TELNET PROTOCOL IN C-KERMIT 8.0 AND KERMIT 95 2.00
Author:
Jeffrey Altman
The Kermit Project
Columbia University
Most recent update:
3 December 2001
CONTENTS
1. [1]INTRODUCTION
2. [2]SUPPORTED TELNET OPTIONS
3. [3]TELNET OPTION MANAGEMENT
4. [4]TELNET COMMAND SUMMARY
5. [5]DIAGNOSING AND FIXING PROBLEMS CONNECTING TO TELNET SERVERS
_________________________________________________________________
1. INTRODUCTION
[ [6]Top ] [ [7]Contents ] [ [8]Next ]
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 [9]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 [10]C-Kermit 7.0 and [11]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 [12]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.
[ [13]C-Kermit ] [ [14]Kermit 95 ] [ [15]Kermit Home ]
________________________________________________________________________
2. SUPPORTED TELNET OPTIONS
[ [16]Top ] [ [17]Contents ] [ [18]Next ] [ [19]Previous ]
AUTHENTICATION (Authenticate end user to host) [[20]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) [[21]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) [[22]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) [[23]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) [[24]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) [[25]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) [[26]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) [[27]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) [[28]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) [[29]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) [[30]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) [[31]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) [[32]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) [[33]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.
[ [34]C-Kermit ] [ [35]Kermit 95 ] [ [36]Kermit Home ]
________________________________________________________________________
3. TELNET OPTION MANAGEMENT
[ [37]Top ] [ [38]Contents ] [ [39]Next ] [ [40]Previous ]
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).
[ [41]C-Kermit ] [ [42]Kermit 95 ] [ [43]Kermit Home ]
________________________________________________________________________
4. TELNET COMMAND SUMMARY [ [44]Top ] [ [45]Contents ] [ [46]Next ] [
[47]Previous ]
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:
1. NTLM (windows only)
2. Kerberos 5
3. Kerberos 4
4. Secure Remote Password
5. 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 [48]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.
[ [49]C-Kermit ] [ [50]Kermit 95 ] [ [51]Kermit Home ]
________________________________________________________________________
5. DIAGNOSING AND FIXING PROBLEMS CONNECTING TO TELNET SERVERS
[ [52]Top ] [ [53]Contents ] [ [54]Previous ]
SECTION CONTENTS
5.1. [55]BUG: Telnet Server does not respond to telnet options.
5.2. [56]BUG: Server sends a subnegotiation without negotiating the option
5.3. [57]BUG: Server sends DO TERM_TYPE but then never asks for the terminal
type.
5.4. [58]BUG: Server negotiates BINARY mode in one direction but uses it in
both.
5.5. [59]PROBLEM: Long login times
5.6. [60]PROBLEM: The Telnet Server skips "login:" prompt
5.7. [61]BUG: The host echos input but never negotiates WILL ECHO
5.8. [62]BUG: BSDI BSD/OS 3.1 Telnetd improperly implements WILL BINARY mode
5.9. [63]PROBLEM: The host supports Telnet AUTH but you wish to login manual
ly
5.10. [64]PROBLEM: Applications on the host are unable to open the X Windows
DISPLAY
5.11. [65]PROBLEM: The Telnet Server is the Microsoft Windows 2000 Telnet Ser
vice
5.12. [66]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.
[ [67]C-Kermit ] [ [68]Kermit 95 ] [ [69]Kermit Home ]
______________________________________________________________________
5.1 BUG: Telnet Server does not respond to telnet options.
[ [70]Top ] [ [71]Contents ] [ [72]Section Contents ] [
[73]Next ]
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
[ [74]C-Kermit ] [ [75]Kermit 95 ] [ [76]Kermit Home ]
______________________________________________________________________
5.2 BUG: Server sends a subnegotiation without negotiating the option
[ [77]Top ] [ [78]Contents ] [ [79]Section Contents ] [
[80]Next ] [ [81]Previous ]
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
[ [82]C-Kermit ] [ [83]Kermit 95 ] [ [84]Kermit Home ]
______________________________________________________________________
5.3 BUG: Server sends DO TERM_TYPE but then never asks for the terminal
type.
[ [85]Top ] [ [86]Contents ] [ [87]Section Contents ] [
[88]Next ] [ [89]Previous ]
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.
[ [90]C-Kermit ] [ [91]Kermit 95 ] [ [92]Kermit Home ]
______________________________________________________________________
5.4 BUG: Server negotiates BINARY mode in one direction but uses it in
both.
[ [93]Top ] [ [94]Contents ] [ [95]Section Contents ] [
[96]Next ] [ [97]Previous ]
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.
