Não deu certo

modprobe ipt_MASQUERADE
modprobe iptable_nat
insmod ip_nat_ftp
insmod ip_conntrack_ftp

echo 1 > /proc/sys/net/ipv4/ip_forward

iptables -F
iptables -t nat -F
iptables -X
iptables -X -t nat
iptables -Z

iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE
iptables -A FORWARD -p tcp -s 0/0 -d 0/0 --dport 25 -j ACCEPT
iptables -A FORWARD -p tcp -s 0/0 -d 0/0 --dport 110 -j ACCEPT
iptables -A FORWARD -p tcp -s 0/0 -d 0/0 --dport 53 -j ACCEPT
iptables -A FORWARD -p udp -s 0/0 -d 0/0 --dport 53 -j ACCEPT

--------------
com as regras acima o computador nao consiguiu chegar até o destino, ou seja, as maquinas que estao atraz desse roteador nao conseguiu nem pingar o server de email.

Adicione essas linhas logo antes do MASQUERADE:

iptables -t nat -A PREROUTING -d 127.0.0.0/32 -j DNAT --to $IP_INTERNO
iptables -t nat -A POSTROUTING -s $IP_INTERNO -j SNAT --to $IP_EXTERNO

Troque pelos respctivos nomes e deve resolver....

modprobe ipt_MASQUERADE
modprobe iptable_nat
insmod ip_nat_ftp
insmod ip_conntrack_ftp

echo 1 > /proc/sys/net/ipv4/ip_forward

iptables -F
iptables -t nat -F
iptables -X
iptables -X -t nat
iptables -Z

iptables -t nat -A PREROUTING -d 127.0.0.0/32 -j DNAT --to 10.0.0.0/8
iptables -t nat -A POSTROUTING -s 10.0.0.0/8 -j SNAT --to 200.195.24.47

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

iptables -A FORWARD -p tcp -s 0/0 -d 0/0 --dport 25 -j ACCEPT
iptables -A FORWARD -p tcp -s 0/0 -d 0/0 --dport 110 -j ACCEPT
iptables -A FORWARD -p tcp -s 0/0 -d 0/0 --dport 53 -j ACCEPT
iptables -A FORWARD -p udp -s 0/0 -d 0/0 --dport 53 -j ACCEPT

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Ficaria assim?

Isso!!! <IMG SRC="images/forum/icons/icon_smile.gif">

O script retornou esse erro:
--------------------------------------------
[root@gateway rc.d]# ./rc.firewall_4
Using /lib/modules/2.4.18-2cl/kernel/net/ipv
insmod: a module named ip_nat_ftp already ex
Using /lib/modules/2.4.18-2cl/kernel/net/ipv
insmod: a module named ip_conntrack_ftp alre
iptables v1.2.4: Bad IP address `10.0.0.0/8&acute;
--------------------------------------------

Onde errei?

Lugar nenhum, ele apenas avisou que os módulos que vc mandou carregar já estão ativos....