In the present day, I see telnet primarily used as a diagnostic tool for HTTP, SMTP, and other protocols. I often use it myself in diagnosing server problems. Lately, I've found my phone to be an ideal device for using telnet for these purposes because it can easily connect to the local network for test as well as using its only Internet connection to connect to a server from a point outside the local network.
The client was quickly and easily written in less than a day using C# and the .NET Compact Framework. Within this document, I'll highlight the most important parts of creating the client.
I have targeted a Windows Mobile 5 Professional device when writing this application. I've tested the program on both the HTC Wizard (branded Cingular 8125) and an iPaq 4705. I attempted to run the program on a Motorola Q but was unable to track down a thread related exception that occurred (unfortunately this exception would also cause Visual Studio to close). So for now I believe this program will not yet run on devices with no touch screen. To run this program, your device must have the 2.0 Compact Framework. If it is not installed, Visual Studio will install the framework to your device if you allow Visual Studio to debug the program on your Windows Mobile 5 device.
The UI was designed to be capable of being displayed on a device with 240x240 resolution but should really be run on a device with at least 320x240 resolution. Ideally it should be run on a VGA resolution device if the partially implemented Terminal Emulator feature is used.
Not All Devices Are Created Equal
.NET allows this program to be run on several types of devices. Unfortunately I found that not all features of the .NET 2.0 Framework are supported on all devices that run the framework. This is further complicated by the fact that the IDE will not generate errors when you use unsupported features (and I can't realistically expect it to since it can't know all the features of all the devices on which it will be run).
An example of a feature that won't work across all devices is the ToolBar control. This control will render fine on the desktop and on Windows Mobile 5 Professional devices but cause an
UInsupportedException on the Moto Q. I also found that the "Save File Dialog" is not available on both platforms.
Establishing a Connection
All of the code for establishing a telnet session can be found in the
TelnetSession class. The main objects of interest in the class are instances of the
Socket class and
NetworkStream class. The creation of the socket is done in the
CreateSocket method. This method takes as its arguments the address or host name of the target machine and the remote port number to use. The method resolves the IP address of the target machine and attempts to make a connection to it. If successful, the socket created will be returned.
Socket CreateSocket(string machineAddress, int port)
IPHostEntry iphostEntry = null;
iphostEntry = Dns.GetHostEntry(machineAddress);
foreach (IPAddress address in iphostEntry.AddressList)
IPEndPoint ipe = new IPEndPoint(address.Address, port);
Socket tempSocket = new Socket
Once a socket is created, a
NetworkStream object is wrapped around it so that we have a stream interface to the socket. Then a
StreamWriter are created which refer to the network stream and give us a natural way to interact with the connection.
Reading and Writing Data
Writing data to the connection is incredibly simple. Use the
Write method on the
StreamWriter object to send an array of bytes over the stream.
public void Send(byte message)
Reading is a little more involved. While one could call the
StreamReader.Read method, this is undesirable since it will block a thread until the read buffer is full. If a server is returning a small amount of bytes, a program could be stuck waiting for more bytes even if there are none. Because of the asynchronous nature of the connection, we have no idea how many bytes are going to be received and when.
To resolve the issue of not knowing when data will be received, a new thread is created that will wait for data to be received and trigger an event when data is available. Within the thread, the
DataAvailable property of the
NetworkStream object is checked. If no data is available, the thread will just sleep for a while. If data is available, it is immediately read and converted to a
string is added to a queue and then the
TelnetSession object sends a notification through the
BlockReceived event to inform subscribers that the data is available. The resulting
strings are read through the
ReadBlock method. Note that
ReadBlock returns any number of lines of text.
The thread on which this occurs is marked as a background thread. This helps to ensure that the thread will be terminated when the main thread of the application terminates. Otherwise there would be the risk that the user closes the program but the program continues to run.
Displaying Information to the User
The program supports two ways of displaying information to the user. The first (and most simple) way is to directly write the received
strings to a
textbox. Since Windows Mobile Devices may have small amounts of memory, as more information is added to the
textbox, text is erased from the beginning of the
textbox to limit its size. For most telnet sessions, this will be a sufficient display technique.
The second way of displaying data involves a class in the program called
TelnetTerminal. This class emulates the behavior of a display terminal with a fixed width and height. I've got the terminal set for 80 columns of width and 25 rows for its height (these dimensions can be changed through the objects constructor). The terminal can recognize certain escape sequences for moving the cursor around within itself and clearing the screen. While the
TerminalEmulator class was inspired by Ecma 48, it is in no way a full implementation. The terminal is largely experimental but was left within the program for anyone that wishes to extend its functionality. The
TerminalEmulator will work best on VGA capable Windows Mobile Devices.
Entering and Saving Connection Information
Connection information is entered through the
ConnectionSettings form. The form asks for the name of the machine to connect to, the port to use, whether or not input will be buffered, and a few other things. The details of this information are shown below:
- Connection name – A name to give to the connection
- Address – The machine name or IP address of the target machine
- Port – The remote port on which to establish the connection
- Buffer Input – If enabled, will allow input to be accumulated into a
textbox and all of the text is sent at once when the enter key is pressed. Otherwise each key press is immediately transmitted.
- Local Echo – If enabled, all transmitted information is immediately displayed on the terminal's screen.
- Terminal Type – Allows the user to select the simple terminal interface (Character Buffer) or the experimental terminal interface (Terminal Emulator, best used on VGA capable devices)
The form can be accessed through the menu sequence File->New Connection. When a new connection is created, information on the current connection is used to populate the default values for each field. Saved connection information is serialized to an XML file, so you can easily change the connection information using a text editor too.
In testing this program, I did find that when using certain phone carriers I could not open connections over certain ports. It is important to take note that not all ports may be allowed by your carrier.
Three Ways to Debug
There are three ways to debug this program. The suggested way is to connect your Windows Mobile 5 device to the machine and then press F5 to begin deploying and running the application. You will be prompted with a screen allowing you to select a target device. The first option labelled "Windows Mobile 5.0 Pocket PC Device" will copy the program to your device and the .NET Compact Framework if necessary. The second method is to select one of the emulators from when the program is deployed.
The third way to test is to run the program on your workstation and attach to it from within Visual Studio. To do this, compile the program and then navigate to the location where the final binary is stored. Run it and then within Visual Studio, select the menu sequence Debug->Attach to Process. You can then select J2i.Net.Telnet.exe as the process to attach to and debug.
Debugging a HTTP Request
The simplest connection to debug would be an HTTP request. To do this, create a new connection and specify a Web server name as the address (such as www.microsoft.com), enter 80 for the port, and select the Character Buffer terminal type. After you select "OK", the session will be started. Enter
GET / and then press the return key twice. You should receive the raw response that the server would give you for its root page.
Conclusions and Future Features
I believe that in its current form, the program is fine for use as a diagnostic tool. I may later decide to add the ability to log the information sent over a session or may further implement the Terminal Emulator feature if I find that it is needed or demanded. I plan to look further into what is preventing the application from running on a Motorola Q. I may also add user interfaces to allow properties of the terminal (such as the text color) to be customized.
- 5th August, 2007 - Article published