debian, problem with DNS

The configuration file /etc/resolv.conf contains information that allows a computer connected to a network to resolve names into addresses.

Change it to, for example, Google's DNS servers:

nameserver 8.8.8.8
nameserver 8.8.4.4

Your problem sounds like a case of not having the DNS server set. Normally, you would want to edit /etc/resolv.conf, but in Debian (and Ubuntu) this file could be overwritten. The official solution to this is to install the resolvconf package: apt-get install resolvconf

Then edit (with root permissions) either /etc/resolvconf/resolv.conf.d/head or /etc/resolvconf/resolv.conf.d/tail adding

nameserver 8.8.8.8
nameserver 8.8.4.4

and then run resolvconf -u to update the file. This will keep your changes from being overwritten.