TELNET(1)

📦
version 0.17.41+0.2 · 2 versions
, in ubuntu jammy

TELNET(1) 💖✨

okay bestie, listen — this is from the General Commands Manual, and it's the whole vibe of talking to another computer over the network. let's go 🦋

✨ NAME ✨

telnet — user interface to the TELNET protocol 🌸

✨ SYNOPSIS ✨

telnet [-468EKLadr] [-S tos] [-X authtype] [-b address] [-e escapechar]
       [-l user] [-n tracefile] [-z option] [host [port]]

✨ DESCRIPTION ✨

So telnet is literally the command for interactive communication with another host using the TELNET protocol 💅

It starts you off in command mode, where it shows you a telnet prompt that looks like "telnet> ". And here's the cute part — if you invoke telnet with a host argument, it just goes ahead and runs an open command implicitly, no asking. (the description of that is down below, keep reading bestie) 👑

Options:

  • -4 — Force IPv4 address resolution. old-school IP energy ✨
  • -6 — Force IPv6 address resolution. futuristic IP energy ✨
  • -8 — Request 8-bit operation. This makes it try to negotiate the TELNET BINARY option for both input and output. Heads up: by default telnet is not 8-bit clean, so this is a whole upgrade 😌
  • -E — Disables the escape character functionality entirely — that is, it sets the escape character to no character'' ``. gone. poof. 🫥
  • -K — Specifies no automatic login to the remote system. we're doing this manually, bestie 🙅‍♀️
  • -L — Specifies an 8-bit data path on output. This causes the TELNET BINARY option to be negotiated on just output. (so half the 8-bit glow-up, if u will) 💅
  • -X atype — Disables the atype type of authentication.
  • -a — Attempt automatic login. Right now, this sends the user name via the USER variable of the NEW-ENVIRON option — but only if the remote system supports it. The username is retrieved via getlogin(3). slay 🔥
  • -b address — Use bind(2) on the local socket to bind it to a specific local address. home address, but for sockets 🏠
  • -d — Sets the initial value of the debug toggle to TRUE. debug mode from the jump 🐛
  • -r — Emulate rlogin(1). In this mode, the default escape character is a tilde. Also the interpretation of the escape character changes:
    • an escape character followed by a dot causes telnet to disconnect from the remote host 🚪
    • a ^Z instead of a dot suspends telnet 💤
    • a ^] (the default telnet escape character) generates a normal telnet prompt

and important: these codes are accepted only at the beginning of a line. no mid-sentence chaos bestie ⚠️

  • -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. ur little teleport button ✨
  • -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. bestie privileges unlocked 👑
  • -n tracefile — Opens tracefile for recording trace information. See the set tracefile command below. receipts, but saved to disk 📝
  • -z option — Set SSL (Secure Socket Layer) parameters. The default is to negotiate via telnet protocol if SSL is available at server side and then to switch it on. In this mode you can connect to both conventional and SSL enhanced telnetd's.
⚠️ If the connection is made to localhost and -z secure is not set, then SSL is not enabled. lowkey important, no cap 💀

The SSL parameters are:

  • debug — Send SSL related debugging information to stderr.
  • authdebug — Enable authentication debugging.
  • ssl — Negotiate SSL at first, then use TELNET protocol. In this mode you can connect to any server directly supporting SSL, like Apache-SSL. The TELNET protocol negotiation is done encrypted. A typical example is the call telnet -z ssl mail.google.com https. this is THE iconic move 🔐
  • nossl, !ssl — switch off SSL negotiation
  • certrequired — server certificate is mandatory
  • secure — Don't switch back to unencrypted mode (no SSL) if SSL is not available. no downgrades, periodt 💅
  • verbose — Be verbose about certificates etc. tell me EVERYTHING 🗣️
  • verify=int — Set the SSL verify flags. (See SSL_VERIFY_* in openssl/ssl.h.)
  • cacert=CA_file — This is used for verification of whatever certificate the remote server cares to send as identifier.
  • cert=cert_file — Present the certificate(s) in cert_file to the server. They are in PEM-format, and the first identifies you as a client.
  • key=key_file — Use the key(s) in key_file in case a key is not stored together with the certificate.
  • cipher=ciph_list — Set the preferred ciphers to ciph_list. The environment variable SSL_CIPHER serves the same purpose. (See openssl/ssl.h.)
  • host — Specifies a host to contact over the network. who are we calling today bestie 📞
  • 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 that fails, then telnet reverts 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. it's giving teamwork ✨

In “character at a time” mode, most text typed is immediately sent to the remote host for processing. instant. no chill. 💨

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 🤫

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 u can be lazy and it still works, iconic ✨)

  • 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.
  • close — Close the connection to the remote host, if any, and return to command mode. bye bestie 👋
  • display argument ... — Display all, or some, of the set and toggle values (see below).
  • 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. it's giving "we're SO done here" 💅
  • 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.
    • 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 — turns on the TELNET LINEMODE option, or if the remote side is giving "idk what that is," it tries to slide into "old-line-by-line" mode instead.
  • isig (-isig) — tries to enable (or disable) the TRAPSIG mode of the LINEMODE option. Heads up bestie: LINEMODE has to be enabled first or this does nothing.
  • edit (-edit) — tries to enable (or disable) the EDIT mode of the LINEMODE option. Also needs LINEMODE enabled. No LINEMODE, no vibes.
  • softtabs (-softtabs) — tries to enable (or disable) the SOFT_TAB mode of the LINEMODE option. Same deal, LINEMODE must be on.
  • litecho (-litecho) — tries to enable (or disable) the LIT_ECHO mode of the LINEMODE option. Yep, you guessed it, LINEMODE required.
  • ? — prints out the help info for the mode command. Self-care.

