I startuped named once to see if it there would happen problems (default configuration - no problem happened). But now the clients connecting through squid cannot resolve IP adresses. The DNS was provided by the modem and still is.

In squid, I have a line: dns_nameservers with the IP address of the modem. Everything was running fine util this test.

This was just supposed to be a test. Now, clients connecting through squid do not connect at all. Once I go to the client and unconfigure proxy in IE. It works...

Does anybody know what could be happening?

squid picks up dns servers from /etc/resolv.conf, if the server you started is in there it is possible squid is now using it but it is unable to resolve names.

Sorry! My fault.
When I edited the firewall, I accidentally commented squid´s redirection.
Thank you anyway.