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How do I find the network interface that's connected to the Internet?

This machine could typically be connected via {eth0,eth1,usb0,wlan0}. The best I could think of is:

sudo route | grep default | awk '{print $NF}'

Update: My favoured solution is:

 $(for i in `ip r`; do echo $i; done | grep -A 1 dev | tail -n1)
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The Internet should be accessed normally via default gateway. You can use the command sudo route | grep UG | awk '{print $NF}' – Khaled Nov 26 '11 at 11:42
@Khaled, there can be more than one interfaces using gateways(Flag G). So your answer is wrong. – Sachin Divekar Nov 26 '11 at 12:04
possible duplicate of How do i get the default gateway in LINUX given the destination? – quanta Nov 26 '11 at 12:25
12:02 <biz> hendry: route -n | awk '/^0\.0\.0\.0/{print $NF}' or ip route list | sed -n 's/default.*dev ([^[:space:]]\+)/\1/p' or parsing /proc/net/route on Linux... it seems there's no portable way to do that – hendry Nov 26 '11 at 14:00
Thinking of tweaking my question to find the IP of the connected interface too. – hendry Nov 27 '11 at 12:30
up vote 2 down vote accepted

i think it will be better to use iproute2 instead old and bad-working route.

ip r | sed -n '/^de/s/.*dev //p'
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+1. I agree. Though I have provided answer using route command, We should use iproute2 commands wherever possible. – Sachin Divekar Nov 26 '11 at 20:22
x220:~$ ip r | sed -n '/^de/s/.*dev //p' eth0 metric 202 I think the regexp can be improved. Gah, why is it soo difficult to parse? – hendry Nov 27 '11 at 12:22
Your solution isn't perfect, I have a "metric 202" suffix. I've added my solution in the question above and accepted your answer, since I've grateful you have showed me ip r. :) – hendry Nov 29 '11 at 7:00
nice idea, but awk seems the better tool for that task: ip r | awk '/^de/{print $NF}' – ThorstenS Nov 29 '11 at 7:43

route -n | awk '$1 ~ / {print $NF}' will give you interface with default gateway which is most probably the interface through which you are accessing internet.

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