Telnet using Java
This article will demonstrate how using the SSH Factory Telnet
component you can establish an interactive Telnet session with
a Telnet server. This article will also serve as a prerequisite
to an upcoming article titled Scripting Telnet sessions using
Java which will demonstrate how to automate Telnet commands
in a batch like process. To see what else iNet Factory has to
offer Download a FREE 30 day SSH Factory
Evaluation.
The Telnet client component provides a method for communicating
with a Telnet server. The process for establishing an interactive
session with a Telnet server using the Telnet component is as
follows:
- Creating a new Telnet instance
- Implementing a TelnetListener
- Registering a TelnetListener
- Establishing a connection
- Performing option negotiation
- Receiving data
- Sending data
- Releasing a connection
Each of these processes is described in the sections below.
To create a new Telnet instance ensure that the com.jscape.inet.telnet
package is included in your import statements and create a new
Telnet instance providing the Telnet server hostname as an argument.
Telnet telnet = new Telnet(hostname);
The Telnet component is one of the few components in SSH Factory
which MUST have a registered event listener. The reason for this
will be clear later. The following is a sample implementation
of the TelnetListener interface. A TelnetListener instance must
be created and registered with the Telnet instance prior to invoking
the Telnet#connect method to ensure that all data sent by the
Telnet server is captured.
public class MyTelnetListener implements TelnetListener {
public void connected(TelnetConnectedEvent event) {
}
public void disconnected(TelnetDisconnectedEvent event) {
}
public void doOption(DoOptionEvent event) {
}
public void dontOption(DontOptionEvent event) {
}
public void willOption(WillOptionEvent event) {
}
public void wontOption(WontOptionEvent event) {
}
public void doSubOption(DoSubOptionEvent event) {
}
public void dataReceived(TelnetDataReceivedEvent event) {
}
}
To register a listener invoke the Telnet#addTelnetListener method
providing a TelnetListener instance as its argument.
Telnet telnet = new Telnet(hostname);
MyTelnetListener listener = new MyTelnetListener();
telnet.addTelnetListener(listener);
Once a Telnet instance has been created and a TelnetListener
registered you may establish a connection to the Telnet server
by invoking the Telnet#connect method.
Telnet telnet = new Telnet(hostname);
MyTelnetListener listener = new MyTelnetListener();
telnet.addTelnetListener(listener);
telnet.connect();
Upon establishing a connection the process of option negotiation
will begin. Option negotiation is a communications process for
the Telnet client and the Telnet server to come up with a set
of agreed upon protocols. An example of option negotiation is
agreeing upon the terminal emulation (e.g. vt100, xterm, dumb)
to use in the Telnet session.
Option negotiation as its name implies is optional and may be
initiated by either the client or server. This does not mean however
that option negotiation may be ignored. For example, in the event
that the server attempts to perform option negotiation the client
must respond by either accepting or rejecting the option request.
Capturing option negotiation data from the Telnet server is accomplished
using the TelnetListener class. In order to capture this data
you must register an instance of the TelnetListener class with
the Telnet instance as shown earlier.
In performing option negotiation there are four (5) Telnet protocol
commands that can be used by the client and server.
- DO OPTION - Requests to enable an option.
- DONT OPTION - Refuses offer to enable an option.
- WILL OPTION - Offers to enable an option.
- WONT OPTION - Refuses request to enable an option.
- SUB OPTION - For option subnegotiation
For the purposes of this article we will refuse all options both
requested and offered by the Telnet server. This in effect will
give us a basic Telnet client that is capable of exchanging data
with the Telnet server. In order to capture and refuse options
requested or offered by the Telnet server you will need to overload
the TelnetListener#doOption and TelnetListener#willOption methods
as follows:
public void doOption(DoOptionEvent event) {
// refuse any options requested by Telnet server
telnet.sendWontOption(event.getOption());
}
public void willOption(WillOptionEvent event) {
// refuse any options offered by Telnet server
telnet.sendDontOption(event.getOption());
}
Once option negotiation has been completed you may receive data
sent by the Telnet server. To receive data, overload the TelnetListener#dataReceived
method as follows:
public void dataReceived(TelnetDataReceivedEvent event) {
// print data received from Telnet server to console
System.out.println(event.getData());
}
To send data to the Telnet server you will first obtain an OutputStream
from the Telnet instance. Given this OutputStream you can then
send data to the Telnet server as follows:
// get output stream
output = telnet.getOutputStream();
// sends all data entered at console to Telnet server
while ((input = reader.readLine()) != null) {
if (connected) {
((TelnetOutputStream) output).println(input);
} else {
break;
}
}
In the example above the input comes from the console and is
redirected to the Telnet server using the TelnetOutputStream#println
method. The TelnetOutputStream class is used as it automatically
appends a \r\n (carriage return line feed) to the end of the data
sent. This is required by the Telnet server for it to know when
it may begin processing the data received.
