Linux – What does the eth0 interface name mean in Linux

How can I test if an Internet connection is available via a certain interface? e.g. is my GSM(wwan0) modem connected to the Internet? If I attempt to run ping www.google.com - How do I know what interface is being used?

You could specify the interface.

ping -I $INTERFACE $DOMAIN_OR_IP_ADDRESS

e.g:

ping -I wwan0 8.8.4.4

Using a known IP address will eliminate any potential DNS issues while troubleshooting.

How can I force connectivity via a certain interface if more than one exist?

You can set the default route. Static addressing with Debian, right? Open /etc/network/interfaces with a text editor. Find the desired network interface and add the option gateway x.x.x.x, where x.x.x.x is the gateway. Save and restart networking:

/etc/init.d/networking restart

Otherwise, I believe one would need to use more advanced policy-based routing for this.

Solution:

auto lo
iface lo inet loopback

allow-hotplug eth0
iface eth0 inet dhcp

allow-hotplug wlan0
iface wlan0 inet dhcp
wpa-ssid "myssid"
wpa-psk "mypasswd"

The differences between auto and allow-hotplug are explained well in Good detailed explanation of /etc/network/interfaces syntax:

auto interface – Start the interface(s) at boot. That’s why the lo interface uses this kind of linking configuration.

allow-hotplug interface – Start the interface when a "hotplug" event is detected. In the real world, this is used in the same situations as auto but the difference is that it will wait for an event like "being detected by udev hotplug api" or "cable linked".