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The Telnet Protocol control enables an application to connect to
a Telnet server, which provides an interactive terminal session
similar to how character based consoles and terminals work. The
user can login, enter commands and interact with applications
programmatically or in conjunction with the terminal emulation
control.
The following methods are available for use by your
application:
Initialize
Initialize the control and load the Windows Sockets library for the
current process. This method is normally not used if the control is
placed on a form in languages such as Visual Basic. However, if the
control is being created dynamically using a method similar to
CreateObject, then the application must call this method to
initialize the component before setting any properties or calling
any other methods in the control.
Connect
Establish a connection to the server. Once the connection has been
established, the other methods in the control may be used to
interact with the server.
Disconnect
Disconnect from the server and release any resources that have been
allocated for the client session. After this method is called, the
client session is no longer valid.
Uninitialize
Unload the Windows Sockets library and release any resources that
have been allocated for the current process. This is the last
method call that the application should make prior to terminating.
This is only necessary if the application has previously called the
Initialize method.
Input and Output
Once connected to the Telnet server, any output generated by a
program on the server will be sent as data for the client to read.
Any input to the program is sent by the client and received and
processed by the server. The following methods are used:
Read
Reads any output that has been generated by the program executing
on the server. When the client first connects, the server typically
executes a login program that requests the users authenticate
themselves by entering a user name and password. Once the user has
logged in, they are usually given a command line prompt where they
can enter commands to be executed on the server. If the server
closes the connection, the Read method will indicate that with an
error result and the client can disconnect from the server at that
point.
Write
Send data to the Telnet server which will be received as input to
the program. If the local echo option is enabled, then the client
is also responsible for writing the input data to the display
device, if there is one. If local echo is not enabled, the server
will automatically echo back any characters written as data to be
read by the client.
Telnet Modes
Telnet supports several modes of operation and the option
negotiation phase, which occurs when a connection is established,
is handled automatically by the control. There are two key modes
which affect how the client session works:
Binary
If this property is set to True, the data between the client and
server is not buffered and the high bit is not removed from any
characters. If the application is executing a program which uses
text mode windowing features (i.e.: it draws boxes on the display)
then this mode must be enabled to ensure that the client processes
the data correctly and it isn't buffered a line at a time. If this
mode is disabled, then the data exchanged between the client and
server will be buffered a line at a time and any 8bit characters
will be stripped. This mode is enabled by default.
LocalEcho
If this property is set to True, it is the responsibility of the
client to echo any data that it is sending to the server. For
example, if the character "A" is sent to the server, the
application must also send the character "A" to whatever interface
the user is interacting with, such as a terminal emulation window.
The default mode is for this option to be disabled, which means
that the server will echo back any data that is sent to it.
Command Processing
The Telnet protocol can be used to connect to a server, log in
and execute one or more commands, process the output from those
commands and display it to an end-user using a graphical interface.
The user never sees or interacts with the actual terminal session.
The Telnet interface provides methods which can simplify this kind
of application, reducing the amount of code needed to process the
data stream returned by the server.
Login
This method is used to automatically log a user in, using the
specific user name and password. This method is specifically
designed for UNIX based servers or Windows servers which emulate
the same basic login sequence.
Search
This method is used to search for a specific character or sequence
of characters in the data stream returned by the server. The
control will accumulate all of the data received up to the point
where the character sequence is encountered. This can be used to
capture all of the output from a command, or search for specific
results returned by the command as it executes on the server.
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