>Ola marcio. Sim se trata de NAT a regra e:

>#iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE

>obs: eth1 <-- placa da internet

Ol� Celso. Esta regra n�o funcionou. E tem mais: Eu tente configurar o squid 
como proxy transparente e tamb�m n�o funcionou. Estou usando o Fedora 2. 
Segue abaixo o meu squid.conf e o arquivo que estou usando como firewall

http_port 3128
hierarchy_stoplist cgi-bin ?
acl QUERY urlpath_regex cgi-bin \?
no_cache deny QUERY
cache_mem 32 MB
cache_swap_low 90
cache_swap_high 95
maximum_object_size 4096 KB
minimum_object_size 0 KB
maximum_object_size_in_memory 8 KB
cache_dir ufs /var/spool/squid 100 16 256
cache_access_log /var/log/squid/access.log
cache_log /var/log/squid/cache.log
cache_store_log /var/log/squid/store.log
debug_options ALL,1
client_netmask 255.255.255.0
hosts_file /etc/hosts
auth_param basic children 5
auth_param basic realm Squid proxy-caching web server
auth_param basic credentialsttl 2 hours
refresh_pattern ^ftp:           1440    20%     10080
refresh_pattern ^gopher:        1440    0%      1440
refresh_pattern .               0       20%     4320
acl all src 0.0.0.0/0.0.0.0
acl manager proto cache_object
acl localhost src 127.0.0.1/255.255.255.255
acl to_localhost dst 127.0.0.0/8
acl SSL_ports port 443 563
acl Safe_ports port 80          # http
acl Safe_ports port 21          # ftp
acl Safe_ports port 443 563     # https, snews
acl Safe_ports port 70          # gopher
acl Safe_ports port 210         # wais
acl Safe_ports port 1025-65535  # unregistered ports
acl Safe_ports port 280         # http-mgmt
acl Safe_ports port 488         # gss-http
acl Safe_ports port 591         # filemaker
acl Safe_ports port 777         # multiling http
acl CONNECT method CONNECT
http_access allow manager localhost
http_access deny manager

http_access deny !Safe_ports
http_access deny CONNECT !SSL_ports

acl empresa src 192.168.0.0/255.255.255.0
acl sites_negados dstdomain -i "/etc/squid/sites_proibidos.txt"
acl palavras_negadas url_regex -i "/etc/squid/palavras_proibidas.txt"

http_access deny sites_negados all
http_access deny palavras_negadas all
http_access allow empresa all

http_access allow localhost
http_access deny all

http_reply_access allow all
icp_access allow all
cache_effective_user squid
cache_effective_group squid
 
httpd_accel_host virtual
httpd_accel_port 80
httpd_accel_single_host on
httpd_accel_with_proxy on
httpd_accel_uses_host_header on

client_db on
netdb_low 900
netdb_high 1000
coredump_dir /var/spool/squid

##########################################################################

firewall

/sbin/modprobe ip_conntrack
/sbin/modprobe ip_conntrack_ftp
/sbin/modprobe ip_tables
/sbin/modprobe ipt_state
/sbin/modprobe iptable_nat
/sbin/modprobe ip_nat_ftp
/sbin/modprobe ipt_LOG
/sbin/modprobe ipt_REJECT
/sbin/modprobe ipt_MASQUERADE

/bin/echo "1" > /proc/sys/net/ipv4/ip_forward
/bin/echo "1" > /proc/sys/net/ipv4/ip_dynaddr
/bin/echo "1" > /proc/sys/net/ipv4/icmp_echo_ignore_all

/sbin/iptables -F
/sbin/iptables -F -t nat
/sbin/iptables -X
/sbin/iptables -X -t nat

/sbin/iptables -P INPUT ACCEPT
/sbin/iptables -P FORWARD ACCEPT
/sbin/iptables -P OUTPUT ACCEPT

/sbin/iptables -A FORWARD -i eth0 -o eth1 -m state --state ESTABLISHED,RELATED 
-j ACCEPT
/sbin/iptables -A FORWARD -i eth1 -o eth0 -m state --state NEW -j ACCEPT

/sbin/iptables -A INPUT -s 127.0.0.1 -j ACCEPT
/sbin/iptables -A OUTPUT -s 127.0.0.1 -j ACCEPT
/sbin/iptables -A FORWARD -s 127.0.0.1 -j ACCEPT

#
# Liberando o DHCP para a rede interna
#
/sbin/iptables -A INPUT -p tcp --dport 67 -i eth1 -s 192.168.0.0/24 -j ACCEPT
/sbin/iptables -A INPUT -p udp --dport 67 -i eth1 -s 192.168.0.0/24 -j ACCEPT
/sbin/iptables -A INPUT -p tcp --dport 68 -i eth1 -s 192.168.0.0/24 -j ACCEPT
/sbin/iptables -A INPUT -p udp --dport 68 -i eth1 -s 192.168.0.0/24 -j ACCEPT

#
# Redirecionando trafego web para o SQUID
#
#/sbin/iptables -t nat -A PREROUTING -p tcp --dport 80 -j DNAT 
--to-destination 192.168.0.1:3128
/sbin/iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 80 -j REDIRECT 
--to-ports 3128
/sbin/iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 443 -j REDIRECT 
--to-port 3128

#
# Mascaramento e forwarding...
#
/sbin/iptables -t nat -A POSTROUTING -d 0/0 -o eth0 -j MASQUERADE
/sbin/iptables -A FORWARD -o eth0 -d 0/0 -j ACCEPT


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