How can I tell if a network cable has been unplugged?

Question

I have a C# application that should only be used when the network is down, but am afraid users will just unplug the network cable in order to use it.

Is there a way to detect if the network cable has been unplugged?

Thanks

Answer 1

The network card will report this as a state. Tools like ethtool can display this (Link up), but that is only available for Linux/Unix.

If you can enumerate the installed network cards with a Windows API, I'm sure you'll find the flag for "link up" somewhere in there.

Answer 2

If I really wanted to use your application and whether it will work depends on something like this, I would always be able to find a way to trick your application. Are you sure there's no better solution?

Answer 3

In my humble opinion, there is no certain way to distinguish between a network down and an unplugged cable. And even if there is a way, there is also a way to work around it.

Let's assume that you have a solution and let's look at some situations:

• There is no network traffic, the cable is not unplugged from the computer: it may be unplugged at the other end.
• There is no network traffic, the cable is unplugged: but this has always been the case, the laptop is connected via Wi-Fi, which is down at the moment.
• There are several network interfaces, only the one connected to WAN is down: should your app work?
• The network is actually down, in the sense you mean: someone has managed to reboot the router continuously for using your app.

Answer 4

Some network drivers are able to detect this. However you'd need to use unmanaged code to access them from C# (which may be very difficult/impossible) and the solution may not be reliable for all network adapters.

Answer 5

You could use IsNetworkAlive(). Although technically it doesn't check link state, it's probably better since it can detect wireless and dialup connectivity as well. Here's an example:

using System.Runtime.InteropServices;
class Program
{
    [DllImport("sensapi.dll")]
    static extern bool IsNetworkAlive(out int flags);

    static void Main(string[] args)
    {
        int flags;
        bool connected = IsNetworkAlive(out flags);

    }
}

The flags param returns whether the connection is to the internet or just a LAN. I'm not 100% sure how it knows, but I'd bet it just looks to see if there is a default gateway set.

Answer 6

How about pinging the default gateway?

There is some code here that gets the default gateway from the registry.