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?

Accepted Answer

It will probably 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.

The above solution will give you a valid/connected interface, and is good enough for 99% of situations. That said, if you have multiple valid interfaces that traffic can be routed through, and you need to be 100% accurate, the way to do this uses `GetBestInterface` to find an interface for routing to a specific IP address. This additionally handles the case where you might have a specific address range routed through a different adapter (e.g. `10.*.*.*` going through a VPN, everything else going to your router)

```
[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;
}
```

These two examples could be wrapped up into a helper class and used in the appropriate cases: that you do, or do not have a destination address in mind already.

Get the Default Gateway

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