mjhjmtu

I have eth0 and wlan0 according to ifconfig and I can ping google.com. How can I find out (with a normal user, not root) what interface is active, as in, what interface did the ping (or whatever, ping is not mandatory) use?

NOTE: using Ubuntu 11.04 or Fedora 14

You can use route to find your default route:

$ route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.1.0 * 255.255.255.0 U 1 0 0 eth0
link-local * 255.255.0.0 U 1000 0 0 eth0
default 192.168.1.1 0.0.0.0 UG 0 0 0 eth0

The Iface column in the line with destination default tells you which interface is used.

My version which is basically based on this and this:

route | grep '^default' | grep -o '[^ ]*$'

And this, experimentally, for macOS:

route -n get default | grep 'interface:' | grep -o '[^ ]*$'

Running ifconfig will give you the information you need.

The active interface will have an inet addr and will show a record of transmitted data, like so:

RX bytes:1930741 (1.8 Mb) TX bytes:204768 (199.9 Kb)

You can also use the ip addr command and any inactive interfaces will be designated as having: NO-CARRIER.

Get the default network interface typically used to route to the "remaining" internet in opposite to DMZ, private network, VM host etc. which are usually routed explicitly.

$ ip -4 route ls | grep default | grep -Po '(?<=dev )(S+)'
eth0

On GNU/Linux systems:

#!/bin/sh

# host we want to "reach"
host=google.com

# get the ip of that host (works with dns and /etc/hosts. In case we get 
# multiple IP addresses, we just want one of them
host_ip=$(getent ahosts "$host" | awk 'print $1; exit')

# only list the interface used to reach a specific host/IP. We only want the part
# between dev and src (use grep for that)
ip route get "$host_ip" | grep -Po '(?<=(dev )).*(?= src| proto)'