Managed alternative for GetBestInterface?

Question

I have absolutely no background on programming in C (or any other unmanaged languages for that matter) yet I would like to use the GetBestInterface function from the IP Helper API in my .NET application. I tried to understand how P/invoke calls can be used to make calls to native methods, but there are many things that just don't make sense to me (like how types in managed code map to unmanaged types).

Is there an alternative function somewhere hidden in the System.Net namespace that does roughly the same thing? Or could I write my own alternative using existing base classes combined with some magic? Because that's basically what it seems to me: magic. There is no real explanation for how the method accomplishes what it does as far as I can tell...

EDIT

I just discovered the LocalEndPoint property on the System.Net.Sockets.Socket class which I think could be quite useful in this. To my understanding it will do the best-interface-picking on my behalf and I'll just need to get the NIC that corresponds to the local endpoint.

http://pinvoke.net/default.aspx/iphlpapi/GetBestInterface.html — Hans Passant, Apr 28 '12 at 00:20
@HansPassant the index that it returns is useless without also implementing GetAdaptersInfo, which is even more beyond my abilities. — Steven Liekens, Apr 28 '12 at 01:14

score 3 · Accepted Answer · answered Jun 06 '19 at 20:32

I also required GetBestInterface since I needed a solution which works if the remote address to probe against is not reachable (something Steven's solution would have problems with), and to be 100% compatible with a native application I was porting to C#.

Luckily, you don't have to port GetAdaptersInfo and all the corresponding IP_ADAPTER_INFO and sub-struct clutter to find which network interface has the index returned by GetBestInterface, since .NET can retrieve that index from interfaces too already.

So, first port the GetBestInterface method as follows:

[DllImport("iphlpapi")]
private static extern int GetBestInterface(uint dwDestAddr, ref uint pdwBestIfIndex);

Then, write a wrapper function as follows:

private static NetworkInterface GetBestNetworkInterface(IPAddress remoteAddress)
{
    // Get the index of the interface with the best route to the remote address.
    uint dwDestAddr = BitConverter.ToUInt32(remoteAddress.GetAddressBytes());
    uint dwBestIfIndex = 0;
    uint result = GetBestInterface(dwDestAddr, ref dwBestIfIndex);
    if (result != 0)
        throw new NetworkInformationException((int)result);

    // Find a matching .NET interface object with the given index.
    foreach (NetworkInterface networkInterface in NetworkInterface.GetAllNetworkInterfaces())
        if (networkInterface.GetIPProperties().GetIPv4Properties().Index == dwBestIfIndex)
            return networkInterface;

    throw new InvalidOperationException($"Could not find best interface for {remoteAddress}.");
}

You can then simply call the method like this - and don't forget unreachable hosts now also work:

NetworkInterface bestNetworkInterface = GetBestNetworkInterface(IPAddress.Parse("8.8.8.8"));

score 1 · Answer 2 · answered Apr 28 '12 at 10:57

Alrighty I've put something together myself that works by piggybacking on the framework's ability to do the interface-picking on its own. Essentially it just connects a socket, then uses the LocalEndPoint property on the socket to get the NIC that it's using. Probably not the greatest way to do it, but it works for me.

Using the following import statements:

Imports System.Net
Imports System.Net.NetworkInformation
Imports System.Net.Sockets

And my super awesome method:

Private Function GetBestInterfaceManaged(ByVal address As IPAddress, ByVal port As Integer) As NetworkInterface

    Dim socket As Socket = New Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.IP)
    Dim outgoingInterface As NetworkInterface = Nothing

    Try
        socket.Connect(New IPEndPoint(address, port))

        If socket.Connected Then ' find the outgoing NIC

            Dim interfaces As List(Of NetworkInterface) = NetworkInterface.GetAllNetworkInterfaces.ToList()

            For Each nic As NetworkInterface In interfaces
                Dim properties As IPInterfaceProperties = nic.GetIPProperties

                For Each unicastAddress In properties.UnicastAddresses
                    If unicastAddress.Address.Equals(DirectCast(socket.LocalEndPoint, IPEndPoint).Address) Then
                        outgoingInterface = nic
                    End If
                Next
            Next

            If outgoingInterface Is Nothing Then
                Console.WriteLine("Darn... it didn't work!")
            Else
                Console.WriteLine("Outgoing interface: {0}", outgoingInterface.Name)
            End If

            Return outgoingInterface
        End If
    Catch ex As SocketException
        Console.WriteLine(ex.Message)
    End Try
    Return Nothing
End Function

This will do the trick:

Sub Main()
    Dim hostEntry As IPHostEntry = Dns.GetHostEntry("www.stackoverflow.com")
    Dim NIC As NetworkInterface = GetBestInterfaceManaged(hostEntry.AddressList.First, 80)
    ' Other code goes down here
End Sub

Managed alternative for GetBestInterface?

2 Answers2