How to check if a port is open on a particular IP address

Question

Hi all,

I have an application which need to send data to particular system on specific port. But the application gets hang if the target system port is closed.

I Want to know if a particular Port is open on a specific IP address so that i can continue my next task.

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PBL (Visual Studio 2010 Ultimate)

Answer 1

We need to see how you are opening the port.  When posting code make sure that the names are changed if needed.

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Serial Port      Random       Check Internet Connection      Microsoft® Community Contributor 2011

Answer 2

dbasnett,

Below is the class that i am using for TCP chat.

This i could able to get and using in my application from other post

http://social.msdn.microsoft.com/Forums/en-US/vbgeneral/thread/435cce94-3a5f-4635-bac3-8e1964422539

Public Class TCP_Messaging
    Public Event DataReceived(ByVal txt As String)
    Public Event Connection(ByVal cStatus As Boolean)
    Public Event sendOK(ByVal sStatus As Boolean)
    Public Event recOK(ByVal sReceived As Boolean)

    Private serverRuns As Boolean
    Private server As TcpListener
    Private sc As SynchronizationContext
    Private isConnected, receiveStatus, sendStatus As Boolean

    Public Sub connect(ByVal thisaddress As String, ByVal thisport As Integer)

        sc = SynchronizationContext.Current

        Try

            server = New TcpListener(IPAddress.Parse(thisaddress), thisport)
            'MsgBox("connected...")


        Catch ex As Exception

            MsgBox("server create: " & ex.Message, MsgBoxStyle.Exclamation)

        End Try
        Try
            With server
                .Start()
                .BeginAcceptTcpClient(New AsyncCallback(AddressOf DoAccept), server)
                isConnected = True
            End With
        Catch ex As Exception
            MsgBox("server listen: " & ex.Message, MsgBoxStyle.Exclamation)
            isConnected = False
        Finally
            RaiseEvent Connection(isConnected)
        End Try
    End Sub

    Public Sub disconnect()

        Try

            isConnected = False
            server.Stop()

        Catch ex As Exception

            MsgBox("disConnect server: " & ex.Message, MsgBoxStyle.Exclamation)
            isConnected = True

        Finally

            RaiseEvent Connection(isConnected)

        End Try


    End Sub

    Public Sub SendData(ByVal txt As String, ByVal targetAddress As String, ByVal targetPort As Integer)

        Dim ClientSocket = New TcpClient
        ClientSocket.SendTimeout = 20
        With ClientSocket
            Try
                .Connect(IPAddress.Parse(targetAddress), targetPort)
                Dim data() As Byte = Encoding.ASCII.GetBytes(txt)
                .NoDelay = True
                .GetStream.Write(data, 0, data.Length)
                .GetStream.Close()

                .Close()
                sendStatus = True

            Catch ex As Exception
                
                'MsgBox("sendData: " & ex.Message, MsgBoxStyle.Exclamation)
                

                sendStatus = False

            Finally

                RaiseEvent sendOK(sendStatus)

            End Try

        End With

    End Sub
    Private Sub DoAccept(ByVal ar As IAsyncResult)

        Dim sb As New StringBuilder
        Dim Buff() As Byte
        Dim DataLen As Integer

        Dim Listner As TcpListener
        Dim ClientSocket As TcpClient

        If Not isConnected Then Exit Sub

        Try
            Listner = CType(ar.AsyncState, TcpListener)
            ClientSocket = Listner.EndAcceptTcpClient(ar)
            ClientSocket.ReceiveTimeout = 50

        Catch ex As ObjectDisposedException
            MsgBox("DoAccept ObjectDisposedException " & ex.Message, MsgBoxStyle.Exclamation)
            Exit Sub
        End Try

        Try

            With ClientSocket

                DataLen = 0

                While DataLen = 0

                    DataLen = .Available

                End While

                Buff = New Byte(DataLen - 1) {}

                .GetStream.Read(Buff, 0, Buff.Length)
                sb.Append(Encoding.ASCII.GetString(Buff, 0, Buff.Length))
                .Close()