[ [98]C-Kermit ] [ [99]Kermit 95 ] [ [100]Kermit Home ]
______________________________________________________________________
5.5 PROBLEM: Long login times
[ [101]Top ] [ [102]Contents ] [ [103]Section Contents ] [
[104]Next ] [ [105]Previous ]
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).
[ [106]C-Kermit ] [ [107]Kermit 95 ] [ [108]Kermit Home ]
______________________________________________________________________
5.6 PROBLEM: The Telnet Server does not display a "login:" prompt
[ [109]Top ] [ [110]Contents ] [ [111]Section Contents ] [
[112]Next ] [ [113]Previous ]
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
[ [114]C-Kermit ] [ [115]Kermit 95 ] [ [116]Kermit Home ]
______________________________________________________________________
5.7 BUG: The host echos input but never negotiates WILL ECHO
[ [117]Top ] [ [118]Contents ] [ [119]Section Contents ] [
[120]Next ] [ [121]Previous ]
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
[ [122]C-Kermit ] [ [123]Kermit 95 ] [ [124]Kermit Home ]
______________________________________________________________________
5.8 BUG: BSDI BSD/OS 3.1 Telnetd improperly implements WILL BINARY mode
[ [125]Top ] [ [126]Contents ] [ [127]Section Contents ] [
[128]Next ] [ [129]Previous ]
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
[130]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
[ [131]C-Kermit ] [ [132]Kermit 95 ] [ [133]Kermit Home ]
______________________________________________________________________
5.9 PROBLEM: The host supports Telnet AUTH but you wish to login manually
[ [134]Top ] [ [135]Contents ] [ [136]Section Contents ] [
[137]Next ] [ [138]Previous ]
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.
[ [139]C-Kermit ] [ [140]Kermit 95 ] [ [141]Kermit Home ]
______________________________________________________________________
5.10 PROBLEM: Applications on the host are unable to open the X Windows
DISPLAY
[ [142]Top ] [ [143]Contents ] [ [144]Section Contents ] [
[145]Next ] [ [146]Previous ]
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
[ [147]C-Kermit ] [ [148]Kermit 95 ] [ [149]Kermit Home ]
______________________________________________________________________
5.11 PROBLEM: The Telnet Server is the Microsoft Windows 2000 Telnet
Service
[ [150]Top ] [ [151]Contents ] [ [152]Section Contents ] [
[153]Next ] [ [154]Previous ]
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
[ [155]C-Kermit ] [ [156]Kermit 95 ] [ [157]Kermit Home ]
______________________________________________________________________
5.12 PROBLEM: The Telnet Server is Seattle Lab's SLNet Telnet Server for NT
[ [158]Top ] [ [159]Contents ] [ [160]Section Contents ] [
[161]Previous ]
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 [162]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 S
E
TELNET SENT SB AUTHENTICATION IS NULL NULL IAC SE
TELNET SENT WONT AUTHENTICATION
Problem 2:
As per [163]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 [164]RFC 855, a telnet option state command
(WILL, WONT, DO, DONT) MUST NOT be sent unless it
results in a change of state. [165]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
[ [166]C-Kermit ] [ [167]Kermit 95 ] [ [168]Kermit Home ]
________________________________________________________
C-Kermit 8.0 Telnet / Columbia University / kermit@columbia.edu / 12
November 2001
References
1. http://www.columbia.edu/kermit/telnet80.html#intro
2. http://www.columbia.edu/kermit/telnet80.html#options
3. http://www.columbia.edu/kermit/telnet80.html#managing
4. http://www.columbia.edu/kermit/telnet80.html#summary
5. http://www.columbia.edu/kermit/telnet80.html#problems
6. http://www.columbia.edu/kermit/telnet80.html#top
7. http://www.columbia.edu/kermit/telnet80.html#contents
8. http://www.columbia.edu/kermit/telnet80.html#options
9. ftp://ftp.isi.edu/in-notes/rfc854.txt
10. http://www.columbia.edu/kermit/ckermit.htm
11. http://www.columbia.edu/kermit/k95next.html
