TELNET PROTOCOL IN C-KERMIT 8.0 AND KERMIT 95 2.0
- Author:
- Jeffrey Altman
The Kermit Project
Columbia University
- Most recent update:
- 4 June 2002
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- INTRODUCTION
- SUPPORTED TELNET OPTIONS
- TELNET OPTION MANAGEMENT
- TELNET COMMAND SUMMARY
- DIAGNOSING AND FIXING PROBLEMS CONNECTING TO TELNET SERVERS
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The Telnet protocol is one of the original protocols developed for the
ARPANET, the precursor to today's Internet. Telnet has evolved since the
early 1970s due to the extensibility provided by its "option" model. To quote
RFC854:
"The purpose of the TELNET Protocol is to provide a fairly general,
bi-directional, eight-bit byte oriented communications facility. Its
primary goal is to allow a standard method of interfacing terminal
devices and terminal-oriented processes to each other. It is
envisioned that the protocol may also be used for terminal-terminal
communication ("linking") and process-process communication
(distributed computation)."
Not so long ago the requirements for a Telnet client were fairly minimal:
support echo management, window size notification, terminal type negotiation,
and perhaps the transmission of environment variables from the client to the
server. Option negotiations were not time sensitive nor were they
interdependent. Everyone was happy as long as each option specification was
followed and infinite negotiation loops were avoided.
This simplicity began to change with the introduction of telnet options
that provide for mutual authentication, data encryption, transport layer
security, and synchronization of remote processes. The new options have order
and timing dependencies that require increased sophistication from both client
and server even though the original Telnet protocol specification did not
change.
Prior to C-Kermit 7.0
and K95 1.1.19,
Kermit implemented Telnet protocol by opening a connection to the host and
then transmitting the options that it supported. What happened next was
determined by how the connection was being used. If the user told Kermit to:
TELNET host
then, immediately after the initial telnet options were transmitted, the terminal
emulator started and began processing the incoming data. The rest of the Telnet
protocol implementation was purely reactive (with minor exceptions such as
window-size changes): when a Telnet option was received it would be processed
and a response sent as required.
However, if the user said:
SET HOST host
then, after the initial telnet options were transmitted, Kermit would wait for the
next command from the user. If a CONNECT command was next the behavior would
be the same as for TELNET host. However, if Kermit was executing a
script containing a series of INPUT and OUTPUT commands, the incoming telnet
option negotiations would be processed while waiting for INPUT.
This was adequate when there were no ordering or timing requirements for the
Telnet negotiations. But with the introduction of authentication, encryption,
transport-layer security, and the Kermit option for managing the states of the
Kermit server on both the workstation and host it is
necessary for Telnet negotiations to take place before the TELNET command
enters the terminal emulator or the SET HOST command completes and allows any
subsequent INPUT and OUTPUT commands to execute.
The timing requirements for the telnet options supported by Kermit are as
follows:
- START_TLS (Transport Layer Security) must be negotiated or refused before
any other option.
- AUTH (Authentication) must be negotiated or refused before ENCRYPT. AUTH
must also be negotiated before the login process is initiated.
- ENCRYPT (Encryption) must be negotiated/refused in both directions before
it is safe to transmit any data that might be considered private, including
Telnet options such as terminal type, location, xdisplay, or environment
variables. ENCRYPT may not be negotiated if START_TLS has been negotiated
or if AUTH has not been.
- FORWARD_X (Forward X Windows Systems data with X authorization) must only
be used on encrypted connections.
- KERMIT (RFC2839/2840 Internet Kermit Service) must wait for a response to
any request for the peer to either turn on or off the Kermit Server
capabilities in order to facilitate automatic uploading or downloading of
files or processing of remote commands.
- NEW_ENV (Transmission of Environment Variables to the Host) must be
negotiated before the login process is initiated if the USER variable
is to be requested from the client.
The result is that Kermit must, to the best of its ability, attempt to
process all of the above options before TELNET enters CONNECT mode or SET
HOST completes to process the next command. Therefore it might take Kermit
longer to make a connection to a host than before.
The reality is actually far different. Even if the CONNECT mode or first
INPUT command was executed sooner no user data could be received until the
Telnet negotiations were complete. In addition, the timing of the initial
INPUT command used to require that the length of time it takes to process the
Telnet negotiations be factored in. This is no longer necessary and was
inappropriate in the first place. A login script should not have to be
modified for different connection types; the telnet negotiations should be
transparent to the script. In C-Kermit 7.0 and Kermit 95 1.1.18 they are.
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- AUTHENTICATION (Authenticate end user to host)
[RFC2941]
- The
AUTHENTICATION option is used to determine which if any authentication method
such as Kerberos 4, Kerberos 5, Secure Remote Password, etc, should be used to
authenticate the user to the host.
- BINARY (Binary Transmission Mode)
[RFC856]
- When a
telnet session is initiated, the connection is in Network Virtual Terminal
(NVT) mode. NVT mode provides for special treatment of the carriage return
(CR) control character to provide for deterministic parsing of the input
stream. Every CR that is transmitted must be followed by a line feed (LF)
control character or a NUL control character. This enables an NVT to
distiguish between the Carriage Return function and the End of Line indicator.
This works fine for textual data. But in transmission of random binary data
there is the possibility that the sequence CR NUL might be misinterpreted.
Binary mode removes the ambiguity by removing the requirement that
CR be followed by either LF or NUL. It is negotiatied separately
in each direction of data transmission. Binary transmission mode is
not required for transferring files with Kermit protocol but it might be
required when transfering files with Xmodem, Ymodem, or Zmodem.
