Система- RH Enterprise Linux AS (2.4.21-27.EL)
Сеть: eth0 192.168.1.111 для выхода в интернет
      eth1 10.0.0.1 для подключения к локальной сети

http_port  3128 
acl lan src  192 . 168 . 1 . 111   10 . 0 . 0 . 0 / 24 
http_access allow localhost
http_access allow lan
http_access deny all

httpd_accel_host virtual
httpd_accel_port  80 
httpd_accel_with_proxy  on
httpd_accel_uses_host_header on

# iptables -L -t nat
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination
DNAT       tcp  --  anywhere             anywhere           tcp dpt:http to:192.168.1.111:3128
REDIRECT   tcp  --  anywhere             anywhere           tcp dpt:http redir ports 3128

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination
MASQUERADE  all  --  anywhere             anywhere

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

iptables --table nat --append POSTROUTING --out-interface eth0 -j MASQUERADE
iptables --append FORWARD --in-interface eth1 -j ACCEPT
iptables -A INPUT -i eth1 -j ACCEPT
iptables -A OUTPUT -o eth1 -j ACCEPT
iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 80 -j DNAT --to 192.168.1.111:3128
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport  80  -j REDIRECT --to-port  3128