Everytime you connect to an ISP, it is necesary to have configured DNS name resolution, so your computer can find IP addresses associated to a computer name.
IP addresses of your DNS servers are placed into the /etc/resolv.conf file.
In a standalone computer connecting to Internet, this file usually contains the IP addresses of your ISP's DNS servers:
#/etc/resolv.conf file for ISPname nameserver 111.222.333.444 nameserver 222.333.444.555
In a proxy/firewall computer, this file usually contains its own IP address (or the loopback address, 127.0.0.1), and this computer includes a DNS server that translates DNS names to IP addresses by querying external DNS servers.
#/etc/resolv.conf file for local DNS resolution nameserver 127.0.0.1
Installation of a local DNS server is out of the scope of this document. There is a lot of documentation about this, but a good and quick approach can be found in the DNS-Howto ( http://www.linuxdoc.org/HOWTO/DNS-HOWTO.html).