Binary mode is one of the most frequently misimplemented telnet options.
Many implementation will negotiate Binary mode in only one direction
but apply it in both. Kermit provides workarounds for
these problems with its SET TELNET BUG BINARY-ME-MEANS-U-TOO and
SET TELNET BUG BINARY-U-MEANS-ME-TOO commands.
Kermit also provides the SET TELNET BINARY-TRANSFER-MODE command to
automatically enter binary mode at the start of a file transfer
and return to NVT mode when the transfer is completed.
- COM-PORT-CONTROL (Remote Serial Port device control)
[RFC2217]
- This
option is designed for use with reverse terminal servers and other network
services that share serial line devices such as modems across the network. A
long term issue has been the need for the network administrator to
pre-configure the modem and serial line parameters to meet the requirements of
the client applications. Client applications are able to adjust the modem
configuration by issuing modem specific command sequences. Many options
require the modem and serial line device to be adjusted simultaneously. Two
examples are the use of dynamic speed matching and flow control protocols.
When the com-port control option is negotiated the telnet client is able to
treat the remote serial line device as if it were locally attached. This
allows Kermit's SHOW COMMUNICATIONS, SET FLOW, SET SPEED, SET PARITY, SET
STOP-BITS, SET MODEM SPEED-MATCHING and WAIT commands to remotely control or
respond to changes in the modem or serial line properties.
- ECHO (Echo Mode)
[RFC857]
- When a telnet session is initiated, data is not echoed by the receiver.
This means that a telnet client must echo each character locally as
it is being sent to the host. While this reduces network traffic it
can cause problems with terminal emulation and echoing of sensitive data.
The echo option allows the each side to specify that it intends to
echo the data that it receives. Normally this would be used to negotiate
that the server should echo the data it receives from the client. While it
is possible for the client to state that it will echo the data received
from the server this makes no sense and if negotiatied could result in
an infinite loop of a single character being echoed back and forth.
As a piece of telnet trivia, the BSD 4.2 telnet client would echo
incoming data sent by the server if the host requested it. Kermit
will always respond WONT ECHO to a DO ECHO request when it is the client.
- ENCRYPTION (Encrypt session)
[RFC2946]
- The ENCRYPTION option is used in conjunction with the AUTHENTICATE option
to encrypt all the data transmitted during the session if START_TLS is not
negotiated. The ENCRYPTION option must be negotiated separately in each
direction.
- FORWARD X (Forward X Windows Systems data with X authorization)
[Internet-Draft]
- The FORWARD X option (developed by the Kermit Project) allows the
telnet server to redirect all output from X Windows clients and
transmit it across an encrypted telnet connection. The telnet client then
forwards the data to the local X Windows server. Connections from the
X Windows client to the Telnet Server are authorized using the strongest
X authorization method supported by both X Windows client and the Telnet
Server. Unauthorized connections are rejected.
- KERMIT (Synchronize Kermit File Transfers)
[RFC2840]
- The Kermit option (developed by the Kermit Project) is designed to
allow a Kermit file-transfer client and a Kermit server to synchronize
their operations. This allows a change in "mode" of the server to
automatically switch the client into the complementary mode, and vice
versa.
- LOGOUT (Logout user from host)
[RFC727]
- Some operating systems such as VMS support the notion of a login session
that can continue across separate telnet connections. If a telnet
connection is prematurely interrupted by a network failure, the user
may reconnect to a pre-existing session on their next login attempt.
The Telnet Logout option is sent by the telnet client just before the
tcp/ip socket is closed to indicate to the host that the connection is
being intentionally terminated by the user and is not being closed
due to a network error.
- NAWS (Negotiate About Window Size)
[RFC1073]
- The Negotiate About Windows Size (NAWS) lets the client report its current
Window size to the host. Every time the client's window size changes, the new
size is reported to the host automatically. It is not possible for the host
to report a window size to the client.
- NEW ENVIRONMENT (Report Environment to host)
[RFC1572]
- The NEW ENVIRONMENT option is used by the client to reply to requests
from the server for either all or specified environment variables such
as DISPLAY, USERNAME, ACCOUNT, JOB, PRINTER, and SYSTEMTYPE. When the
NEW ENVIRONMENT option is used to transmit the username, many telnet
servers skip their login or username prompt and go directly to the
password prompt.
- SEND LOCATION (Send Terminal Location)
[RFC779]
- The Send Location option provides the host with a method for requesting
the location of the telnet client. When a location string has been
specified with the SET TELNET LOCATION command, Kermit transmits
this string to the host upon request.
- START TLS (Transmit Telnet over TLS)
[Internet-Draft]
- The START TLS option is used by the client and server to determine whether
the telnet session should be restarted after first establishing a TLSv1
session. TLS provides strong encryption and optionally authenticates the
client and the server using X.509 certificates or Kerberos 5 credentials.
START_TLS can be used with the AUTHENTICATE option. TLS connections may
implement data compression. When negotiatied START_TLS replaces the
ENCRYPTION option.
- SUPPRESS GO AHEAD (Suppress Go Ahead commands)
[RFC858]
- When a telnet session is initiated, all data transmitted by the sender is
to be followed by a Go Ahead (GA) command sequence. This is to enable telnet
to be used over half-duplex (two-way alternate) connections, and it gives the
telnet partner permission to transmit. But to our knowledge, all telnet
sessions used over the Internet are full duplex connections. The Suppress Go
Ahead (SGA) option is negotiated in both directions to suppress the
transmission of the GA commands and treat the connection as full duplex
(two-way simultaneous).
