • autodecrypt — when the TELNET ENCRYPT option is negotiated, by default the actual encryption (decryption) of the data stream does not start automatically. The autoencrypt (autodecrypt) command states that encryption of the output (input) stream should be enabled as soon as possible.
note that this flag exists only if encryption support is enabled. 🔐
  • autologin — if the remote side supports the TELNET AUTHENTICATION option, telnet tries to use it to do automatic authentication. If the TELNET AUTHENTICATION option is not supported, your login name gets propagated using the TELNET NEW-ENVIRON option instead. Setting this flag is the same thing as specifying the a option to the open command or on the command line. 🪪
  • autosynch — if autosynch and localchars are both TRUE, then when you type either the intr or quit characters (see set above for descriptions of the intr and quit characters), the resulting telnet sequence that gets sent is followed by the TELNET SYNCH sequence. This procedure should cause the remote system to start throwing away all previously typed input until both of those telnet sequences have been read and acted upon. The initial value of this toggle is FALSE. 🌊
  • binary — enable or disable the TELNET BINARY option on both input and output. 🎯
  • inbinary — enable or disable the TELNET BINARY option on input. 📥
  • outbinary — enable or disable the TELNET BINARY option on output. 📤
  • crlf — if this is TRUE, then carriage returns get sent as <CR><LF>. If it's FALSE, then carriage returns get send as <CR><NUL>. The initial value for this toggle is FALSE. ↩️
  • crmod — toggles carriage return mode. When this mode is enabled, most carriage return characters received from the remote host get mapped into a carriage return followed by a line feed. This mode does not affect the characters you type, only the ones coming back from the remote host. And honestly? This mode is not very useful unless the remote host only ever sends carriage return but never line feed. The initial value for this toggle is FALSE. 🛤️
  • debug — toggles socket level debugging (useful only to the super user, so if that's not you, mind your business 💅). The initial value for this toggle is FALSE. 🐛
  • encdebug — turns on debugging information for the encryption code. Note that this flag only exists if encryption support is available. 🔓
  • localchars — if this is TRUE, then the flush, interrupt, quit, erase, and kill characters (see set above) are recognized locally, and transformed into (hopefully) appropriate TELNET control sequences (respectively ao, ip, brk, ec, and el; see send above). The initial value for this toggle is TRUE in "old line by line" mode, and FALSE in "character at a time" mode. When the LINEMODE option is enabled, the value of localchars is ignored, and assumed to always be TRUE. And if LINEMODE has ever been enabled, then quit is sent as abort, and eof and susp are sent as eof and susp, see send above). 🌈
  • netdata — toggles the display of all network data (in hexadecimal format). The initial value for this toggle is FALSE. 🔢
  • options — toggles the display of some internal telnet protocol processing (the stuff having to do with telnet options). The initial value for this toggle is FALSE. ⚙️
  • prettydump — when the netdata toggle is enabled, if prettydump is also enabled the output from the netdata command gets formatted in a much more user-readable way. Spaces are put between each character in the output, and the beginning of telnet escape sequences are preceded by a '*' to help you locate them. so iconic of it. 🎀
  • skiprc — when the skiprc toggle is TRUE, telnet does not read the telnetrc files. The initial value for this toggle is FALSE. 🙈
  • termdata — toggles the display of all terminal data (in hexadecimal format). The initial value for this toggle is FALSE. 🖥️

verbose_encrypt

When the verbose_encrypt toggle is TRUE, TELNET spills the tea 💬 — it prints out a message every single time encryption is enabled or disabled. The initial value for this toggle is FALSE, bestie. It's giving quiet luxury. 🎀

Heads up: this flag only exists if encryption support is available. No encryption build, no flag, no drama. 💅

?

Displays the legal toggle commands. Basically the toggle list's own little help menu — ask nicely and it shows you the whole lineup. ✨

z

Suspend telnet. 😴 Just... pause it. Put it in timeout.

Real talk though: this command only works when the user is using csh(1). Any other shell and it's simply not happening, no cap. 😭

! [command]

Execute a single command in a subshell on the local system. 🖥️ If command is omitted, then an interactive subshell is invoked. So ! with stuff after it runs one thing locally, and bare ! drops you into a whole interactive subshell. Yes bestie, the local system, not the remote one — read it again, it matters. 🫶

? [command]

Get help. 🆘 With no arguments, telnet prints a help summary. If a command is specified, telnet will print the help information for just that command. So like ? open gives you the lore on open specifically and nothing else. Iconic. 👑

🌍 ENVIRONMENT 🌍

Telnet uses at least the HOME, SHELL, DISPLAY, and TERM environment variables. Other environment variables may be propagated to the other side via the TELNET NEW-ENVIRON option. 👀

📁 FILES 📁

  • /etc/telnetrc — global telnet startup values
  • ~/.telnetrc — user customized telnet startup values

📜 HISTORY 📜

The Telnet command appeared in 4.2BSD. Vintage. Old money. Literally a classic. 🕰️✨

📝 NOTES 📝

On some remote systems, echo has to be turned off manually when in “old line by line” mode. 🙈 So if things look weird, that's why — go turn it off yourself.

In “old line by line” mode or LINEMODE the terminal's eof character is only recognized (and sent to the remote system) when it is the first character on a line. 🦋 Position matters, fr fr.

💀 BUGS 💀

The source code is not comprehensible. That's it. That's the whole bug report. Not me opening that C file and immediately closing it. 😭💀

And that's the whole page, bestie — this one's from Linux NetKit (0.17), dated August 15, 1999, and it's TELNET(1). Slay. 🎀👑

📦 where this page comes from