Specifying what network interface an UDP multicast should go to in .NET

c# .net sockets udp multicast
Markus Roth picture Markus Roth · Feb 3, 2010 · Viewed 14.9k times · Source

On a computer with both an active Wireless Card and a LAN-Port with a crossover cable hooked up to another machine running the same application, we need to send a UDP multicast over the LAN wire to the other computer. Using C# Sockets, Windows seems to try to route the message over the WLAN adapter every time.

Is there a way to specify what network interface to send a UDP multicast on?

Answer

Yury Schkatula picture Yury Schkatula · Jan 30, 2013

Just as addendum to Nikolai answer: the problem with KB318911 is a dirty trick that user must provide necessary adapter index. While looking how to retrieve this adapter index I figured out such recipe:

NetworkInterface[] nics = NetworkInterface.GetAllNetworkInterfaces();
foreach (NetworkInterface adapter in nics)
{
  IPInterfaceProperties ip_properties = adapter.GetIPProperties();
  if (!adapter.GetIPProperties().MulticastAddresses.Any())
    continue; // most of VPN adapters will be skipped
  if (!adapter.SupportsMulticast)
    continue; // multicast is meaningless for this type of connection
  if (OperationalStatus.Up != adapter.OperationalStatus)
    continue; // this adapter is off or not connected
  IPv4InterfaceProperties p = adapter.GetIPProperties().GetIPv4Properties();
  if (null == p)
    continue; // IPv4 is not configured on this adapter

  // now we have adapter index as p.Index, let put it to socket option
  my_sock.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.MulticastInterface, (int)IPAddress.HostToNetworkOrder(p.Index));
}

Full note at http://windowsasusual.blogspot.ru/2013/01/socket-option-multicast-interface.html

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