- TERMINAL TYPE (Negotiate Terminal Type)
[RFC1091]
- The Terminal Type option allows the client and server to agree to a common
terminal type that they both support. C-Kermit reports the value of the local
TERM environment variable. Since Kermit 95 supports more than 30 terminal
types, it continues to offer additional terminal types to the host until the
host decides which one to use.
- XDISPLOC (Report X Window Display location)
[RFC1096]
- The X Windows display option is used to report to the host the address of
the local X Windows Server. Kermit sends the contents of the local DISPLAY
environment variable or the string specified by the SET TELNET ENVIRONMENT
DISPLAY command. XDISPLOC is not used if FORWARDX is negotiated.
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One of the benefits of processing all the Telnet options during the SET HOST
and TELNET commands is that it is now possible to configure policy
requirements for a valid connection. This capability is necessary when the
connection must be secure (authenticated and encrypted) or else fail.
Policies are specified with the new command:
- SET TELOPT [ switch ] option local-mode
- SET TELOPT [ switch ] option remote-mode
- SET TELOPT [ switch ] option local-mode remote-mode
- Which of the SET TELOPT command forms is used is dependent on the telnet
option. Some options, such as authentication, terminal type and window size,
are negotiated in one direction and others, such as binary, encryption and
kermit are negotiated separately in each direction.
For each option, the mode can be:
- ACCEPTED
- Kermit does not offer the option but if the peer requests it
Kermit agrees to use it.
- REFUSED
- Kermit does not offer the option and if the peer requests it
Kermit refuses to use it.
- REQUESTED
- Kermit requests the option but agrees not to use it if the peer
refuses it.
- REQUIRED
- Kermit requests the option and terminates the connection if
the peer refuses it.
The optional can be:
- /CLIENT
- Specifies that the command is being used to set the configuration for when
Kermit is the Telnet client. This is the default when Kermit is not acting
as an Internet Kermit Service.
- /SERVER
- Specifies that the command is being used to set the configuration
for when Kermit is the Telnet server. Kermit is a telnet server when it
is accepting incoming connections with SET HOST * or when it is acting as
an Internet Kermit Service. This is the default when Kermit is acting as
an Internet Kermit Service.
The options that can be configured and their default settings, as viewed by
SHOW TELOPT, are:
Telnet Option Me (client) U (client) Me (server) U (server)
000 BINARY ACCEPTED ACCEPTED ACCEPTED ACCEPTED
WONT DONT
001 ECHO REFUSED ACCEPTED REQUESTED REFUSED
WONT DONT
003 SUPPRESS-GO-AHEAD ACCEPTED ACCEPTED REQUESTED REQUESTED
WONT DONT
023 SEND-LOCATION REQUESTED REFUSED REFUSED REFUSED
WONT DONT
024 TERMINAL-TYPE REQUESTED REFUSED REFUSED REQUESTED
WONT DONT
031 NAWS REQUESTED REFUSED REFUSED REQUESTED
WONT DONT
035 XDISPLOC REFUSED REFUSED REFUSED REFUSED
WONT DONT
037 AUTHENTICATION REQUESTED REFUSED REFUSED REQUESTED
WONT DONT
038 ENCRYPTION REQUESTED REQUESTED REQUESTED REQUESTED
WONT DONT
039 NEW-ENVIRONMENT REQUESTED REFUSED REFUSED REQUESTED
WONT DONT
044 COM-PORT-CONTROL REQUESTED REFUSED REFUSED REFUSED
WONT DONT
046 START-TLS ACCEPTED REFUSED REFUSED REQUESTED
WONT DONT
047 KERMIT REQUESTED REQUESTED REQUESTED REQUESTED
WONT DONT
049 FORWARD-X REFUSED ACCEPTED REFUSED REFUSED
WONT DONT
The second line listed for each option shows the currently negotiated state
of the option for the current connection.
The SET TELOPT command should be used instead of the older commands:
SET TELNET AUTHENTICATION {ACCEPTED, REFUSED, REQUESTED, REQUIRED }
SET TELNET BINARY-MODE {ACCEPTED, REFUSED, REQUESTED, REQUIRED }
SET TELNET ENCRYPTION {ACCEPTED, REFUSED, REQUESTED, REQUIRED }
SET TELNET ENVIRONMENT {ON, OFF }
SET TELNET NAWS {ACCEPTED, REFUSED, REQUESTED, REQUIRED }
SET TELNET XDISPLAY-LOC {ACCEPTED, REFUSED, REQUESTED, REQUIRED }
which are now considered obsolete (but still supported).
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The Telnet protocol is used by Kermit when establishing the vast majority of
its connections with the SET HOST or TELNET commands. The format of the SET
HOST command is
SET HOST [ switches ] hostname-or-address [ service ] [ protocol-switch ]
Telnet protocol is used when the protocol-switch is one of the following:
- /TELNET
- Send initial Telnet negotiations even if this is not a Telnet port.
- /NO-TELNET-INIT
- Use Telnet protocol, but do not initiate any telnet negotiations unless the
server sends a telnet first. For use with non-Telnet servers which expect the
client to use Telnet NVT mode.
- /SSL-TELNET
- Perform SSL negotiations and if successful start Telnet negotiations.
- /TLS-TELNET
- Perform TLS negotiations and if successful start Telnet negotiations.
