[SOLVED] Easy way to automatically switch between two network adapters depending on which of the two has internet access

jnihil · 07-11-2016, 08:20 PM

I'd responded to you thinking you were using a couple of dial-up modems. Editing my post.

Yes, if you are connected to a DSL router via Ethernet, then the link state would be the same.
It might be useful if you explain to us exactly what your setup is, rather than us guessing what you are using.

You had initially written:

Quote:

I have two network adapters connecting to different isp's.

and now you are telling us you are using a DSL router?

Quote:

...ethernet cable that leads to the adsl router

Ulysses_ · 07-11-2016, 09:31 PM

No, I am using 2 adsl routers, one from each isp. Would rather not mess with these routers, better to mess with the PC's routing functionality.

jnihil · 07-11-2016, 09:54 PM

Right.
So write yourself a script which runs in a loop, testing connectivity of remote resources (such as 8.8.8.8), and modifies the default route depending on the connectivity.
That shouldn't be so hard. The information you require is already in this thread.

Ulysses_ · 07-12-2016, 04:08 AM

That gc_timeout setting above seems to do just what I want automatically according to the docs. But what should the routing table look like for this to work?

Ulysses_ · 07-12-2016, 06:05 PM

Made a script in the end. The way to switch the default route is like this:

To switch to eth1:
ip route change default via 192.168.1.1 dev eth1

To switch back to eth0:
ip route change default via 192.168.0.1 dev eth0

So this script is quite robust, no matter which isp fails it will do the right thing to get back online.

Code:

#!/bin/sh

PINGTARGET=8.8.8.8
PINGINTERVAL=3
PINGWAITFORRESPONSE=3

while [ 1 ]
do
   echo Waiting for ping to fail
   while [ 1 ] 
   do
      ping -c 1 -W $PINGWAITFORRESPONSE $PINGTARGET >/dev/null 2>/dev/null || break
      sleep $PINGINTERVAL
   done

   echo Switching to eth1
   ip route change default via 192.168.1.1 dev eth1 2>/dev/null

   echo Waiting for ping to fail
   while [ 1 ] 
   do
      ping -c 1 -W $PINGWAITFORRESPONSE $PINGTARGET >/dev/null 2>/dev/null || break
      sleep $PINGINTERVAL
   done

   echo Switching to eth0
   ip route change default via 192.168.0.1 dev eth0 2> /dev/null

   sleep 1
done

Ulysses_ · 07-12-2016, 06:10 PM

It would be nice if it switched back to eth0 when this isp is back online as this isp is faster. But how can a script test this without disrupting your connectivity while using a browser?

jnihil · 07-12-2016, 06:49 PM

You could setup a static route for 8.8.8.8/32 pointed to eth0 and 8.8.4.4/32 for eth1.
Then you can ping 8.8.8.8 for ISP1 test, and 8.8.4.4 for ISP2, regardless of where the default route is pointing at the time.