If, like it just happened to me tonight, your ISP DNS collapse, you can use openDNS. Just manually change the DNS in /etc/resolv.conf
nameserver 208.67.222.222 nameserver 208.67.220.220
Many thanks Stemp for the tip :)
Edit: ne t’inquiètes pas Stemp, je sais que tu as toujours raison :-þ