- TELNET /AUTH:type /ENCRYPT:type /USERID:name /PASSWORD:string host port
- The TELNET command is a shortcut for making interactive connections.
It is the equivalent of specifying:
SET TELOPT AUTH ...
SET TELNET AUTH TYPE ...
SET TELOPT ENCRYPT ...
SET TELNET ENCRYPT TYPE ...
SET LOGIN USERID ...
SET LOGIN PASSWORD ...
SET HOST /CONNECT host port /TELNET
- /AUTH:type
- Is equivalent to SET TELNET AUTH TYPE type and
SET TELOPT AUTH REQUIRED with the following exceptions. If the type
is AUTO, then SET TELOPT AUTH REQUESTED is executed and if the type
is NONE, then SET TELOPT AUTH REFUSED is executed. If START_TLS is
negotiated, REQUIRED becomes REQUESTED.
- /ENCRYPT:type
- Is equivalent to SET TELNET ENCRYPT TYPE type
and SET TELOPT ENCRYPT REQUIRED REQUIRED with the following exceptions.
If the type is AUTO then SET TELOPT AUTH REQUESTED REQUESTED is executed
and if the type is NONE then SET TELOPT ENCRYPT REFUSED REFUSED is
executed. If START_TLS is negotiated, REQUIRED becomes REFUSED.
- /USERID:[name]
- This switch is equivalent to SET LOGIN USERID name or SET TELNET
ENVIRONMENT USER name. If a string is given, it sent to host during
Telnet negotiations; if this switch is given but the string is omitted, no
user ID is sent to the host. If this switch is not given, your current
USERID value, \v(userid), is sent. When a userid is sent to the host
it is a request to login as the specified user.
- /PASSWORD:[string]
- This switch is equivalent to SET LOGIN PASSWORD. If a string is given,
it is treated as the password to be used (if required) by any Telnet
Authentication protocol (Kerberos Ticket retrieval, Secure Remote
Password, or X.509 certificate private key decryption.) If no password
switch is specified a prompt is issued to request the password if one
is required for the negotiated authentication method.
For TCP/IP TELNET connections, which are in NVT (ASCII) mode by default:
- SET LOGIN USERID name
- If a name is given, it sent to host during Telnet negotiations; if
this switch is given but the string is omitted, no user ID is sent to the host.
If this command is not given, your current USERID value, \v(userid),
is sent. When a userid is sent to the host it is a request to login as the
specified user.
- SET LOGIN PASSWORD password
- If a is given, it is treated as the password to be used (if
required) by any Telnet Authentication protocol (Kerberos Ticket retrieval,
Secure Remote Password (SRP), or X.509 certificate private key decryption.)
If no password is specified a prompt is issued to request the password if
one is required for the negotiated authentication method.
- SET TELNET AUTHENTICATION TYPE -
{AUTOMATIC, KERBEROS_IV, KERBEROS_V, NTLM, SSL, SRP, NONE } [ { ... } ... ]
- AUTOMATIC allows the host to choose the preferred type of authentication.
Other values allow a specific authentication method to be used. AUTOMATIC
is the default. Available options can vary depending on configuration;
type SET TELNET AUTHENTICATION TYPE ? for a list.
When the type is AUTOMATIC and Kermit is accepting incoming connections
the supported authentication methods will be offered in the following
order:
- NTLM (windows only)
- Kerberos 5
- Kerberos 4
- Secure Remote Password
- SSL
This is the equivalent of the command:
SET TELNET AUTH TYPE NTLM KRB5 KRB4 SRP SSL
- SET TELNET AUTHENTICATION FORWARDING { ON, OFF }
- Set this to ON to forward Kerberos V ticket-granting-tickets to the host
after authentication is complete. OFF by default.
- SET TELNET AUTHENTICATION ENCRYPT-FLAG { ANY, NONE, TELOPT }
- Use this command to specify which AUTH telopt encryption flags may be
accepted in client mode or offered in server mode. The default is ANY.
- SET TELNET AUTHENTICATION HOW-FLAG { ANY, ONE-WAY, MUTUAL }
- Use this command to specify which AUTH telopt how flags may be
accepted in client mode or offered in server mode. The default is ANY.
- SET TELNET BINARY-TRANSFER-MODE { ON, OFF }
- Set this command to ON if you want to force Kermit to negotiate
Telnet Binary in both directions when performing file transfers.
Default is OFF. Alias SET TELNET BINARY-XFER-MODE.
- SET TELNET BUG BINARY-ME-MEANS-U-TOO { ON, OFF }
- Set this to ON to try to overcome TELNET binary-mode misnegotiations by
C-Kermit's TELNET partner.
- SET TELNET BUG BINARY-U-MEANS-ME-TOO { ON, OFF }
- Set this to ON to try to overcome TELNET binary-mode misnegotiations by
C-Kermit's TELNET partner.
- SET TELNET BUG INFINITE-LOOP-CHECK { ON, OFF }
- Set this to ON to prevent Kermit from responding to a telnet negotiation
sequence that enters an infinite loop. The default is OFF because this
should never occur.
- SET TELNET BUG SB-IMPLIES-WILL-DO { ON, OFF }
- Set this to ON to allow Kermit to respond to telnet sub-negotiations if
the peer forgets to respond to WILL with DO or to DO with WILL before
sending a SB (subnegotiation).
- SET TELNET DEBUG ON
- Displays all TELNET negotiations in full detail.