            End With

            receiveStatus = True

        Catch ex As TimeoutException    'Time out exception

            MsgBox("doAcceptData timeout: " & ex.Message, MsgBoxStyle.Exclamation)
            receiveStatus = False
            ClientSocket.Close()
            Exit Sub

        Catch ex As Exception

            MsgBox("doAcceptData: " & ex.Message, MsgBoxStyle.Exclamation)
            receiveStatus = False
            ClientSocket.Close()
            Exit Sub

        Finally

            RaiseEvent recOK(receiveStatus)

        End Try
        ' post data
        sc.Post(New SendOrPostCallback(AddressOf OnDataReceived), sb.ToString)
        ' start new read
        server.BeginAcceptTcpClient(New AsyncCallback(AddressOf DoAccept), server)

    End Sub

    Public Sub OnDataReceived(ByVal state As Object)

        RaiseEvent DataReceived(state.ToString)

    End Sub

    Public Sub receiveData(ByVal txt As String) Handles Me.DataReceived

'Frm_MessagePopUp is the form to display the received message in Label : lbl_msg

        If Frm_MessagePopUp.Visible Then
            Frm_MessagePopUp.Dispose()
        End If

        Frm_MessagePopUp.lbl_msg.Text = txt

        Frm_MessagePopUp.Show()

    End Sub

End Class

In the form load event i am using below code

Dim WithEvents myChat As New TCP_Messaging

Private Sub MDI_ShowTime_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        InitilizeMessaging()    'Initilize the Messaging service by opening the ports for socket connection over LAN

    End Sub


Private Sub InitilizeMessaging()

        strHostName = Dns.GetHostName       'Retriving the current system name

        Dim ipEntry As IPHostEntry = Dns.GetHostEntry(strHostName)

        IPaddr = ipEntry.AddressList

        Dim i As Integer

        For i = 0 To IPaddr.Length - 1

            If IPaddr(i).AddressFamily = AddressFamily.InterNetwork Then

                Exit For

            End If
        Next

        myChat.connect(IPaddr(i).ToString, 1863)

    End Sub

I could able to send and receive the message without any hang  if the application is running on both the ends (system1 and system2). The problem arises when the application is closed at system1 while system2 is trying to send the message to system1. it hangs for nearly 18sec, which was explained in below link

http://support.microsoft.com/kb/200770/en-us?fr=1

So i was trying to check if the port (for ex 1863 which i used in code) on receiving system is open, so that i can continue sending message else skip that process and continue other process in the code.

if i am not clear let me know, i will try to explain where you have any doubt.

All your support is most valuable.

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PBL (Visual Studio 2010 Ultimate)

Answer 3

It seems that you are opening and closing the connection before every send.  Why not open the connection and pass messages as needed.  Both ends should be able to terminate the connection.

 

I'll try to look at this in more detail, but that was what jumped out at me.

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Serial Port      Random       Check Internet Connection      Microsoft® Community Contributor 2011

Answer 4

It seems that you are opening and closing the connection before every send.  Why not open the connection and pass messages as needed.  Both ends should be able to terminate the connection.

 

I'll try to look at this in more detail, but that was what jumped out at me.

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Serial Port      Random       Check Internet Connection      Microsoft® Community Contributor 2011

dbasnett,

I am not opening and closing the connection before every send.

You can see at my form_load when InitilizeMesaging is called it opens a connection on the system where the application is launched. with which its in open mode to receive the messages. when we need to send the message to other system, it connects to that system and same port and sends messages. this is done at below code...