To release an established connection simply invoke the Telnet#disconnect
method as follows:
telnet.disconnect();
Below is the fully functional source code for an interactive
Telnet client written using the SSH Factory Telnet component.
This same example is available in the SSH Factory evaluation
download in the examples directory.
001 import com.jscape.inet.telnet.*;
002 import java.io.*;
003
004 public class TelnetExample extends TelnetAdapter {
005
006 private Telnet telnet = null;
007 private OutputStream output = null;
008 private static BufferedReader reader = null;
009 private boolean connected = false;
010
011 public TelnetExample(String hostname) throws IOException {
012
013 String input = null;
014 // create new Telnet instance
015 telnet = new Telnet(hostname);
016
017 // register this class as TelnetListener
018 telnet.addTelnetListener(this);
019
020 // establish Telnet connection
021 telnet.connect();
022 connected = true;
023
024 // get output stream
025 output = telnet.getOutputStream();
026
027 // sends all data entered at console to Telnet server
028 while ((input = reader.readLine()) != null) {
029 if (connected) {
030 ((TelnetOutputStream) output).println(input);
031 } else {
032 break;
033 }
034 }
035 }
036
037 /** Invoked when Telnet socked is connected.
038 * @see TelnetConnectedEvent
039 * @see Telnet#connect
040 */
041 public void connected(TelnetConnectedEvent event) {
042 System.out.println("Connected");
043 }
044
045 /**
046 * Invoked when Telnet socket is disconnected. Disconnect can
047 * occur in many circumstances including IOException during socket read/write.
048 * @see TelnetDisconnectedEvent
049 * @see Telnet#disconnect
050 */
051 public void disconnected(TelnetDisconnectedEvent event) {
052 connected = false;
053 System.out.print("Disconnected. Press enter key to quit.");
054 }
055
056 /**
057 * Invoked when Telnet server requests that the Telnet client begin performing specified <code>TelnetOption</code>.
058 * @param event a <code>DoOptionEvent</code>
059 * @see DoOptionEvent
060 * @see TelnetOption
061 */
062 public void doOption(DoOptionEvent event) {
063 // refuse any options requested by Telnet server
064 telnet.sendWontOption(event.getOption());
065 }
066
067 /**
068 * Invoked when Telnet server offers to begin performing specified <code>TelnetOption</code>.
069 * @param event a <code>WillOptionEvent</code>
070 * @see WillOptionEvent
071 * @see TelnetOption
072 */
073 public void willOption(WillOptionEvent event) {
074 // refuse any options offered by Telnet server
075 telnet.sendDontOption(event.getOption());
076 }
077
078 /**
079 * Invoked when data is received from Telnet server.
080 * @param event a <code>TelnetDataReceivedEvent</code>
081 * @see TelnetDataReceivedEvent
082 */
083 public void dataReceived(TelnetDataReceivedEvent event) {
084 // print data received from Telnet server to console
085 System.out.print(event.getData());
086 }
087
088 /**
089 * Main method for launching program
090 * @param args program arguments
091 */
092 public static void main(String[] args) {
093 try {
094 reader = new BufferedReader(new InputStreamReader(System.in));
095
096 // prompt user for Telnet server hostname
097 System.out.print("Enter Telnet server hostname (e.g. 10.0.0.1): ");
098 String hostname = reader.readLine();
099
100 // create new TelnetExample instance
101 TelnetExample example = new TelnetExample(hostname);
102 } catch (Exception e) {
103 e.printStackTrace(System.out);
104 }
105 }
106
107 }
- Line 1. Add necessary import statements.
- Line 4. Implement the TelnetAdapter class to capture events
fired by the Telnet class. TelnetAdapter is an abstract implementation
of the TelnetListener class.
- Line 15. Create a new Telnet instance using provided hostname.
- Line 18. Register Telnet event listener.
- Line 21. Establish connection with Telnet server.
- Line 25. Get OutputStream for writing data to Telnet server.
- Line 28. Read data from console and send to Telnet server.
- Line 41. Capture when connection is established.
- Line 51. Capture when connection is released.
- Line 62. Refuse options requested by Telnet server.
- Line 73. Refuse options offered by Telnet server.
- Line 83. Print data received from Telnet server to console.
- Line 92. main() method for starting Telnet client.
In this article you learned how to establish an interactive Telnet
session with a Telnet server using the Telnet component provided
in SSH Factory. The Telnet component in SSH Factory makes this
easy removing the complexities of the Telnet protocol. To see
what else SSH Factory has to offer Download
a FREE 30 day SSH Factory Evaluation.
|