- SET TELNET DELAY-SB { ON, OFF }
- When ON, telnet subnegotiation responses are delayed until after all
authentication and encryption options are either successfully negotiated
or refused. This ensures that private data is protected. When OFF, telnet
subnegotiation responses are sent immediately. The default is ON.
- SET TELNET ECHO { LOCAL, REMOTE }
- C-Kermit's initial echoing state for TELNET connections, LOCAL by default.
After the connection is made, TELNET negotiations determine the echoing.
- SET TELNET ENCRYPTION TYPE { AUTOMATIC, CAST128_CFB64,
CAST128_OFB64,CAST5_40_CFB64, CAST5_40_OFB64, DES_CFB64, DES_OFB64,
DES3_CFB64, DES3_OFB64, NONE }
- AUTOMATIC allows the host to choose the preferred type of encryption.
Other values allow a specific encryption method to be specified.
AUTOMATIC is the default. The list of options will vary depending
on the encryption types selected at compilation time.
When the type is AUTOMATIC and Kermit is accepting incoming connections
the supported encryption methods will be offered in the following order:
DES3_CFB64
CAST128_CFB64
DES_CFB64
CAST5_40_CFB64
DES3_OFB64
CAST128_OFB64
DES_OFB64
CAST5_40_OFB64
- SET TELNET ENVIRONMENT [ USERVAR ]
{ variable-name [ value ] }
- This feature lets Kermit send the values of certain environment variables
to the other computer if it asks for them. The variable-name can be any
of the "well-known" variables "USER", "JOB", "ACCT", "PRINTER",
"SYSTEMTYPE", or "DISPLAY". The default values are taken from your
environment; use this command to change or remove them. If
USERVAR
is specified you can include any non-standard environment variables.
The most commonly used variables and their default values are:
- USER:
- Telnet servers that request this value will use it as the name of the
account to be accessed and will therefore not prompt you for a username.
If a password is required to access the specified user's account only
the password prompt will be issued. The default value is the name of
the user on the local machine. This value may also be set with the
command SET LOGIN USER name.
- DISPLAY:
- This variable is used to specify the location of the X Windows Server
to be used by X Windows client applications executed on the remote host.
This value is used by three telnet options: ENVIRONMENT, XDISPLOC, and
FORWARD-X. The default value is retrieved from the value of the DISPLAY
environment variable in the local environment.
- PRINTER:
- This variable is used to specify the location of the LPR printer to be
used for printing files on the host. This variable is not set by
default.
- SYSTEM:
- This variable is used to specify the operating system type of the local
machine. The default value is determined by Kermit. Typical values
are "UNIX", "VMS", "WIN32", and "OS2".
See RFC1572 for further
details.
- SET TELNET LOCATION [ text ]
- Location string to send to the Telnet server if it asks. By default this
is picked up from the LOCATION environment variable. Give this command
with no text to disable this feature. This string is sent either by using the
SNDLOC option or as USERVAR "LOCATION" within the NEW-ENV option.
- SET TELNET NEWLINE-MODE { NVT, BINARY-MODE } { OFF, ON, RAW }
- Determines how carriage returns are handled on TELNET connections.
There are separate settings for NVT (ASCII) mode and binary mode.
ON (default for NVT mode) means CRLF represents CR.
OFF means CR followed by NUL represents CR.
RAW (default for BINARY mode) means CR stands for itself.
- SET TELNET PROMPT-FOR-USERID prompt
- Specifies a custom prompt to be used when prompting for a userid.
Kermit will prompt for a userid if "set login userid {}"
has been issued prior to a telnet authentication negotiation for
an authentication type that requires the transmission of a name.
(e.g. Secure Remote Password).
- SET TELNET REMOTE-ECHO { ON, OFF }
- Applies only to incoming connections created with:
"set host * port /telnet".
This command determines whether Kermit will actually echo characters
received from the remote when it has negotiated to do so. The default
is ON. Remote echoing may be turned off when it is necessary to read
a password with the INPUT command.
- SET TELNET SFU-COMPATIBILITY-MODE { ON, OFF }
- Turns on the SFUTLNTVER and SFUTLNTMODE environment variables for use
on connections to Microsoft's Services for Unix and Windows 2000 Telnet
Servers.
- SET TELNET TERMINAL-TYPE name
- The terminal type to send to the remote TELNET host. If none is given,
your current SET TERMINAL TYPE value is sent, e.g. VT220.
- SET TELNET TRANSFER-MODE { ON, OFF }
- When ON (OFF by default) and BINARY negotiations are not REFUSED Kermit
will attempt to negotiate BINARY mode in each direction before the start
of each file transfer. After the transfer is complete BINARY mode will
be restored to the pre-transfer state.
- SET TELNET WAIT-FOR-NEGOTIATIONS { ON, OFF }
- Each Telnet option must be fully negotiated either On or Off before the
session can continue. This is especially true with options that require
subnegotiations such as Authentication, Encryption, and Kermit; for
proper support of these options Kermit must wait for the negotiations to
complete. Of course, Kermit has no way of knowing whether a reply is
delayed or not coming at all, and so will wait forever before continuing
the session. If you know that Kermit's Telnet partner will not be sending
the required replies, you should instruct Kermit to REFUSE specific
options with the SET TELOPT command. If you do not know which command
the host is not responding to set this option of OFF.
