thanks for your answer.
I already had tried with that configuration without success, but today I discovered that there was a configuration problem on my squid proxy server, because of upgrading it from version 2.5 to version 2.6 it changed the syntax on squid.conf file about the transparent proxy feature.
With squid proxy port=8080, and squid 2.5 version we need the following configuration:
in squid 2.6 version the following command is enough:
http_port 8080 transparent
and the other commands are not recognized.