heimdal-telnetd man page

telnetd — DARPA TELNET protocol server

Synopsis

telnetd [-BeUhkln] [-D debugmode] [-S tos] [-X authtype] [-a authmode] [-rlowpty-highpty] [-u len] [-debug] [-L /bin/login] [-y] [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.
otp
Only allow authenticated connections (as with -a user) and also logins with one-time passwords (OTPs). This option will call login with an option so that only OTPs are accepted. The user can of course still type secret information at the prompt.
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
Ignored.
-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.
-e
require encryption to be turned on (in both direction) by the client and disconnects if the client tries to turn the encryption off (in either direction).
-h
Disables the printing of host-specific information before login has been completed.
-k
-l
Ignored.
-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 tos
-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 gethostbyaddr(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.
-L pathname
Specify pathname to an alternative login program.
-y
Makes telnetd not warn when a user is trying to login with a cleartext password.

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.

Files

/etc/services
/etc/inittab
(UNICOS systems only)
/etc/iptos
(if supported)

See Also

telnet(1), login(1)

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 4.2 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).

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.

Info

September 19, 2006