TELNETD(8)

📦 version 1.0.3, in opensuse-leap 16.0

🌸 NAME 🌸

okay so bestie, welcome to TELNETD(8) from the System Manager's Manual 💖. This page is all about telnetd — the DARPA TELNET protocol server. slay.

✨ SYNOPSIS ✨

       telnetd [-a authmode] [-B] [-D] [debugmode] [-e] [-h] [-Iinitid] [-l]
       [-k] [-n] [-rlowpty-highpty] [-s] [-S tos] [-U] [-X authtype] [-w
       [ip|maxhostlen[,[no]striplocal]]] [-debug [port]]

🦋 DESCRIPTION 🦋

The telnetd command is a server which supports the DARPA standard TELNET virtual terminal protocol. 💅 Telnetd is normally invoked by the internet server (see inetd(8) for requests to connect to the TELNET port as indicated by the /etc/services file (see services(5))). The -debug option may be used to start up telnetd manually, instead of through inetd(8). If started up this way, port may be specified to run telnetd on an alternate TCP port number. 🫶

The telnetd command accepts the following options:

  • -a authmode — this option may be used for specifying what mode should be used for authentication. Note that this option is only useful if telnetd has been compiled with support for the AUTHENTICATION option. There are several valid values for authmode:
    • debug — turns on authentication debugging code. 🐛
    • valid — only allow connections when the remote user can provide valid authentication information to identify the remote user, and is allowed access to the specified account without providing a password.
    • user — only allow connections when the remote user can provide valid authentication information to identify the remote user. The login(1) command will provide any additional user verification needed if the remote user is not allowed automatic access to the specified account.
    • other — only allow connections that supply some authentication information. This option is currently not supported by any of the existing authentication mechanisms, and is thus the same as specifying -a valid.
    • none — this is the default state. Authentication information is not required. If no or insufficient authentication information is provided, then the login(1) program will provide the necessary user verification.
    • off — this disables the authentication code. All user verification will happen through the login(1) program.
  • -B — specifies bftp server mode. In this mode, telnetd causes login to start a bftp(1) session rather than the user's normal shell. In bftp daemon mode, normal logins are not supported, and it must be used on a port other than the normal TELNET port. 👑
  • -D debugmode — this option may be used for debugging purposes. This allows telnetd to print out debugging information to the connection, allowing the user to see what telnetd is doing. There are several possible values for debugmode:
    • options — prints information about the negotiation of TELNET options.
    • report — prints the options information, plus some additional information about what processing is going on.
    • netdata — displays the data stream received by telnetd.
    • ptydata — displays data written to the pty.
    • encrypt — enables encryption debugging code.
    • exercise — has not been implemented yet. 💀
  • -debug — enables debugging on each socket created by telnetd (see SO_DEBUG in socket(2)). 🔥
  • -e — this option causes telnetd to refuse unencrypted connections. 🚫
  • -h — disables the printing of host-specific information before login has been completed.
  • -I initid — this option is only applicable to UNICOS systems prior to 7.0. It specifies the ID from /etc/inittab to use when init starts login sessions. The default ID is fe.
  • -k — this option is only useful if telnetd has been compiled with both linemode and kludge linemode support. If the -k option is specified, then if the remote client does not support the LINEMODE option, then telnetd will operate in character at a time mode. It will still support kludge linemode, but will only go into kludge linemode if the remote client requests it. (This is done by the client sending DONT SUPPRESS-GO-AHEAD and DONT ECHO.) The -k option is most useful when there are remote clients that do not support kludge linemode, but pass the heuristic (if they respond with WILL TIMING-MARK in response to a DO TIMING-MARK) for kludge linemode support. 🧚
  • -l — specifies line mode. Tries to force clients to use line-at-a-time mode. If the LINEMODE option is not supported, it will go into kludge linemode.
  • -n — disable TCP keep-alives. Normally telnetd enables the TCP keep-alive mechanism to probe connections that have been idle for some period of time to determine if the client is still there, so that idle connections from machines that have crashed or can no longer be reached may be cleaned up. 🫶
  • -r lowpty-highpty — this option is only enabled when telnetd is compiled for UNICOS. It specifies an inclusive range of pseudo-terminal devices to use. If the system has sysconf variable _SC_CRAY_NPTY configured, the default pty search range is 0 to _SC_CRAY_NPTY; otherwise, the default range is 0 to 128. Either lowpty or highpty may be omitted to allow changing either end of the search range. If lowpty is omitted, the - character is still required so that telnetd can differentiate highpty from lowpty.
  • -s — this option is only enabled if telnetd is compiled with support for SecurID cards. It causes the -s option to be passed on to login(1), and thus is only useful if login(1) supports the -s flag to indicate that only SecurID validated logins are allowed, and is usually useful for controlling remote logins from outside of a firewall. 🔐
  • -S tos — the page lists this one but gives no description here, bestie.
  • -U — this option causes telnetd to refuse connections from addresses that cannot be mapped back into a symbolic name via the gethostbyaddr(3) routine. 🌈
  • -w [ip|maxhostlen[,[no]striplocal]] — controls the form of the remote hostname passed to login(1). Specifying ip results in the numeric IP address always being passed to login(1). Specifying a number, maxhostlen, sets the maximum length of the hostname passed to login(1) before it will be passed as a numeric IP address. If maxhostlen is 0, then the system default, as determined by the utmp or utmpx structures, is used. The nostriplocal and striplocal options, which must be preceded by a comma, control whether or not the local host domain is stripped from the remote hostname. By default, the equivalent of striplocal is in effect. 💖
  • -X authtype — this option is only valid if telnetd has been built with support for the authentication option. It disables the use of authtype authentication, and can be used to temporarily disable a specific authentication type without having to recompile telnetd.

