I have the same kind of problem with KNOPPIX V5.0 DVD. The KNOPPIX V4.0.2 was working correctly. So I suspect it is the newer kernel in combination with a cheap ADSL-router (Etech), I think the two do not interact properly when they do DHCP configuration.

I use the following script to connect to the internet. In my case the router is set on 10.10.10.100.
Code:
#!/bin/bash

ifconfig eth0 10.10.10.105 up
#ifconfig
route add default gw 10.10.10.100
#route

echo nameserver 217.149.196.6 > /etc/resolv.conf
echo nameserver 217.149.192.6 >> /etc/resolv.conf
cat /etc/resolv.conf
Once run I'm connected to the internet perfectly fine.

This way I can continue to use the newest KNOPPIX, but I'm interested in where the problem comes from. Anyone??