Connected to DHCP server but cannot access internet

I can suddenly no longer access the internet with my Debian Etch (default kernel + updates). I get "Page not found" in Firefox and ping www.somesite.com says "unknown host". ifdown eth0 && ifup eth0 and the thereby invoked dhclient work as expected, that is I get a valid IP address by the DHCP server etc. I can also ping the DHCP server, its just the internet that doesn't work. The firewall is switched off. What should I do next to diagnose the problem? I know that the DHCP server is not at fault, because other operating systems on my computer can reach the internet (and Debian used to as well!).

With best regards,
Rehceb Rotkiv