Criar arquivo /etc/iptables.rules e incluir as regras abaixo:

*filter
 
# Allows all loopback (lo0) traffic and drop all traffic to 127/8 that doesn't use lo0
-A INPUT -i lo -j ACCEPT
-A INPUT ! -i lo -d 127.0.0.0/8 -j REJECT
 
# Accepts all established inbound connections
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
 
# Allows all outbound traffic
# You could modify this to only allow certain traffic
-A OUTPUT -j ACCEPT
 
# Allows HTTP and HTTPS connections from anywhere (the normal ports for websites)
-A INPUT -p tcp --dport 80 -j ACCEPT
-A INPUT -p tcp --dport 443 -j ACCEPT
 
# Allows SSH connections for script kiddies
# THE -dport NUMBER IS THE SAME ONE YOU SET UP IN THE SSHD_CONFIG FILE
-A INPUT -p tcp -m state --state NEW --dport 22022 -j ACCEPT
 
# Now you should read up on iptables rules and consider whether ssh access 
# for everyone is really desired. Most likely you will only allow access from certain IPs.
 
# Allow ping
-A INPUT -p icmp -m icmp --icmp-type 8 -j ACCEPT
 
# Reject all other inbound - default deny unless explicitly allowed policy:
-A INPUT -j REJECT
-A FORWARD -j REJECT
 
COMMIT

* O exemplo acima só libera SSH, HTTP e HTTPS, você pode adicionar outras portas logo abaixo das linhas que liberam as portas 80 e 443

Logo após crie o arquivo /etc/network/if-pre-up.d/iptables com o conteúdo descrito abaixo:

#!/bin/bash
iptables-restore < /etc/iptables.rules

Dê permissão de execução ao arquivo acima e o execute:

# chmod +x /etc/network/if-pre-up.d/iptables
# /etc/network/if-pre-up.d/iptables

No próximo boot, as regras de iptables serão aplicadas automaticamente.

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

CentOS

# sysctl -w net.ipv4.ip_forward=1

Para persistir, edite /etc/sysctl.conf:

net.ipv4.ip_forward = 1

Use o comando abaixo para habilitar as alterações:

# sysctl -p /etc/sysctl.conf

Roteamento da rede interna eth1 para a eth0:

# /sbin/iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
# /sbin/iptables -A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT
# /sbin/iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT

• http://www.revsys.com/writings/quicktips/nat.html
• https://www.centos.org/docs/5/html/5.2/Deployment_Guide/s1-firewall-ipt-fwd.html