Criando um Gateway Rapidamente.

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Forest Criando um gateway rapidamente, neste TipConf mostrarei os caminhos para setar um gateway rapidamente com regras em ComandLine.
Eu utilizo-me deste cenário para implementar alguns testes na minha rede no VirtualBox.
Criarei dois subcenários, o primeiro quando desejo permitir o acesso entre as redes.

Inventário:
S/O : Debian 8.5 Kernel 3.16 [default]
eth0: Internet
eth1: 172.16.0.0/24 Lan1
eth2: 172.16.1.0/24 Lan2
Cenário 01:
Um ambiente onde as redes 172.16.0.0/24 e 172.16.1.0/24 se comuniquem.

root# echo 1 > /proc/sys/net/ipv4/ip_forward
root# iptables -t nat -A POSTROUTING -s 172.16.0.0/24 -o eth0 -j MASQUERADE
root# iptables -t nat -A POSTROUTING -s 172.16.1.0/24 -o eth0 -j MASQUERADE
root# iptables -nvL FORWARD && iptables -nvL POSTROUTING -t nat 


stdout>
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination     
Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target      prot opt in      out     source              destination     
    0     0 MASQUERADE  all  --  *      eth0    172.16.0.0/24        0.0.0.0/0       
    0     0 MASQUERADE  all  --  *      eth0    172.16.1.0/24        0.0.0.0/0   
stdout>

Cenário 02:
Um ambiente onde as redes 172.16.0.0/24 e 172.16.1.0/24 <b>não</b> se comuniquem.

root# echo 1 > /proc/sys/net/ipv4/ip_forward
root# iptables -t nat -A POSTROUTING -s 172.16.0.0/24 -o eth0 -j MASQUERADE
root# iptables -t nat -A POSTROUTING -s 172.16.1.0/24 -o eth0 -j MASQUERADE
root# iptables -A FORWARD -s 172.16.0.0/24 -d 172.16.1.0/24 -j DROP
root# iptables -A FORWARD -s 172.16.1.0/24 -d 172.16.0.0/24 -j DROP
root# iptables -nvL FORWARD && iptables -nvL POSTROUTING -t nat 

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination       
    0     0 DROP       all  --  *      *       172.16.0.0/24        172.16.1.0/24     
    0     0 DROP       all  --  *      *       172.16.1.0/24        172.16.0.0/24     
Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination       
    0     0 MASQUERADE  all  --  *      eth0    172.16.0.0/24        0.0.0.0/0         
    0     0 MASQUERADE  all  --  *      eth0    172.16.1.0/24        0.0.0.0/0

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