Silverlight - Policy Server in VB.NET

Silver Light Policy Server

For Socket based communications in Silverlight, we need to run Policy Server at Socket Listening host. Policy Server will send the Policy XML file to silverlight client similar to flash crossdomain.xml file, which Silverlight also uses for its HTTP request. Without Policy Server, Silverlight sockets cannot connect to a Server Application.

Why we need a separate Policy file for sockets ?

The Crossdomain.xml file can be directly accessed by Clients using HTTP request from related web server, where as for sockets based communication server,  web server is not in the picture. So for security purposes, similar to crossdomain file, there needs a policy file to be downloaded & which will be issued by Policy Server.

Ports

Policy Server Listening Port: 943

Silverlight Sockets Allowed Ports: 4502 to 4534

How it will work ?

1. Our Policy Server listens in Port: 943. 

2. Silverlight client will connect to our policy server & send a request "<policy-file-request/>"

3. Once this request is received, Policy Server should send the Policy XML file in that connection & close that connection request.

Sample Policy XML File

clientaccesspolicy.xml

<?xml version="1.0" encoding="utf-8" ?>
<access-policy>
    <cross-domain-access>
        <policy>
            <allow-from>
                <domain uri="*" />
           </allow-from>
            <grant-to>
                <socket-resource port="4502-4534" protocol="tcp"/>
            </grant-to>
</policy>
   </cross-domain-access>

</access-policy>

domain uri - Allows the domain from where it can be called. "*" will allow all domains.

To allow only a particular domain Ex: uri="http://srimax.com"/>

Socket-Resource port can be used to indicate the allowed ports.

To allow only a particular port: port="4512" ...

Policy Server
PolicyServerService.vb

Imports System

Imports System.IO

Imports System.Net.Sockets

Imports System.Text

Public Class PolicyServerService

    Public sSocket As Socket

    Private PolicyFile As String = String.Empty

    Private Const POLICYREQUEST As String = "<policy-file-request/>"

    Private Const POLICYPORT As Integer = 943

    Private Const POLICYFILENAME As String = "clientaccesspolicy.xml" 

    Public Sub New()

        ' Read Policy File details

        PolicyFile = Path.GetDirectoryName(System.Reflection.Assembly.GetExecutingAssembly().Location) & "\" & POLICYFILENAME

        ' Establish the local endpoint for the socket.

        Dim ipLocal As New System.Net.IPEndPoint(System.Net.IPAddress.Any, POLICYPORT)

        sSocket = New Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp)

        sSocket.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.NoDelay, 0)

sSocket.Bind(ipLocal)

        sSocket.Listen(10)

        'Start an asynchronous socket to listen for connections.

        sSocket.BeginAccept(New AsyncCallback(AddressOf onConnect), Nothing)

    End Sub

 

    Private Sub onConnect(ByVal asyn As IAsyncResult)

        Dim objPolicyConnection As New PolicyConnection

        Try

            With objPolicyConnection

                .cSocket = sSocket.EndAccept(asyn)

                .cSocket.BeginReceive(.buffer, 0, .buffer.Length, SocketFlags.None, New AsyncCallback(AddressOf onDataArrival), objPolicyConnection)

            End With 

            'continue listen for connections.

            sSocket.BeginAccept(New AsyncCallback(AddressOf onConnect), Nothing)

        Catch exNull As System.NullReferenceException

        Catch ex As Exception

            Throw New Exception(ex.Message)

        End Try

    End Sub 

    Private Sub onDataArrival(ByVal asyn As IAsyncResult)

        Dim strData As String = String.Empty

        Dim policyFile As String = String.Empty

        Dim ibRead As Integer = 0

        Dim objPolicyConnection As PolicyConnection = CType(asyn.AsyncState, PolicyConnection)

        Try

            With objPolicyConnection

                If .cSocket.Connected Then ibRead = .cSocket.EndReceive(asyn)

                If ibRead > 0 Then

receivedText.Append(Encoding.ASCII.GetString(.buffer, 0, ibRead))

                strData = .receivedText.ToString()

                If strData.Length < POLICYREQUEST.Length Then

                    ' if not full request received

                    .cSocket.BeginReceive(.buffer, 0, .buffer.Length, SocketFlags.None, New AsyncCallback(AddressOf onDataArrival), objPolicyConnection)

                    Return

                End If

                If StringComparer.InvariantCultureIgnoreCase.Compare(strData, POLICYREQUEST) <> 0 Then

                    ' if not matching with Policy Request, then exit

                    .cSocket.Close()

                    Return

                End If

                'Sending Policy File

                .cSocket.BeginSendFile(policyFile, New AsyncCallback(AddressOf onSend), objPolicyConnection)

            End With

        Catch ex As Exception

            objPolicyConnection.cSocket.Close()

        End Try

    End Sub 

    Private Sub onSend(ByVal asyn As IAsyncResult)

        Dim objPolicyConnection As PolicyConnection = CType(asyn.AsyncState, PolicyConnection)

        ' Finish File Send & Close Connection since no further communication needed

        Try

            objPolicyConnection.cSocket.EndSendFile(asyn)

        Catch ex As Exception

        Finally

            objPolicyConnection.cSocket.Close()

        End Try

    End Sub

End Class

Public Class PolicyConnection

    Public cSocket As Socket

    Private Const POLICYREQUEST As String ="<policy-file-request/>"

    Public buffer(POLICYREQUEST.Length) As Byte

    Public receivedText As New StringBuilder

End Class

This Policy Server Service class can be called from our server application or can be a standalone PolicyServer application by just creating object.
Private objPolicyServer As PolicyServerService
Private Sub StartPolicyServer()
 objPolicyServer = New PolicyServerService()
End Sub

Download

The source code for the above can be downloaded from here

Conclusion
Thus we can have a stand-alone policy server or integerated policy server created in VB.NET.

Happy Silverlight Sockets Programming!