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SECTION CONTENTS
5.1. BUG: Telnet Server does not respond to telnet options.
5.2. BUG: Server sends a subnegotiation without negotiating the option
5.3. BUG: Server sends DO TERM_TYPE but then never asks for the terminal type.
5.4. BUG: Server negotiates BINARY mode in one direction but uses it in both.
5.5. PROBLEM: Long login times
5.6. PROBLEM: The Telnet Server skips "login:" prompt
5.7. BUG: The host echos input but never negotiates WILL ECHO
5.8. BUG: BSDI BSD/OS 3.1 Telnetd improperly implements WILL BINARY mode
5.9. PROBLEM: The host supports Telnet AUTH but you wish to login manually
5.10. PROBLEM: Applications on the host are unable to open the X Windows DISPLAY
5.11. PROBLEM: The Telnet Server is the Microsoft Windows 2000 Telnet Service
5.12. PROBLEM: The Telnet Server is Seattle Lab's SLNet Telnet Server for NT
After we replaced the Telnet engine and added support for the new
functionality we found many Telnet servers that do not adhere to the Telnet
protocol as described in the IETF RFCs. Here are some of the most common
bugs and the workarounds that Kermit provides the user to avoid them.
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- Description:
- The telnet protocol requires that all initial requests for action (WILL or
DO) must be responded to either in the affirmative (DO or WILL) or in the
negative (DONT or WONT). A negative response is required for all
unrecognized options. A failure to respond to a Telnet option may result
in the peer waiting forever.
This behavior is known to exist in the following telnet servers:
- AOS/VS II Release 2.20.00.39
- IBM OS/2 Warp 4.0 (all versions not configured with authentication modules)
- Symptom:
- Kermit connects to the host, waits two minutes, and then reports a telnet
protocol error followed by the list of outstanding negotiations. Kermit
has sent a Telnet option such as WILL AUTH and is waiting for the required
response which never comes. In C-Kermit you will see "Negotiations...."
with many dots. In K95 you will see the "spinner" twirl.
Example:
DNS Lookup... Trying xxx.xxx.xxx.xxx... Reverse DNS Lookup... (OK)
Negotiations... ?Telnet Protocol Timeout
?Telnet waiting for response to WILL TERMINAL-TYPE
?Telnet waiting for response to WILL NAWS
?Telnet waiting for response to WILL AUTHENTICATION
?Telnet waiting for response to WILL NEW-ENVIRONMENT
**** AOS/VS II Release 2.20.00.39 / Press NEW-LINE to begin logging on ****
Too slow - input timed out
- Workaround:
- For each of the telnet options listed as missing a response issue a
SET TELOPT option REFUSED [ REFUSED ]
command. For instance:
SET TELOPT TERMINAL-TYPE REFUSED
SET TELOPT NAWS REFUSED
SET TELOPT AUTHENTICATION REFUSED
SET TELOPT NEW-ENVIRONMENT REFUSED
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- Description:
- The telnet protocol requires that subnegotiations not be sent until the
peers have agreed to WILL or DO the specified option. All subnegotiations
received in a WONT or DONT state are to be ignored.
- Symptom:
- There are two possibilities. In the first, the host sends a subnegotiation
such as
IAC SB TERM_TYPE SEND IAC SE
without Kermit attempting to use the telnet option.
IAC WILL TERM_TYPE
IAC DO TERM_TYPE
In this case the subnegotiation request will be ignored and the
subnegotiation response
IAC SB TERM_TYPE IS type IAC SE
will not be sent. How this affects the sender of the illegal subnegotation
is unknown.
In the second scenario, Kermit sends
IAC WILL TERM_TYPE
But the host sends the subnegotiation without responding to the request with:
IAC DO TERM_TYPE
This scenario degenerates into the previous bug. (See "Telnet Server does
not respond to telnet options it does not recognize.")
- Workaround:
- For the first scenario, there is nothing that can be done. Kermit is
already ignoring the subnegotiations and there is nothing that Kermit
can do to force the host to adhere to the protocol. If you have a
support contract with the maker of the Telnet Server, file a report.
For the second scenario, Kermit implements a workaround which is on by
default:
SET TELNET BUG SB-IMPLIES-WILL-DO ON
This causes Kermit to treat:
IAC SB TERM_TYPE SEND IAC SE
as if it were:
IAC WILL TERM_TYPE
IAC SB TERM_TYPE SEND IAC SE
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- Description:
- Although it is not required by the Telnet Terminal-Type RFC, it makes
sense that if a server asks the client to negotiate the terminal
type, that it will actually go through with the negotiation.
- Symptom:
- Kermit reports that Terminal Type negotiation is in use but the terminal
type is not configured properly on the host.
- Workaround:
- There isn't much that can be done other than to instruct Kermit to:
SET TELOPT TERMINAL-TYPE REFUSE
so it doesn't appear to the user that the terminal type has
indeed been negotiated.
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- Description:
- When either the client or the server says it WILL BINARY and the peer
accepts, it is an indication that CR is to be sent without a following
NUL or LF by the sender of WILL BINARY. A misunderstanding about the
meaning of this negotiation can prevent files from being transfered as
the packet lengths and checksums will not match.
- Symptom:
- File transfers fail, reporting checksum or packet length errors.
- Workaround:
- Use SET TERMINAL DEBUG ON to determine which direction the host is
negotiating BINARY mode in.
Then use either:
SET TELNET BUG BINARY-ME-MEANS-U-TOO ON
or:
SET TELNET BUG BINARY-U-MEANS-ME-TOO ON
to instruct Kermit to follow the broken behavior.
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- Description:
- A connection is made to the Telnet Server but then it takes 30
to 60 seconds for a login prompt, or disconnects without displaying a prompt.
