How to answer vb.net TCP client using netcat?

Question

Actually I create an connection between windows-clients and a linux server using vb.net for the client and netcat for the server. The problem is that I cannot receive anything using client_receive(), because I cannot send back data from the server to the client inside the open netcat socket connection. Also I am looking for a way to not directly respond to an client - rather time-independent. The connection needs to keep open and the server can say anything to an connected client using e.g. echo '$message' > /dev/tcp/client.ip/4000 in combination with screen. The important thing is that it needs to be the same connection, no new.

With the following code I am able to send strings to an port netcat is listening to:

 Dim stream As NetworkStream
    Dim streamw As StreamWriter
    Dim streamr As StreamReader
    Dim myclient As New TcpClient
    Dim IP As String = "example.com"
    Dim Port As Integer = 4000

Public Sub client_send(ByVal text As String)
        streamw.WriteLine(text)
        streamw.Flush()
    End Sub
    Public Function client_receive() As String
        client_receive = streamr.ReadLine
    End Function



Private Sub Welcome_Load(sender As Object, e As EventArgs) Handles MyBase.Load

        Try
            myclient.Connect(IP, Port)
            stream = myclient.GetStream
            streamw = New StreamWriter(stream)
            streamr = New StreamReader(stream)

        Catch
        End Try

End Sub

Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles iamonline.Tick
        client_send("ItzMe")
        MessageBox.Show(client_receive)
    End Sub

We send ItzMe every 5 seconds to the port 4000/tcp. In the netcat screen we see:

ItzMe
ItzMe
...

Update #1: Changed Timer1

Update #2: Detailed the question

score 2 · Accepted Answer · 2017-07-10 17:44:25Z

2

Try this:

while true ; do nc -l -p 4000 -c 'echo -e "message received"'; done

Using socat, you can create a simple echo server:

socat -v tcp-l:4000,fork exec:'/bin/cat'

Alternatively if you do not have the -c parameter:

mkfifo fifo
cat fifo  | nc -k -l 4000 -v | cat  > fifo

edited Jul 10, 2017 at 17:44

answered Jul 10, 2017 at 15:00

user8201141

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6 Comments

user8201141 Over a year ago

while true; do echo -e "ur welcome!\n" | nc -k -l -p 4000 done. Sorry I forgot to add the keepalive earlier.

realKSMC Over a year ago

Tried while true; do echo -e "ur welcome!\n" | nc -k -l -p 4000; done. Textbox appears two times, but the second one is empty and finally GUI freezes and netcat session get a total of 3 times ItzMe.

realKSMC Over a year ago

Also I am looking for a way to not directly respond to an client - rather time-independent. The connection needs to keep open and the server can say anything to the client using e.g. screen. Hope you understand :)

realKSMC Over a year ago

- updated the question to explain more what the goal is.

user8201141 Over a year ago

I have incorporated a echo server solution using socat or nc.

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