 Public Sub SendData(ByVal txt As String, ByVal targetAddress As String, ByVal targetPort As Integer)

        Dim ClientSocket = New TcpClient
        ClientSocket.SendTimeout = 20
        With ClientSocket
            Try
                .Connect(IPAddress.Parse(targetAddress), targetPort)
                Dim data() As Byte = Encoding.ASCII.GetBytes(txt)
                .NoDelay = True
                .GetStream.Write(data, 0, data.Length)
                .GetStream.Close()

                .Close()
                sendStatus = True

            Catch ex As Exception
                
                'MsgBox("sendData: " & ex.Message, MsgBoxStyle.Exclamation)
                

                sendStatus = False

            Finally

                RaiseEvent sendOK(sendStatus)

            End Try

        End With

    End Sub

if the target system is not running the application then the systems will try to connect but fails @ below line of statment

.Connect(IPAddress.Parse(targetAddress), targetPort)

 

Hope i you understood how the connection is made...

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PBL (Visual Studio 2010 Ultimate)

Answer 5

It sure looks to me like you are creating a new socket, connecting, sending, and closing every time.  Maybe I should not have said open...

@AP - not me, and certainly not for that. 

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Serial Port      Random       Check Internet Connection      Microsoft® Community Contributor 2011

Answer 6

Form with a button and two textboxes (IP_HostTextBox and PortTextBox)

 

 

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim ipa As System.Net.IPAddress = Nothing
        Dim portno As Integer = CInt(PortTextBox.Text)
        If System.Net.IPAddress.TryParse(IP_HostTextBox.Text, ipa) Then
            ipa = System.Net.IPAddress.Parse(IP_HostTextBox.Text)
        Else
            ipa = System.Net.Dns.GetHostAddresses(IP_HostTextBox.Text)(0)
        End If
        Try
            Dim sock As System.Net.Sockets.Socket = New System.Net.Sockets.Socket(System.Net.Sockets.AddressFamily.InterNetwork, System.Net.Sockets.SocketType.Stream, System.Net.Sockets.ProtocolType.Tcp)
            sock.Connect(ipa, portno)
            If sock.Connected = True Then
                'Port is in use and connection is successful
                MessageBox.Show("Port is Open")
                sock.Close()
            End If
        Catch ex As Exception
            MessageBox.Show("Port is Closed")
        End Try
    End Sub

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        IP_HostTextBox.Text = "ftp-linux.cc.gatech.edu" ' used as a test, known reliable host
         PortTextBox.Text = "21" ' Standard FTP port for above host
     End Sub
End Class

You can put an IP address in that TextBox also. No real error checking is implemented.

Answer 7

Devon,

I checked your suggestion and its working fine.

Can u suggest any other alternate for my process using sockets instead mine.

If so i will open an other thread for this.

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PBL (Visual Studio 2010 Ultimate)

Answer 8

Nothing that I'd call very reliable. I sometimes use plain old telnet

telnet hostname port#

then if it connects at all I try a few keys, like

HELO

LIST

HELP

QUIT

and see if whatever is on the other end sends anything back. Sometimes telnet connects, the server sends back a response that clears the screen, then nothing happens for a long time until the server has had enough and disconnects. Not very useful.

 

Answer 9

Because Telnet is a protocol and you are not responding properly to the messages required for the correct connection.

RFC 854

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Serial Port      Random       Check Internet Connection      Microsoft® Community Contributor 2011

Answer 10

If you get this:

C:\>telnet Z.Z.Z.Z PPPP
Connecting To Z.Z.Z.Z...Could not open connection to the host, on port pppp: Connect failed

It's likely that there is nothing listening at Z.Z.Z.Z on port pppp

If you get a response of any kind, text or screen clear etc., there is something there and the port is open.

Answer 11

Devon,

I tried implementing ur suggestion in my chat . I am uploading my chat program in the below link.

https://skydrive.live.com/#cid=58C45CD8088B910B&id=58C45CD8088B910B%21129

when i launch the application and press connect button with the default values(Source & target are the same system). it conects to the target IP and port

if i send message by hitting enter at the message text box (tbin). i could able to send the message

in the message window it shows out message with -----> and incoming message with <----

But i am not able to send message after i click the button Chk Port (where i applied your code to check, it reports correctly if the tatget port is open/close). Can u check the code and update where i am going wrong (is ur socket.close is closing the port on target machine...?).

 

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PBL (Visual Studio 2010 Ultimate)