🌸 open — let's go meet somebody 🌸

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

Opens a connection to the named host. If you don't give a port number, telnet tries to reach a telnet daemon at the standard port (23). The host spec can be a host name or an IP address. The -l option lets you name a user to pass along to the remote system, exactly like the -l command-line option.

Now this part is lowkey important, so listen up: when you connect to ports other than the telnet port, telnet does not attempt telnet protocol negotiations. That's a feature, not a bug, bestie — it means you can connect to services that don't speak telnet protocol without making a whole mess. If you do want protocol negotiation anyway, you force it by putting a dash before the port number.

Once the connection is up, any commands tied to that remote host in /etc/telnetrc and in the user's .telnetrc file get executed, in that order. 💅

The format of the telnetrc files goes like this:

  • Lines starting with #? Ignored. Blank lines? Also ignored.
  • The rest of the file is hostnames plus sequences of telnet commands to use with that host.
  • Commands go one per line, indented by whitespace. Lines that begin without whitespace get read as hostnames.
  • Lines starting with the special hostname DEFAULT 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 has to match exactly with whatever was specified on the command line. If no port was specified on the command line, then the value telnet is used.
  • When you connect to a particular host, the commands associated with that host get executed.

🎀 quit — we're done here 🎀

Closes any open session and exits telnet. An end of file condition on input, while you're in command mode, will trigger this too. Bye bestie 🫶

✨ send — beam special sequences into the void ✨

send arguments

Sends one or more special telnet protocol character sequences to the remote host. Here are the codes you can specify (and yes, you can use more than one in a single command):

  • abort — sends the TELNET ABORT (Abort Processes) sequence.
  • ao — sends the TELNET AO (Abort Output) sequence, which should make the remote system flush all output going from the remote system to your terminal.
  • ayt — sends the TELNET AYT (Are You There?) sequence. The remote system may or may not choose to respond. Honestly, relatable.
  • 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 make the remote system erase the last character entered.
  • el — sends the TELNET EL (Erase Line) sequence, which should make the remote system erase the line you're currently entering.
  • 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. It's giving decoration.
  • getstatus — if the remote side supports the TELNET STATUS command, getstatus sends the subnegotiation requesting that the server send its current option status. Snooping, but politely.
  • ip — sends the TELNET IP (Interrupt Process) sequence, which should make the remote system 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 one makes the remote system discard all previously typed (but not yet read) input. It's sent as TCP urgent data — and it 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 — toggling the variables 💜

set argument value
unset argument value

The set command sets any one of a bunch of telnet variables to a specific value, or to TRUE. The special value off turns off the function associated with the variable — which is the same thing as using the unset command. The unset command disables, or sets to FALSE, any of the specified variables. You can interrogate the values of variables with the display command. 👑

The variables listed here can be set or unset, but not toggled. Also, any of the variables for the toggle command can be explicitly set or unset too.

  • 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 values 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 chillin' in localchars mode (peep toggle localchars down below bestie) and you hit the quit character, a TELNET BRK sequence gets yeeted to the remote host (see send brk above). The initial value for the quit character is literally just your terminal's quit character. 💅
  • reprint — if telnet is out here operating in LINEMODE or "old line by line" mode, then this character is taken to be the terminal's reprint character. And the initial value for the reprint character? You guessed it, it's taken to be the terminal's reprint character. ✨
  • rlogin — this is the rlogin mode escape character. Setting it enables rlogin mode, exactly like the r command-line option (q.v.). iconic behavior fr fr 🎀
  • 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. 💀
  • 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. 💀
  • susp — if telnet is in localchars mode, or LINEMODE is enabled, and the suspend character is typed, 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 — this is the file where all the output from netdata or option tracing being TRUE gets written. And if you set it to "-", then tracing information goes straight to standard output (which is the default, btw). 👑
  • worderase — if telnet is operating in LINEMODE or "old line by line" mode, then this character is taken to be the terminal's worderase character. Initial value for the worderase character is taken to be the terminal's worderase character. 🧚
  • ? — Displays the legal set (unset) commands. short n sweet, we love her 🌸

🌈 slc state 🌈

The slc command (Set Local Characters) is how you set or change the state of the special characters when the TELNET LINEMODE option has been enabled. So like, what even are special characters? They're the characters that get mapped to TELNET command sequences (think ip or quit) or to line editing characters (think erase and kill). And by default? The local special characters are exported. periodt. 💖

