B# .NET Blog

A lightweight POP3 server

public class Pop3Server
{
   private TcpListener listener;
   private Thread tListener;
   private string url;

   public Pop3Server(int port, string url)
   {
      listener = new TcpListener(IPAddress.Loopback, port);
      this.url = url;
   }
}

public void Start()
{
   listener.Start();
   tListener = new Thread(new ThreadStart(Listen));
   tListener.Start();
}

public void Stop()
{
   tListener.Abort();
   while (tListener.ThreadState != ThreadState.Aborted)
      ;
   listener.Stop();
}

To do this, we just use some quick-n-dirty threading stuff. A cleaner approach can be taken to stop the server more smoothly in order to close communications with the server in a friendly way. However, for demonstration purposes this approach should be okay. Here's the Listen method that acts as the server's background listening thread:

Basically, this method sits and wait for incoming client connections (i.e. connections made from the mail client software to localhost:110). When a connection is made, a background session thread is started, passing in the TcpClient object as a parameter through the ParameterizedThreadStart delegate (new in .NET 2.0):

Time has come to start the real work: the Pop3Session class. I've declared this class as a nested private class inside the Pop3Server class. Let's start by examining the class' skeleton:

Basically, this class has two faces. The first one is the TCP socket one, supported by the stream member. The second one is the WCF client one, supported by the svc proxy instance. As long as the local POP3 session lasts, the WCF service session keeps alive too.

On to the Start method which contains the main loop that waits for commands sent by the local mail client software to the localhost:110 local POP3 server:

The first thing the Start method does (which is - for the record - called on a separate thread by the Pop3Server's Listen connection listening loop) is connecting to the target server by calling the svc.Connect method. This method starts a WCF session; recall the WCF service contract defined in part 2:

   [OperationContract(IsInitiating = true)]
   string Connect(string server, int port);

Next, we start to wait for POP3 commands sent by the local mail client. POP3 commands are straightforward to take in because they come in as single-line strings, terminated by a CRLF (\r\n) pair. Therefore, the ReadLine method of the StreamReader attached to the NetworkStream will just do its job fine.

Every command is then forwarded to the ExecuteCommand helper method, defined below:

Again, I've omitted all Console.Write* stuff that prints diagnostic information on the screen. This ExecuteCommand method should be very easy to understand. It performs some basic parsing of the command that's received by our local POP3 server and calls the appropriate method on the WCF service through the svc proxy instance. Two helper methods are required:

As you recall from the service contract, every POP3 equivalent web method takes in all of the parameters required for that command and returns the POP3 response string that was obtained by the target POP3 server. Using the Write method this answer is sent back to the local mail client, with the trailing CRLF pair:

   Write(svc.User(cmds[1]));

In order to run this client, we need some simple host code (read: a Main method):

class Program
{
   static void Main(string[] args)
   {
      int port = 110;
      string url = new BdsSoft.Net.Mail.localhost.Pop3TunnelServiceClient().Endpoint.Address.Uri.ToString();

      Console.WriteLine("WCF POP3 Tunnel Client");
      Console.WriteLine("======================");
      Console.WriteLine();

      Console.WriteLine("Initializing local tunnel endpoint...");
      Console.WriteLine("- Local POP3 server listening on TCP port {0}", port);
      Console.WriteLine("- Forwarding to {0}", url);
      Console.WriteLine();

      new Pop3Server(port, url).Start();
      Console.ReadLine();
   }
}

Configuration

Last but not least, our client needs some configuration. The WCF "Add Service Dialog" will have added an app.config file already but it requires some changes, as shown below. These changes are required to deal with large responses, e.g. in response to a RETR POP3 command that retrieves an entire mail message (the default of 16 KB is not sufficient for our purposes). If you decide to implement this stuff for production purposes, you'll want to change the endpoint address to some remote server of course, not just the local machine which we are using for demo purposes (tunnel server and localhost are the same machine).

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
   <system.serviceModel>
      <bindings>
         <wsHttpBinding>
            <binding name="WSHttpBinding_IPop3TunnelService" closeTimeout="00:01:00"
               openTimeout="00:01:00" receiveTimeout="00:10:00" sendTimeout="00:01:00"
               bypassProxyOnLocal="false" transactionFlow="false" hostNameComparisonMode="StrongWildcard"
               maxBufferPoolSize="524288" maxReceivedMessageSize="2147483647"
               messageEncoding="Text" textEncoding="utf-8" useDefaultWebProxy="true"
               allowCookies="false">
               <readerQuotas maxDepth="32" maxStringContentLength="2147483647" maxArrayLength="16384"
                  maxBytesPerRead="4096" maxNameTableCharCount="16384" /> 
               <reliableSession ordered="true" inactivityTimeout="00:10:00"
                  enabled="false" />
               <security mode="Message">
                  <transport clientCredentialType="Windows" proxyCredentialType="None"
                     realm="" />
                  <message clientCredentialType="Windows" negotiateServiceCredential="true"
                     algorithmSuite="Default" establishSecurityContext="true" />
               </security>
            </binding>
         </wsHttpBinding>
      </bindings>
      <client>
         <endpoint address="http://localhost:8080/BdsSoft.Net.Mail/Pop3TunnelService"
            binding="wsHttpBinding" bindingConfiguration="WSHttpBinding_IPop3TunnelService"
            contract="BdsSoft.Net.Mail.localhost.IPop3TunnelService" name="WSHttpBinding_IPop3TunnelService">
            <identity>
               <userPrincipalName value="something" />
            </identity>
         </endpoint>
      </client>
   </system.serviceModel>
</configuration>