I have a debian install on a machine with 2 eth cards,eth0 and eth1.
eth0 has been configured using pppoeconf for a pppoe connection which
is working fine.
eth1 was meant to be there in order to provide for a nat so that an
internet connection can be provided to another computer also using debian.
The moment I plug in the cable to eth1 , eth0 stops working.
(Is this caused maybe by hotplug ?)
The goal was to actually get eth1 working also without breaking eth0's connection and afterwards make a nat using iptables.
The same thing was done using Slackware and netconfig+pppoeconf and there
were no problems.
The pppoe is up using the pppd service and it was configured using pppoeconf.


I tried to find out the problem by trying to find out which file is beeing
modified when I plug the cable in eth1 so I did
strace ifconfig eth1 2>&1 | grep open
I got a list of files and I suspect /proc/net/if_inet6 is the file
that ifconfig modifies and reads thata from.
So I decided to do sudo lsof | grep if_inet6 in order to find out
what process uses the hotplug feature to autodetect when I put the cable
inside eth1.
Unfortunately I find no process that did this.

What should I do so that the internet connection doesn't drop when I plug in the cable for eth1 ?

hotplug is ancient ... meant for older machines

can you post the contents of

and verify you haven't assigned a default route to both interfaces..

one interface should have a default route the other should have a static route defined.

I've solved the problem

Would be nice to know what you did, or how you solved the issue for the next guy searching for a solution...