http://www.linux.or.jp/JF/JFdocs/TransparentProxy.html
LAN 側からの port 80 へのアクセスを squid (port 3128) を経由させる．
squid の設定
httpd_accel_host virtual httpd_accel_port 80 httpd_accel_with_proxy on httpd_accel_uses_host_header on
iptables の設定
/sbin/iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 80 -j REDIRECT --to-port 3128