- Symptom:
- The host is trying to resolve a host name for the IP Address assigned to
your computer and is unable to.
- Workaround
- Check with your network administrator
or ISP to make sure that the IP address you are using has a valid DNS
entry for reverse lookups (IP address to name).
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- Description:
- The server you are connecting to supports the Telnet environment option
and has been given your username on the workstation during the telnet
option negotiations.
- Workaround:
- If your username on the workstation is not the same as the username
on the host, or if you are using a script that requires a username
or login prompt, use the command:
SET TELNET ENVIRONMENT USER {}
or:
SET LOGIN USERID {}
to specify your name on the host; or disable this option with:
SET TELOPT NEW-ENVIRONMENT REFUSED
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- Description:
- The Telnet protocol requires that all Telnet options be in a state of I
DONT and you WONT until otherwise negotiated. That means that unless a
host says WILL ECHO it should not echo data; the client should echo it
locally.
- Symptom:
- Failure to follow the protocol definition can result in no echoing or
double echoing. This kind of confusion has been seen with two
well-known sites:
- The USA Library of Congress
- Dow Jones News Retrieval
- Workaround:
- SET TELNET ECHO REMOTE
SET TELOPT ECHO REFUSE
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- Description:
- The BSDI telnetd when it negotiaties WILL BINARY (host to client) binary
mode refuses to transmit CR control characters. The man page for telnetd
states, "Binary mode has no common interpretation except between similar
operating systems (Unix in this case)." The implementors clearly have
misread
RFC856
(TELNET BINARY TRANSMISSION) which clearly states that
the only affect that BINARY mode has on the channel is to disable NVT
(network virtual terminal) handling of CR (CR no longer must be followed
by NUL if it is not followed by LF) and that the 8th data bit must not
be stripped.
- Symptom:
- By refusing to transmit CR control characters and instead translating them to LF
the BSDI telnetd causes end of lines to be misinterpreted by the
terminal and for file transfers to become corrupted if the host is
allowed to negotiate WILL BINARY.
- Workaround:
- SET TELOPT BINARY ACCEPT REFUSE
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- Description:
- You are using Kermit to connect to a host that supports Telnet
Authentication except you need to login manually for one of the
following reasons:
- You do not have credentials that match the supported Telnet AUTH
type. For example, the host supports Kerberos 5 but you do not
have a principal defined in the Kerberos realm even though you
have a valid account on the host.
- You wish to login to an Internet Kermit Service anonymously.
- Workaround:
- SET TELOPT AUTH REFUSE
To login anonymously, set the userid to anonymous. This will force
Kermit to refuse to authenticate.
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- Description:
- Some applications such as the editor 'emacs' are dual mode. They execute
either in terminal mode or as an X Windows client. If the application
terminates with an error that it is unable to open the DISPLAY it could
be for one of the following reasons:
- A DISPLAY environment variable is defined in the shell's script that is
executed at login and it points to an invalid value;
- There is a DISPLAY environment variable defined on the local machine
which has been forwarded to the host by Kermit and the specified
DISPLAY is unreachable.
- A SET TELNET ENVIRONMENT DISPLAY command was issued prior to connecting
to the host and the specified DISPLAY value is invalid.
- Workaround:
- If you wish to use the application as an X Windows client you must
have a working X Windows Server running on your local machine and specify
a valid DISPLAY string for your server. This can either be specified on
the host via:
export DISPLAY=host:display[.screen]
or by specifying the display in Kermit with the command
SET TELNET ENVIRONMENT DISPLAY [host:]display[.screen]
If your telnet server supports any of the following telnet options:
- X-Display Location
- Environment Variables
- X-Windows Forwarding
then Kermit will transmit the DISPLAY value to the host during the initial
telnet negotiations.
If you wish to use the application in terminal mode you can prevent Kermit
from transmitting the local DISPLAY value to the host by issuing the
following commands:
SET TELOPT XDISPLOC REFUSE
SET TELOPT FORWARD-X REFUSE
SET TELNET ENVIRONMENT DISPLAY
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- Description:
- The Microsoft Windows 2000 (and NT Services for Unix) Telnet Service is a
bit of a challenge to work with due to limitations that are imposed by the
Windows platform and the choices made by the developers. The Telnet Service
supports three terminal emulations (ANSI, VT100, and VTNT) and two types of
end user login (Telnet AUTH NTLM and plaintext domain\username/password.)
Depending on the choices that are made will determine the levels of
functionality that can be obtained for the service.
- Terminal types:
- ANSI and VT100 are considered to be the same terminal type by Microsoft
even though they have some very significant differences. The Microsoft
ANSI is closest to the Kermit 95 "ANSI-BBS" which should be used in
preference to VT100 when communicating with this service. The VTNT
terminal type is Microsoft specific (and undocumented.) Kermit 95
implements a reverse engineered implementation. VTNT uses raw Win32
data structures to implement transmission of screen snapshots from the
service to the client; and keystroke events from the client to the service.
VTNT is the preferred terminal type to use with the Microsoft Telnet service
provided that you do not need access to Kermit 95 keyboard verbs or any
form of scripting. If Keyboard verbs or scripting are required ANSI or
VT100 must be used.
When using ANSI or VT100 the Backspace key must send BS and not DEL.
ANSI and VT100 do not support color whereas VTNT does.
VTNT supports Unicode characters. ANSI and VT100 only support the local
ANSI code page. You must configure the Kermit local and remote character
sets to properly convert between ANSI code pages.
