Consider the following setup: A windows PC with a LAN interface and a WiFi interface (the standard for any new laptop). Each of the interfaces might be connected or disconnected from a network. I need a way to determine which one of the adapters is the one connected to the internet – specifically, in case they are both connected to different networks, one with connection to the internet and one without.
My current solution involves using IPHelper’s ‘GetBestInterface‘ function and supplying it with the IP address ‘0.0.0.0’.
Do you have any other solutions you might suggest to this problem?
Following some of the answers, let me elaborate:
- I need this because I have a product that has to choose which adapter to bind to. I have no way of controlling the setup of the network or the host where the product will run and so I need a solution that is as robust as possible, with as few assumptions as possible.
- I need to do this in code, since this is part of a product.
@Chris Upchurch: This makes me dependent on google.com being up (usually not a problem) and on any personal firewall that might be installed to allow pinging.
@Till: Like Steve Moon said, relying on the adapter’s address is kind of risky because you make a lot of assumptions on the internal network setup.
@Steve Moon: Looking at the routing table sounds like a good idea, but instead of applying the routing logic myself, I am trying to use ‘GetBestInterface’ as described above. I believe what it should do is exactly what you outlined in your answer, but I am not really sure. The reason I’m reluctant to implement my own ‘routing logic’ is that there’s a better chance that I’ll get it wrong than if I use a library/API written and tested by more ‘hard-core’ network people.
Technically, there is no ‘connected to the Internet’. The real question is, which interface is routeable to a desired address. Right now, you’re querying for the ‘default route’ – the one that applies if no specific route to destination exists. But, you’re ignoring any specific routes.
Fortunately, for 99.9% of home users, that’ll do the trick. They’re not likely to have much of a routing table, and GetBestInterface will automatically prefer wired over wireless – so you should be good. Throw in an override option for the .1% of cases you screw up, and call it a day.
But, for corporate use, you should be using GetBestInterface for a specific destination – otherwise, you’ll have issues if someone is on the same LAN as your destination (which means you should take the ‘internal’ interface, not the ‘external’) or has a specific route to your destination (my internal network could peer with your destination’s network, for instance).
Then again, I’m not sure what you plan to do with this adapter ‘connected to the Internet’, so it might not be a big deal.