C# UDP broadcast client/server does not work

Question

I'm using .NET 2.0 and have created a fairly simple udp broadcast app and UDP listener.

Listener code:

Socket listener = new Socket( AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp );
IPEndPoint localEndPoint = new IPEndPoint( IPAddress.Any, 11000 );
listener.Bind( localEndPoint );
EndPoint ep = (EndPoint)localEndPoint;
Console.WriteLine("Ready to receive…");
byte[] data = new byte[1024];
int recv = listener.ReceiveFrom(data, ref ep);
string stringData = Encoding.ASCII.GetString(data, 0, recv);
Console.WriteLine("received: {0} from: {1}", stringData, ep.ToString());
listener.Close();

Server code:

int groupPort = 11000;
IPEndPoint groupEP = new IPEndPoint( IPAddress.Parse( "255.255.255.255" ), groupPort );

if ( radioButton2.Checked )
{
    groupEP = new IPEndPoint( IPAddress.Broadcast, groupPort );
}
    else if ( radioButton3.Checked )
{
    groupEP = new IPEndPoint( IPAddress.Parse( "172.16.75.15" ), groupPort );
}

Socket socket = new Socket( AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp );
socket.SetSocketOption( SocketOptionLevel.Socket, SocketOptionName.Broadcast, 1 );
socket.SendTo( System.Text.ASCIIEncoding.ASCII.GetBytes( testTextBox.Text ), groupEP );

The server is just a simple windows app with 3 radio buttons, button and a textbox.

When I run the server on a separate computer and choose radioButton3 I receive the message just fine on my client listener (which is running on ip address 172.16.75.15). However if I choose the first or second radiobutton (which creates Broadcast or 255.255.255.255 as the ip address) I get nothing. Now if I run the client on the same pc as the server I can receive the message using those two choices.

I'm not sure what I'm doing wrong or if there could be some kind of firewall preventing UDP messages on the LAN. Any ideas?

Thanks,

Craig

Answer 1

Try a broadcast on the local subnet only. IE if your subnet is 255.255.255.0 try a broadcast of 172.16.75.255. It may be that windows, a router or even network card automatically block universal broadcasts as a preventative measure.

Answer 2

Just a note, but if you are writing a new app from the ground up then you should really be using multicast instead of broadcast.

Answer 3

Is the client on the same physical network as the server? If not, you won't be able to do a local broadcast (255.255.255.255) and will need to do a directed subnet broadcast. You will have to enable your router to allow a directed subnet broadcast (172.16.75.255) before that will work.

Answer 4

I just tried your code and worked for me. The only thing i changed was the IPAddress to match my local IP Address for the server.

Answer 5

Craig, I develop a C# udp client/server tool, using broadcasts, it works ( datagrams are received by all the clients on the same subnet ). Just avoid 255.255.255.255, windows cannot always take the right route. You have to help windows specifying the right subnet. Server : bind the udp socket to 172.16.75.15, and sendto the local broadcast address ( are you sure about 172.16.75.255 ? check your subnet mask ).

Answer 6

I had the similar problem, nothing seemed to work in all the code I saw here. when I started my program there was this firewall window that pops up telling you the firewall has blocked some features.

my problem was I was clicking allow access without ticking the check box that says "Private networks, suck as my home or work network". you can change this later of course in your firewall setting and tick that check box. both de server and the other machine must have that check box checked. Or at least that's what makes my mine work.

Also i had to change my broadcast IP address to for example 192.168.1.255. My router does block the recommended by my book 224.0.0.0 - 239.255.255.255;