  • check — verify the current settings for the current special characters. The remote side is requested to send all the current special character settings, and if there's any discrepancy with the local side, the local side will switch to the remote value. vibe check but for characters ✨
  • export — switch to the local defaults for the special characters. Those local default characters are the ones from your local terminal at the time when telnet was started. 🫶
  • import — switch to the remote defaults for the special characters. The remote default characters are those of the remote system at the time when the TELNET connection was established. 🌟
  • ? — prints out help information for the slc command. slay 💅

🔥 startssl 🔥

Attempt to negotiate telnet-over-SSL (same energy as the -z ssl option). This is highkey useful when you're connecting to non-telnetds like imapd (with the STARTTLS command). But heads up bestie: to control SSL when connecting to an SSL-enabled telnetd, use the auth command instead. iykyk 😌

👑 status 👑

Show the current status of telnet. This includes the name of the remote host (if there even is one) as well as the current mode. main character energy, fr fr 💜

💫 toggle arguments ... 💫

Toggle (back and forth between TRUE and FALSE) a bunch of flags that control how telnet responds to events. You can also set these flags explicitly to TRUE or FALSE using the set and unset commands. And yes, you can toggle more than one flag at once — multitasking icon. The state of these flags can be examined with the display command. Here are the valid flags, no cap: 🦋

  • authdebug — turns on debugging for the authentication code. Ngl this flag only exists if authentication support is enabled. 🙈
  • autoflush — if autoflush and localchars are both TRUE, then when the ao or quit characters are recognized (and transformed into TELNET sequences; see set above for details), telnet 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)). 😭
  • autologin — if the remote side supports the TELNET AUTHENTICATION option, telnet attempts to use it to perform automatic authentication. If the TELNET AUTHENTICATION option is not supported, the user's login name is propagated using the TELNET NEW-ENVIRON option. Setting this flag is the same as specifying the a option to the open command or on the command line. 💅
  • autosynch — if autosynch and localchars are both TRUE, then when either the intr or quit characters is typed (see set above for descriptions of the intr and quit characters), the resulting telnet sequence sent is followed by the TELNET SYNCH sequence. This procedure should cause the remote system to begin throwing away all previously typed input until both of the 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. both. at once. icon. ✨
  • 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 will be sent as <CR><LF>. If this is FALSE, then carriage returns will be send as <CR><NUL>. The initial value for this toggle is FALSE. 🌸
  • crmod — toggle carriage return mode. When this mode is enabled, most carriage return characters received from the remote host will be mapped into a carriage return followed by a line feed. Important: this mode does not affect those characters typed by the user, only those received from the remote host. Also ngl this mode is not very useful unless the remote host only 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). The initial value for this toggle is FALSE. 👑
  • 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. If LINEMODE has ever been enabled, then quit is sent as abort, and eof and suspend 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 (having to do with telnet options). The initial value for this toggle is FALSE. 💜
  • prettydump — when the netdata toggle is enabled, if prettydump is enabled the output from the netdata command will be formatted in a more user-readable format. Spaces are put between each character in the output, and the beginning of telnet escape sequences are preceded by a '*' to aid in locating them. cute and functional 🌈
  • 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. ⭐
  • ? — Displays the legal toggle commands. short queen 💖
  • z — suspend telnet. pausing the whole vibe ✨ heads up though: this one only works when you're using csh(1), so if that's not your shell, bestie, it's giving nothing 😭
  • ! [command] — run a single command in a subshell over on your local system. If you don't hand it a command, it just opens up an interactive subshell instead 🐚 lowkey a whole side quest
  • ? [command] — help time!! 🎀 With no arguments at all, telnet prints a help summary. But if you pass a command, it prints the help info for just that command — targeted, specific, main character energy 👑

🌸 ENVIRONMENT 🌸

telnet uses at least these environment variables, no cap:

  • HOME
  • SHELL
  • DISPLAY
  • TERM

Other environment variables may get propagated over to the other side via the TELNET NEW-ENVIRON option 🌈 (it's giving "sharing is caring" but for env vars).

And the variable SSL_CIPHER gets accessed when setting up encrypted traffic 🔐✨

📁 FILES 📁

  • /etc/telnetrc — global telnet startup values 🌍
  • ~/.telnetrc — your own user customized telnet startup values 💅 (this is your house, your rules)

🕰️ HISTORY 🕰️

The Telnet command appeared in 4.2BSD. ancient. iconic. still here. 👑

📝 NOTES 📝

On some remote systems, echo has to be turned off manually when you're in “old line by line” mode. like, nobody's gonna do it for you bestie 🙈

Also: 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. first or it doesn't count, periodt ⭐

💀 BUGS 💀

The source code is not comprehensible.

that's it. that's the bug. the whole thing. 😭💀 iykyk

okay that's the whole page!! go forth and telnet responsibly, bestie 🫶

and yes, this little legend is from Linux NetKit (0.17), dated August 15, 1999 — an absolute vintage baddie 🎀✨ she's old, she's classic, she's still serving 🌸

📦 where this page comes from