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

12
votes

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?

4
c#.netsocketsudpmulticast
did you find a solution? I have the same Problem. Did the article on MSDN solve your Problem? – Gobliins
@Gobliins Didn't worked for me too :/ Any solution? – J4N
@J4N i did use the udpclient class (but its also possible with socket) to join the multicast address on a specific adapter – Gobliins

4 Answers

16
votes

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

4
votes

You are probably looking for SocketOptionName.MulticastInterface. Here's an article on MSDN that might help you.

Other then that if you update your local routing table to have an exact entry matching the multicast address and pointing to the right interface it should just work.

3
votes

Depending on what you're doing, there's a Win32 method that might help. It'll return the best interface for a given IP address. To get the default one (the 0.0.0.0), which is usually what you want for multicast, it's pretty easy:

P/Invoke signature:

[DllImport("iphlpapi.dll", CharSet = CharSet.Auto)]
private static extern int GetBestInterface(UInt32 DestAddr, out UInt32 BestIfIndex);

Then somewhere else:

// There could be multiple adapters, get the default one
uint index = 0;
GetBestInterface(0, out index);
var ifaceIndex = (int)index;

var client = new UdpClient();
client.Client.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.MulticastInterface, (int)IPAddress.HostToNetworkOrder(ifaceIndex));

var localEndpoint = new IPEndPoint(IPAddress.Any, <port>);
client.Client.Bind(localEndpoint);

var multicastAddress = IPAddress.Parse("<group IP>");
var multOpt = new MulticastOption(multicastAddress, ifaceIndex);
client.Client.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.AddMembership, multOpt);

var broadcastEndpoint = new IPEndPoint(IPAddress.Parse("<group IP>"), <port>);
byte[] buffer = ...
await client.SendAsync(buffer, buffer.Length, broadcastEp).ConfigureAwait(false);
0
votes

If you are using UDPClient class, then this method forces the IGMP message out of the interface you desire (second parameter), even when binding doesn't work.

receiveUDPClient.JoinMulticastGroup(IPAddress.Parse("239.254.2.1"), IPAddress.Parse("192.168.6.1"));

Otherwise, MulticastOption(IPAddress, IPAddress) will work. The first parameter is the multicast address, the second address forces a localendpoint you specify to be used.