Squid Proxy server (not transparent) working on port 3128 and 80

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http://serverfault.com – I have a Squid Proxy server (Debian) working perfectly with authentication: auth_param basic program /usr/lib/squid/ncsa_auth /etc/squid/passwd I'd like that also client that try to get web pages using the proxy as gateway (port 80) get redirected on port 3128. I've tried iptables: iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 3128 without luck because in that case my port 80 clients get an error: The following error was encountered: Invalid Request Some aspect of the HTTP Request is invalid. Possible problems: Missing or unknown request method Missing (HowTos)