12. http://www.columbia.edu/kermit/cuiksd.html
13. http://www.columbia.edu/kermit/ckermit.html
14. http://www.columbia.edu/kermit/k95.html
15. http://www.columbia.edu/kermit/index.html
16. http://www.columbia.edu/kermit/telnet80.html#top
17. http://www.columbia.edu/kermit/telnet80.html#contents
18. http://www.columbia.edu/kermit/telnet80.html#managing
19. http://www.columbia.edu/kermit/telnet80.html#intro
20. ftp://ftp.isi.edu/in-notes/rfc2941.txt
21. ftp://ftp.isi.edu/in-notes/rfc856.txt
22. ftp://ftp.isi.edu/in-notes/rfc2217.txt
23. ftp://ftp.isi.edu/in-notes/rfc857.txt
24. ftp://ftp.isi.edu/in-notes/rfc2946.txt
25. ftp://ftp.isi.edu/in-notes/rfc2840.txt
26. ftp://ftp.isi.edu/in-notes/rfc727.txt
27. ftp://ftp.isi.edu/in-notes/rfc1073.txt
28. ftp://ftp.isi.edu/in-notes/rfc1572.txt
29. ftp://ftp.isi.edu/in-notes/rfc779.txt
30. http://search.ietf.org/internet-drafts/draft-ietf-tn3270e-telnet-tls-05.txt
31. ftp://ftp.isi.edu/in-notes/rfc858.txt
32. ftp://ftp.isi.edu/in-notes/rfc1091.txt
33. ftp://ftp.isi.edu/in-notes/rfc1096.txt
34. http://www.columbia.edu/kermit/ckermit.html
35. http://www.columbia.edu/kermit/k95.html
36. http://www.columbia.edu/kermit/index.html
37. http://www.columbia.edu/kermit/telnet80.html#top
38. http://www.columbia.edu/kermit/telnet80.html#contents
39. http://www.columbia.edu/kermit/telnet80.html#summary
40. http://www.columbia.edu/kermit/telnet80.html#options
41. http://www.columbia.edu/kermit/ckermit.html
42. http://www.columbia.edu/kermit/k95.html
43. http://www.columbia.edu/kermit/index.html
44. http://www.columbia.edu/kermit/telnet80.html#top
45. http://www.columbia.edu/kermit/telnet80.html#contents
46. http://www.columbia.edu/kermit/telnet80.html#problems
47. http://www.columbia.edu/kermit/telnet80.html#managing
48. ftp://ftp.isi.edu/in-notes/rfc1572.txt
49. http://www.columbia.edu/kermit/ckermit.html
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51. http://www.columbia.edu/kermit/index.html
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53. http://www.columbia.edu/kermit/telnet80.html#contents
54. http://www.columbia.edu/kermit/telnet80.html#summary
55. http://www.columbia.edu/kermit/telnet80.html#x1
56. http://www.columbia.edu/kermit/telnet80.html#x2
57. http://www.columbia.edu/kermit/telnet80.html#x3
58. http://www.columbia.edu/kermit/telnet80.html#x4
59. http://www.columbia.edu/kermit/telnet80.html#x5
60. http://www.columbia.edu/kermit/telnet80.html#x6
61. http://www.columbia.edu/kermit/telnet80.html#x7
62. http://www.columbia.edu/kermit/telnet80.html#x8
63. http://www.columbia.edu/kermit/telnet80.html#x9
64. http://www.columbia.edu/kermit/telnet80.html#x10
65. http://www.columbia.edu/kermit/telnet80.html#x11
66. http://www.columbia.edu/kermit/telnet80.html#x12
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68. http://www.columbia.edu/kermit/k95.html
69. http://www.columbia.edu/kermit/index.html
70. http://www.columbia.edu/kermit/telnet80.html#top
71. http://www.columbia.edu/kermit/telnet80.html#contents
72. http://www.columbia.edu/kermit/telnet80.html#x5
73. http://www.columbia.edu/kermit/telnet80.html#x5.2
74. http://www.columbia.edu/kermit/ckermit.html
75. http://www.columbia.edu/kermit/k95.html
76. http://www.columbia.edu/kermit/index.html
77. http://www.columbia.edu/kermit/telnet80.html#top
78. http://www.columbia.edu/kermit/telnet80.html#contents
79. http://www.columbia.edu/kermit/telnet80.html#x5
80. http://www.columbia.edu/kermit/telnet80.html#x5.3
81. http://www.columbia.edu/kermit/telnet80.html#x5.1
82. http://www.columbia.edu/kermit/ckermit.html
83. http://www.columbia.edu/kermit/k95.html
84. http://www.columbia.edu/kermit/index.html
85. http://www.columbia.edu/kermit/telnet80.html#top
86. http://www.columbia.edu/kermit/telnet80.html#contents
87. http://www.columbia.edu/kermit/telnet80.html#x5
88. http://www.columbia.edu/kermit/telnet80.html#x5.4
89. http://www.columbia.edu/kermit/telnet80.html#x5.2
90. http://www.columbia.edu/kermit/ckermit.html
