TELNETD(8)

TELNETD(8) | System Manager's Manual | TELNETD(8)

# NAME

telnetd — DARPA TELNET protocol server

# SYNOPSIS

/usr/libexec/telnetd | [-Uhlkns46] [-D debugmode] [-S tos] [-X authtype] [-a authmode] [-edebug] [-g gettyent] [-u len] [-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.
  user
    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.
  valid
    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.
-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.
  exercise
    Has not been implemented yet.
-debug
  Enables debugging on each socket created by telnetd (see SO_DEBUG in socket(2)).
-edebug
  If telnetd has been compiled with support for data encryption, then the -edebug option may be used to enable encryption debugging code.
-g gettyent
  Specifies which entry from /etc/gettytab should be used to get banner strings, login program and other information. The default entry is default.
-h
  Disables the printing of host-specific information before login has been completed.
-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 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.
-s
  This option is only enabled if telnetd is compiled with support for secure logins. 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 Kerberos or S/Key validated logins are allowed, and is usually useful for controlling remote logins from outside of a firewall.
-S tos
  This option sets the IP Type-of Service (TOS) option on the connection to the value tos, which may be a numeric TOS value or a symbolic TOS name found in the /etc/iptos file. This option has no effect on NetBSD.
-u len
  This option is used to specify the size of the field in the utmp structure that holds the remote host name. If the resolved host name is longer than len, the dotted decimal value will be used instead. This allows hosts with very long host names that overflow this field to still be uniquely identified. Specifying -u0 indicates that only dotted decimal addresses should be put into the utmp file.
-U
  This option causes telnetd to refuse connections from addresses that cannot be mapped back into a symbolic name via the getnameinfo(3) routine.
-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.
-4
-6
  Specifies address family to be used on -debug mode. During normal operation (called from inetd(8)) telnetd will use the file descriptor passed from inetd(8).

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 a 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, of 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
  Indicates a desire to be able to request the name of the type of terminal that is attached to the client side of the connection.
DO SGA
  Indicates that it does not need to receive IAC GA, the go ahead command.
DO NAWS
  Requests that the client inform the server when the window (display) size changes.
DO TERMINAL-SPEED
  Indicates a desire to be able to request information about the speed of the serial line to which the client is attached.
DO XDISPLOC
  Indicates a desire to be able to request the name of the X windows display that is associated with the telnet client.
DO NEW-ENVIRON
  Indicates a desire to be able to request environment variable information, as described in RFC 1572.
DO ENVIRON
  Indicates a desire to be able to request environment variable information, as described in RFC 1408.
DO LINEMODE
  Only sent if telnetd is compiled with support for linemode, and requests 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 responds with WILL TM, the it is assumed that the client supports kludge linemode. Note that the [-k] option can be used to disable this.
DO AUTHENTICATION
  Only sent if telnetd is compiled with support for authentication, and indicates a willingness to receive authentication information for automatic login.
DO ENCRYPT
  Only sent if telnetd is compiled with support for data encryption, and indicates a willingness to decrypt the data stream.

At the end of a login session, telnetd invokes the ttyaction(3) facility with an action of "telnetd" and user "root" to execute site-specific commands.

# FILES

- /etc/services
- /etc/iptos (if supported)

# SEE ALSO

login(1), skey(1), telnet(1), ttyaction(3)

# 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

# BUGS

Some TELNET commands are only partially implemented.

Because of bugs in the original 4.2BSD telnet(1), telnetd performs some dubious protocol exchanges to try to discover if the remote client is, in fact, a 4.2BSD telnet(1).

Binary mode has no common interpretation except between similar operating systems (UNIX in this case).

The terminal type name received from the remote client is converted to lower case.

telnetd never sends TELNET IAC GA (go ahead) commands.

July 17, 2004 | NetBSD 7.0