- End user login:
- The Microsoft provides two forms of end user login. The first is via the
use of "login:" and "password:" prompts. The username is either the name
of a user with a local account; or a domain\name which specifies a user
with an account in the provided domain. Since the login is performed over
an unencrypted channel the password is easily stolen by monitoring the local
network traffic.
The second method is a proprietary (and undocumented) Telnet authentication
method based upon the NT Lan Manager (NTLM) protocol. This protocol has
also been reverse engineered and implemented in Kermit 95.
NTLM does not produce a shared secret
that can be used for encrypting the connection. NTLM can only be
implemented on Windows 9x, NT, or Windows 2000 so connections from other
operating systems must use plaintext logins.
If NTLM is used, the user can only log into the service with the identity they
are logged into the local workstation. If another username is desired NTLM
must be disabled on the client (SET TELOPT AUTH REFUSE). [K95 1.1.20 only]
Other quirks:
- The Microsoft Telnet Service implements Telnet NAWS (Negotiate About
Window Size) but only listens to it when the connection is initially
established. This has two side effects when used with Kermit. First, the
Telnet Service may completely ignore the screen size reported by Kermit if it
is not sent immediately after the Telnet Service agrees to use NAWS. Second,
the Telnet Service will not recognize changes to the screen size after the
connection is established.
- The Microsoft Telnet Service does not create a proper environment for the
end user. The user's profile, home directory and environment variables are
not loaded onto the system. Applications that require this information may
fail to execute or otherwise run incorrectly.
- The Microsoft Telnet Service only allows a single telnet session to be
running at any one time.
- The Microsoft Telnet Service provides no mechansim for performing file
transfers.
- The Microsoft Telnet Service performs its job by taking snapshots of the
console's active virtual window. This means that it is possible for data
to be lost due to scrolling or other screen updates between snapshots.
This can play havoc with scripts and prevents Kermit from being able to
store data into its scrollback buffers.
- If the NTLM authentication feature is disabled in the registry, the
Telnet Server will not respond to authentication requests causing the bug
entitled Telnet Server does not respond to telnet options.
- Recommendations:
- If using Kermit 95 and scripts are not required:
SET TERMINAL TYPE VTNT
SET TELNET DELAY-SB OFF
SET KEY \264 \8
If scripts are required:
SET TERMINAL TYPE ANSI
SET TELNET DELAY-SB OFF
SET KEY \264 \8
If the Telnet Server has NTLM authentication disabled use the command:
SET TELOPT AUTH REFUSE
or:
TELNET /AUTH:none host
If you are using C-Kermit:
SET TELNET TERMINAL ANSI
SET TELNET DELAY-SB OFF
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Version 2.5 of the SLNet Telnet Server violates serveral
IETF RFCs which hamper the interoperability of SLNet with conforming
Telnet client implementations.
- Problem 1:
- The SLNet Telnet Server negotiates a Telnet AUTH TYPE (99) which has not
been requested from nor registered with IANA.
All telnet option and sub-option values MUST be approved and issued by IANA in
order to facilitate interoperability among products distributed by developers
from all over the world.
When Kermit refuses to perform authentication using the unknown method, the
SLNet Telnet Server does not respond to the refusal:
TELNET SENT WILL AUTHENTICATION
TELNET RCVD DO AUTHENTICATION
TELNET RCVD SB AUTHENTICATION SEND UNKNOWN=99 CLIENT_TO_SERVER|MUTUAL=2 IAC SE
TELNET SENT SB AUTHENTICATION IS NULL NULL IAC SE
TELNET SENT WONT AUTHENTICATION
- Problem 2:
- As per RFC 855,
Telnet requires a default WONT or DONT response for
any telnet option that is not supported or recognized. The SLNet Telnet
server does not respond in either an affirmative or negative manner to
IAC WILL NEW-ENVIRONMENT
The Telnet Server also does not respond to
IAC WILL ENCRYPTION
IAC DO ENCRYPTION
IAC WILL KERMIT
IAC DO KERMIT
IAC DO FORWARD_X
IAC WILL COM-PORT-CONTROL
when initiated by the telnet client.
- Problem 3:
- The SLNet Telnet responds to a request to perform Window Size negotiations
with the wrong negative response.
TELNET SENT WILL NAWS
TELNET RCVD WONT NAWS
The proper response is:
TELNET RCVD DONT NAWS
- Problem 4:
- The SLNet Telnet Server does not keep track of the state of Telnet
Options and transmits requests for Telnet Option state changes that have
already been agreed to. In particular, the SLNet server retransmits
TELNET SENT DO BINARY
TELNET SENT WILL ECHO
TELNET SENT WILL BINARY
after these negotiations were already agreed to. As per
RFC 855, a
telnet option state command (WILL, WONT, DO, DONT) MUST NOT be sent unless it
results in a change of state.
RFC 1143 "The Q Method of
Implementing Telnet Option Negotiation" provides extensive hints on how to
properly construct a telnet state machine to adhere to the rules of the Telnet
protocol.
- Workaround:
- To enable Kermit to establish a connection to this very broken server
Kermit must be told to:
SET TELOPT AUTHENTICATION REFUSE
SET TELOPT KERMIT REFUSE REFUSE
SET TELOPT NEW-ENVIRONMENT REFUSE
SET TELOPT NAWS REFUSE
SET TELOPT FORWARD-X REFUSE
SET TELOPT COM-PORT-CONTROL REFUSE
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C-Kermit 8.0 and Kermit 95 2.0 Telnet Protocol /
Columbia University /
[email protected] /
4 June 2002