Telnetd operates by allocating a pseudo-terminal device (see pty(4)) for a client, then creating a login process which has the slave side of the pseudo-terminal as stdin, stdout and stderr. Telnetd manipulates the master side of the pseudo-terminal, implementing the TELNET protocol and passing characters between the remote client and the login process. ✨

When a TELNET session is started up, telnetd sends TELNET options to the client side indicating a willingness to do the following TELNET options, which are described in more detail below:

DO AUTHENTICATION
WILL ENCRYPT
DO TERMINAL TYPE
DO TSPEED
DO XDISPLOC
DO NEW-ENVIRON
DO ENVIRON
WILL SUPPRESS GO AHEAD
DO ECHO
DO LINEMODE
DO NAWS
WILL STATUS
DO LFLOW
DO TIMING-MARK

The pseudo-terminal allocated to the client is configured to operate in “cooked” mode, and with XTABS and CRMOD enabled (see tty(4)). 🍓

Telnetd has support for enabling locally the following TELNET options:

  • WILL ECHO — when the LINEMODE option is enabled, a WILL ECHO or WONT ECHO will be sent to the client to indicate the current state of terminal echoing. When terminal echo is not desired, a WILL ECHO is sent to indicate that telnetd will take care of echoing any data that needs to be echoed to the terminal, and then nothing is echoed. When terminal echo is desired, a WONT ECHO is sent to indicate that telnetd will not be doing any terminal echoing, so the client should do any terminal echoing that is needed. 🎀
  • WILL BINARY — indicates that the client is willing to send 8 bits of data, rather than the normal 7 bits of the Network Virtual Terminal.
  • WILL SGA — indicates that it will not be sending IAC GA, go ahead, commands.
  • WILL STATUS — indicates a willingness to send the client, upon request, the current status of all TELNET options.
  • WILL TIMING-MARK — whenever a DO TIMING-MARK command is received, it is always responded to with a WILL TIMING-MARK. ⭐
  • WILL LOGOUT — when a DO LOGOUT is received, a WILL LOGOUT is sent in response, and the TELNET session is shut down.
  • WILL ENCRYPT — only sent if telnetd is compiled with support for data encryption, and indicates a willingness to decrypt the data stream. 🔐

Telnetd has support for enabling remotely the following TELNET options:

  • DO BINARY — sent to indicate that telnetd is willing to receive an 8 bit data stream.
  • DO LFLOW — requests that the client handle flow control characters remotely. 🌊
  • DO ECHO — this is not really supported, but is sent to identify a 4.2BSD telnet(1) client, which will improperly respond with WILL ECHO. If a WILL ECHO is received, a DONT ECHO will be sent in response. 😭
  • DO TERMINAL-TYPE — lowkey just saying "hey, I'd love to be able to ask what kind of terminal is hooked up on the client side of this connection 💅"
  • DO SGA — it's telling you it does not need to receive IAC GA, like, the go ahead command? not needed bestie, skip it ✨
  • DO NAWS — requests that the client let the server know whenever the window (display) size changes, so nobody's vibes get cropped 🪟
  • DO TERMINAL-SPEED — indicates a desire to be able to request info about the speed of the serial line the client is attached to 🏃‍♀️💨
  • DO XDISPLOC — indicates a desire to be able to ask for the name of the X windows display that's associated with the telnet client 🖥️
  • DO NEW-ENVIRON — indicates a desire to be able to request environment variable information, exactly as described in RFC 1572 🌱
  • DO ENVIRON — same energy, but for the older flavour: a desire to request environment variable information, as described in RFC 1408 🌿
  • DO LINEMODE — only sent if telnetd was compiled with support for linemode, and it's requesting that the client do line by line processing 📝
  • DO TIMING-MARK — only sent if telnetd is compiled with support for both linemode and kludge linemode, and the client responded with WONT LINEMODE. If the client comes back with WILL TM, then it's assumed that the client supports kludge linemode 🕰️. ngl, the -k option can be used to disable this whole situation.
  • DO AUTHENTICATION — only sent if telnetd is compiled with support for authentication, and it signals a willingness to receive authentication information for automatic login 🔐
  • DO ENCRYPT — only sent if telnetd is compiled with support for data encryption, and it signals a willingness to decrypt the data stream 🛡️

🎀 FILES 🎀

  • /etc/services
  • /etc/inittab (UNICOS systems only)
  • /etc/iptos (if supported)
  • /usr/ucb/bftp (if supported)

🌈 SEE ALSO 🌈

telnet(1), login(1), bftp(1) (if supported)

go read those, iykyk 🫶

⭐ STANDARDS ⭐

  • RFC-854 — TELNET PROTOCOL SPECIFICATION
  • RFC-855 — TELNET OPTION SPECIFICATIONS
  • RFC-856 — TELNET BINARY TRANSMISSION
  • RFC-857 — TELNET ECHO OPTION
  • RFC-858 — TELNET SUPPRESS GO AHEAD OPTION
  • RFC-859 — TELNET STATUS OPTION
  • RFC-860 — TELNET TIMING MARK OPTION
  • RFC-861 — TELNET EXTENDED OPTIONS - LIST OPTION
  • RFC-885 — TELNET END OF RECORD OPTION
  • RFC-1073 — Telnet Window Size Option
  • RFC-1079 — Telnet Terminal Speed Option
  • RFC-1091 — Telnet Terminal-Type Option
  • RFC-1096 — Telnet X Display Location Option
  • RFC-1123 — Requirements for Internet Hosts -- Application and Support
  • RFC-1184 — Telnet Linemode Option
  • RFC-1372 — Telnet Remote Flow Control Option
  • RFC-1416 — Telnet Authentication Option
  • RFC-1411 — Telnet Authentication: Kerberos Version 4
  • RFC-1412 — Telnet Authentication: SPX
  • RFC-1571 — Telnet Environment Option Interoperability Issues
  • RFC-1572 — Telnet Environment Option

that's a whole reading list, bestie — bring a snack 🍓

💀 BUGS 💀

Some TELNET commands are only partially implemented. not fully baked, bestie, sorry 😭
Because of bugs in the original 4.2 BSD telnet(1), telnetd performs some dubious protocol exchanges to try to discover if the remote client is, in fact, a 4.2 BSD telnet(1). it's giving detective work nobody asked for 🙈
Binary mode has no common interpretation except between similar operating systems (Unix in this case). fr fr, don't assume it'll play nice across totally different systems.
The terminal type name received from the remote client is converted to lower case. yeah. it just does that. 🔡
Telnetd never sends TELNET IAC GA (go ahead) commands. never ever, periodt. 👑

and that's the whole page, bestie! 🎀 this was TELNETD(8), dated TELNETD(8) in the footer — mwah, go forth and telnet responsibly 💜✨🦋

📦 where this page comes from