Identifying active network interface

Question

In a .NET application, how can I identify which network interface is used to communicate to a given IP address?

I am running on workstations with multiple network interfaces, IPv4 and v6, and I need to get the address of the "correct" interface used for traffic to my given database server.

Template

Answer 1

The simplest way would be:

UdpClient u = new UdpClient(remoteAddress, 1);
IPAddress localAddr = ((IPEndPoint)u.Client.LocalEndPoint).Address;

Now, if you want the NetworkInterface object you do something like:

foreach (NetworkInterface nic in NetworkInterface.GetAllNetworkInterfaces())
{
   IPInterfaceProperties ipProps = nic.GetIPProperties();
   // check if localAddr is in ipProps.UnicastAddresses
}

Another option is to use P/Invoke and call GetBestInterface() to get the interface index, then again loop over all the network interfaces. As before, you'll have to dig through GetIPProperties() to get to the IPv4InterfaceProperties.Index property).

Langdon

after some research I'm finding that the "best" answer means: 1. The first IP of the adapter with the matching route/default gateway combo with the lowest metric. 2. In case of a metric tie, the adapter with the lowest binding order wins. – Raul

Answer 2

Neither of these will actually give the OP the info he's looking for -- he wants to know which interface will be used to reach a given destination. One way of doing what you want would be to shell out to the route command using System.Diagnostics.Process class, then screen-scrape the output. route PRINT (destination IP) will get you something useable. That's probably not the best solution, but it's the only one I can give you right now.

Implicatory

Answer 3

The info you are after will be in WMI.

This example using WMI may get you most of the way:

using System.Management;
string query = "SELECT * FROM Win32_NetworkAdapterConfiguration";
ManagementObjectSearcher moSearch = new ManagementObjectSearcher(query);
ManagementObjectCollection moCollection = moSearch.Get();// Every record in this collection is a network interface
foreach (ManagementObject mo in moCollection)
{    
    // Do what you need to here....
}

The Win32_NetworkAdapterConfiguration class will give you info about the configuration of your adapters e.g. ip addresses etc.

You can also query the Win32_NetworkAdapter class to find out 'static'about each adapter (max speed, manufacturer etc)

Lalittah

Answer 4

At least you can start with that, giving you all addresses from dns for the local machine.

IPHostEntry hostEntry = Dns.GetHostEntry(Environment.MachineName);

foreach (System.Net.IPAddress address in hostEntry.AddressList)
{
    Console.WriteLine(address);
}

Equitable

Answer 5

Just to give a complete picture: another approach would be to use Socket.IOControl( SIO_ROUTING_INTERFACE_QUERY, ... )

ConferenceXP includes rather comprehensive function wrapping this, works with IPv4/6 and multicast addresses: https://github.com/conferencexp/conferencexp/blob/master/MSR.LST.Net.Rtp/NetworkingBasics/utility.cs#L84

Rev

full code example for querying the routing table by Socket.IOControl can be found here – Austinaustina

Answer 6

this a old thread but today I'm looking for this, but find a better answer, I think.

/// <summary>
/// Get ifIndex of ip (ip address to connect)
/// </summary>
/// <param name="ip">IP Address that want to connect</param>
/// <param name="description">Lan card description</param>
/// <param name="ifIndex">interface index</param>
/// <param name="ifAddress">IP Address of Lan Interface</param>
private static void CheckInterface(string ip, out string description, out int ifIndex, out IPAddress ifAddress)
{
    description = "";
    ifIndex = -1;
    ifAddress = IPAddress.Any;
    var myIp = IPAddress.Parse(ip);
    if (!NetworkInterface.GetIsNetworkAvailable()) return;
    foreach (var adapter in NetworkInterface.GetAllNetworkInterfaces())
    {
        if (!adapter.SupportsMulticast) continue;
        if (adapter.OperationalStatus != OperationalStatus.Up) continue;
        var properties = adapter.GetIPProperties();
        if (!properties.MulticastAddresses.Any()) continue;
        var p = properties.GetIPv4Properties();
        if (p == null) continue;
        foreach (var ipInfo in properties.UnicastAddresses)
        {
            if (ipInfo.Address.AddressFamily == AddressFamily.InterNetwork)
            {
                //Console.WriteLine("Address:{0} == {1}", ipInfo.Address, myIp);
                var adpIp = ipInfo.Address.GetAddressBytes();
                var cmpIp = myIp.GetAddressBytes();
                if ((adpIp[0] == cmpIp[0]) && (adpIp[1] == cmpIp[1]) && (adpIp[2] == cmpIp[2]))
                {
                    description = adapter.Description;
                    ifIndex = p.Index;
                    ifAddress = ipInfo.Address;
                    return;
                    //Console.WriteLine("Name {0} ifIndex {1}", adapter.Description, p.Index);
                }
            }
        }
    }
}

Today, works for me! (when ifIndex is -1, not found an interface)

Bias