Tips Using Telnet with Java

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:

1. Creating a new Telnet instance
2. Implementing a TelnetListener
3. Registering a TelnetListener
4. Establishing a connection
5. Performing option negotiation
6. Receiving data
7. Sending data
8. Releasing a connection

Each of these processes is described in the sections below.

Creating a new Telnet instance
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);

Implementing a TelnetListener
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) {
}
}

Registering a TelnetListener
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);

Establishing a connection
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();

Performing option negotiation
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.

1. DO OPTION – Requests to enable an option.
2. DONT OPTION – Refuses offer to enable an option.
3. WILL OPTION – Offers to enable an option.
4. WONT OPTION – Refuses request to enable an option.
5. 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());
}

Receiving data
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());
}

Sending data

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.

Releasing a connection
To release an established connection simply invoke the Telnet#disconnect method as follows:

telnet.disconnect();

Example

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 }

1. Line 1. Add necessary import statements.
2. Line 4. Implement the TelnetAdapter class to capture events fired by the Telnet class. TelnetAdapter is an abstract implementation of the TelnetListener class.
3. Line 15. Create a new Telnet instance using provided hostname.
4. Line 18. Register Telnet event listener.
5. Line 21. Establish connection with Telnet server.
6. Line 25. Get OutputStream for writing data to Telnet server.
7. Line 28. Read data from console and send to Telnet server.
8. Line 41. Capture when connection is established.
9. Line 51. Capture when connection is released.
10. Line 62. Refuse options requested by Telnet server.
11. Line 73. Refuse options offered by Telnet server.
12. Line 83. Print data received from Telnet server to console.
13. Line 92. main() method for starting Telnet client.

Source :  www.jscape.com

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