91. http://www.columbia.edu/kermit/k95.html
92. http://www.columbia.edu/kermit/index.html
93. http://www.columbia.edu/kermit/telnet80.html#top
94. http://www.columbia.edu/kermit/telnet80.html#contents
95. http://www.columbia.edu/kermit/telnet80.html#x5
96. http://www.columbia.edu/kermit/telnet80.html#x5.5
97. http://www.columbia.edu/kermit/telnet80.html#x5.3
98. http://www.columbia.edu/kermit/ckermit.html
99. http://www.columbia.edu/kermit/k95.html
100. http://www.columbia.edu/kermit/index.html
101. http://www.columbia.edu/kermit/telnet80.html#top
102. http://www.columbia.edu/kermit/telnet80.html#contents
103. http://www.columbia.edu/kermit/telnet80.html#x5
104. http://www.columbia.edu/kermit/telnet80.html#x5.6
105. http://www.columbia.edu/kermit/telnet80.html#x5.4
106. http://www.columbia.edu/kermit/ckermit.html
107. http://www.columbia.edu/kermit/k95.html
108. http://www.columbia.edu/kermit/index.html
109. http://www.columbia.edu/kermit/telnet80.html#top
110. http://www.columbia.edu/kermit/telnet80.html#contents
111. http://www.columbia.edu/kermit/telnet80.html#x5
112. http://www.columbia.edu/kermit/telnet80.html#x5.7
113. http://www.columbia.edu/kermit/telnet80.html#x5.5
114. http://www.columbia.edu/kermit/ckermit.html
115. http://www.columbia.edu/kermit/k95.html
116. http://www.columbia.edu/kermit/index.html
117. http://www.columbia.edu/kermit/telnet80.html#top
118. http://www.columbia.edu/kermit/telnet80.html#contents
119. http://www.columbia.edu/kermit/telnet80.html#x5
120. http://www.columbia.edu/kermit/telnet80.html#x5.8
121. http://www.columbia.edu/kermit/telnet80.html#x5.6
122. http://www.columbia.edu/kermit/ckermit.html
123. http://www.columbia.edu/kermit/k95.html
124. http://www.columbia.edu/kermit/index.html
125. http://www.columbia.edu/kermit/telnet80.html#top
126. http://www.columbia.edu/kermit/telnet80.html#contents
127. http://www.columbia.edu/kermit/telnet80.html#x5
128. http://www.columbia.edu/kermit/telnet80.html#x5.9
129. http://www.columbia.edu/kermit/telnet80.html#x5.7
130. ftp://ftp.isi.edu/in-notes/rfc856.txt
131. http://www.columbia.edu/kermit/ckermit.html
132. http://www.columbia.edu/kermit/k95.html
133. http://www.columbia.edu/kermit/index.html
134. http://www.columbia.edu/kermit/telnet80.html#top
135. http://www.columbia.edu/kermit/telnet80.html#contents
136. http://www.columbia.edu/kermit/telnet80.html#x5
137. http://www.columbia.edu/kermit/telnet80.html#x5.10
138. http://www.columbia.edu/kermit/telnet80.html#x5.8
139. http://www.columbia.edu/kermit/ckermit.html
140. http://www.columbia.edu/kermit/k95.html
141. http://www.columbia.edu/kermit/index.html
142. http://www.columbia.edu/kermit/telnet80.html#top
143. http://www.columbia.edu/kermit/telnet80.html#contents
144. http://www.columbia.edu/kermit/telnet80.html#x5
145. http://www.columbia.edu/kermit/telnet80.html#x5.11
146. http://www.columbia.edu/kermit/telnet80.html#x5.9
147. http://www.columbia.edu/kermit/ckermit.html
148. http://www.columbia.edu/kermit/k95.html
149. http://www.columbia.edu/kermit/index.html
150. http://www.columbia.edu/kermit/telnet80.html#top
151. http://www.columbia.edu/kermit/telnet80.html#contents
152. http://www.columbia.edu/kermit/telnet80.html#x5
153. http://www.columbia.edu/kermit/telnet80.html#x5.12
154. http://www.columbia.edu/kermit/telnet80.html#x5.10
155. http://www.columbia.edu/kermit/ckermit.html
156. http://www.columbia.edu/kermit/k95.html
157. http://www.columbia.edu/kermit/index.html
158. http://www.columbia.edu/kermit/telnet80.html#top
159. http://www.columbia.edu/kermit/telnet80.html#contents
160. http://www.columbia.edu/kermit/telnet80.html#x5
161. http://www.columbia.edu/kermit/telnet80.html#x5.10
162. http://www.iana.org/
163. ftp://ftp.isi.edu/in-notes/rfc855.txt
164. ftp://ftp.isi.edu/in-notes/rfc855.txt
165. ftp://ftp.isi.edu/in-notes/rfc1143.txt
166. http://www.columbia.edu/kermit/ckermit.html
167. http://www.columbia.edu/kermit/k95.html
168. http://www.columbia.edu/kermit/index.html