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Reusing a local address/port when creating sockets that send data


I want to periodically send a TCP packet to a peer, always from the same
source port. That is, each packet will come from my local ip address, port
8801, and go to the peer's ip address, to HIS port 8801.

This means that I need to bind my sender socket to (myaddress,8801), use it
for the send, then shut it down and close it. Then I want to wait, say 30
seconds, and do it all over again. Naturally, the socket's ReuseAddress
property must be set for this to work.

However, in practice, the second time I bind a socket to my local endpoint,
I get a SocketException: "Only one usage of each socket address
(protocol/network address/port) is normally permitted."

Can anyone help me? The following code attempts to send a string containing
the date/time to a server on If I set the "bindIt"
variable to false, the code works fine. If I try to bind (and re-bind) the
socket to the same local endpoint (e.g, I've set bindIt to true), the code
fails with the above exception. Help!


Greg Hassett

using System;
using System.IO;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;

class TcpClientTest
static public void Main()
bool bindIt = true;

IPHostEntry localHostEntry = Dns.GetHostByName(Dns.GetHostName());
IPEndPoint localIPEndPoint = new IPEndPoint
(localHostEntry.AddressList[0], 8801);

while (true)
Console.WriteLine("Press return to send data");

Socket socket = new Socket(AddressFamily.InterNetwork,
SocketType.Stream, ProtocolType.Tcp);

socket.SetSocketOption (SocketOptionLevel.Socket,
SocketOptionName.ReuseAddress, 1);
socket.SetSocketOption (SocketOptionLevel.Socket,
SocketOptionName.Linger, new LingerOption(false, 0));
socket.SetSocketOption (SocketOptionLevel.Socket,
SocketOptionName.DontLinger, 1);

if (bindIt)
socket.Bind (new IPEndPoint (IPAddress.Any, 8801));

socket.Connect (new
IPEndPoint(IPAddress.Parse(""), 8801));

byte[] buf = Encoding.ASCII.GetBytes
socket.Send (buf);

socket.Shutdown (SocketShutdown.Both);
socket.Close ();

Console.Out.WriteLine ("Sent");
catch (Exception ex)
Console.Out.WriteLine (ex.ToString());

Nov 17 '05 #1
0 2701

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