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I've a Debian system without UI, with a GPRS modem attached to it. Now, this system is connected with ethernet through a standard cable. I want to obtain a configuration that allows the computer to access internet via ethernet, if an ethernet cable is connected, otherwise use the GPRS to do so. Pratically speaking, I want the eth0 interface to be the preferable interface if it is on, ppp (only) if former isn't mounted.

How to obtain an easy /etc/network/interfaces configuration to achieve this?


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There are if-up.d /if-down.d directories in /etc/ppp/ . you should put there ifup\ifdown scripts to configure yours default gw.

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Archedius, assuming your debian system has /sbin/ip help and /etc/iprout??/
iproute2 gives you the both a systemwide default gateway and a NIC/interface specific default gateway. It is called source based routing as opposed to destination based routing. The source in your case is the GPRS modem. or download Alexey Kuznetsov's IP Command Reference.

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