Get the Default Gateway

Question

I'm writing a program that shows the user their IP address, Subnet mask and Default gateway. I can get the first two, but for the last one, this is what I turned up:

GatewayIPAddressInformationCollection gwc = 
    System.Net.NetworkInformation.NetworkInterface.GetAllNetworkInterfaces()[0].GetIPProperties().GatewayAddresses;

That, of course, returns a collection of GatewayIPAddressInformation. So, if a computer has multiple gateways, how can I determine which is the default gateway?

In practice, I've only ever seen this collection contain a single entry, but since it's implemented as a collection, it stands to reason that some computers contain multiple gateways, none of which are marked as "Default". So is there a way to determine the default or is it all just guesswork?

Answer 1

It should be the first valid and enabled gateway address of the first enabled network interface:

public static IPAddress GetDefaultGateway()
{
    return NetworkInterface
        .GetAllNetworkInterfaces()
        .Where(n => n.OperationalStatus == OperationalStatus.Up)
        .Where(n => n.NetworkInterfaceType != NetworkInterfaceType.Loopback)
        .SelectMany(n => n.GetIPProperties()?.GatewayAddresses)
        .Select(g => g?.Address)
        .Where(a => a != null)
         // .Where(a => a.AddressFamily == AddressFamily.InterNetwork)
         // .Where(a => Array.FindIndex(a.GetAddressBytes(), b => b != 0) >= 0)
        .FirstOrDefault();
}

I've also added some further commented checks which have been pointed out as useful by other people here. You can check the AddressFamily one to distinguish between IPv4 and IPv6. The latter one can be used to filter out 0.0.0.0 addresses.

That said, the recommended way to do this uses GetBestInterface to find an interface for routing to a specific IP address. If you have a destination IP address in mind already, then it's better to use this - and so I've also included an example of that below:

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

public static IPAddress GetGatewayForDestination(IPAddress destinationAddress)
{
    UInt32 destaddr = BitConverter.ToUInt32(destinationAddress.GetAddressBytes(), 0);

    uint interfaceIndex;
    int result = GetBestInterface(destaddr, out interfaceIndex);
    if (result != 0)
        throw new Win32Exception(result);

    foreach (var ni in NetworkInterface.GetAllNetworkInterfaces())
    {
        var niprops = ni.GetIPProperties();
        if (niprops == null)
            continue;

        var gateway = niprops.GatewayAddresses?.FirstOrDefault()?.Address;
        if (gateway == null)
            continue;

        if (ni.Supports(NetworkInterfaceComponent.IPv4))
        {
            var v4props = niprops.GetIPv4Properties();
            if (v4props == null)
                continue;

            if (v4props.Index == interfaceIndex)
                return gateway;
        }

        if (ni.Supports(NetworkInterfaceComponent.IPv6))
        {
            var v6props = niprops.GetIPv6Properties();
            if (v6props == null)
                continue;

            if (v6props.Index == interfaceIndex)
                return gateway;
        }
    }

    return null;
}