Primeiramente instalamos o pacote iproute 2
# aptitude update
# aptitude install iproute

Após configure as tabelas:

# vim /etc/iproute2/rt_tables
Adicione essas duas linhas no final do arquivo:

200 uplink1
201 uplink2

Vamos configurar nossa interface onde em eth0 192.168.1.1 / eth1 192.168.2.1 são os gateways
# vim /etc/network/interfaces

# LOCALHOST
auto lo
iface lo inet loopback

# LINK 1

auto eth0
iface eth0 inet static
        address 192.168.1.100
        netmask 255.255.255.0

        post-up ip route add 192.168.1.1/32 dev eth1 src 192.168.1.100 table uplink1
        post-up ip route add default via 192.168.1.1 table uplink1
        post-up ip rule add from 192.168.1.100 table uplink1
        post-down ip rule del from 192.168.1.100 table uplink1

# LINK 2

auto eth1
iface eth1 inet static
        address 192.168.2.100
        netmask 255.255.255.0

        post-up ip route add 192.168.2.1/32 dev eth1 src 192.168.2.210 table uplink2
        post-up ip route add default via 192.168.2.1 table uplink2
        post-up ip rule add from 192.168.2.210 table uplink2
        post-down ip rule del from 192.168.2.210 table uplink2

# REDE CLIENTE
auto eth2
iface eth2 inet static
        address 192.168.3.1
        netmask 255.255.255.0

Reinicie o serviço:
# /etc/init.d/networking restart

Agora vamos criar um firewall, criaremos as rotas.

#! /bin/sh

# Ativa forward
echo "1" > /proc/sys/net/ipv4/ip_forward

# Limpa regras
/sbin/iptables -F
/sbin/iptables -X
/sbin/iptables -t nat -F
/sbin/iptables -X -t nat
/sbin/iptables -F -t mangle
/sbin/iptables -X -t mangle

# Carrega modulos
/sbin/modprobe iptable_nat
/sbin/modprobe ip_conntrack_ftp
/sbin/modprobe ip_nat_ftp
/sbin/modprobe ipt_LOG
/sbin/modprobe ipt_REJECT
/sbin/modprobe ipt_MASQUERADE

#Libera navegacao nas interfaces
/sbin/iptables -t nat -A POSTROUTING -s 192.168.30/24 -o eth0 -j MASQUERADE
/sbin/iptables -t nat -A POSTROUTING -s 192.168.30/24 -o eth1 -j MASQUERADE

# Configurações Proxy
# /sbin/iptables -t nat -A PREROUTING -i eth2 -p tcp --dport 80 -j REDIRECT --to-port 3128
# /sbin/iptables -A PREROUTING -t mangle -s 192.168.3.0/24 -d 0/0 -j MARK --set-mark 3
# /sbin/iptables -t mangle -A OUTPUT -p TCP -d ! 192.168.3.0/24 --dport 80 -m owner --uid-owner 23 -j MARK --set-mark 3

# Faz com que o servico dns faca consultas pelo outro link
/sbin/iptables -t mangle -A OUTPUT -p UDP --dport 53 -m owner --uid-owner 25 -j MARK --set-mark 2

# Deleta rotas
route del default

# Load balance
ip route add default scope global nexthop via 192.168.1.1 dev eth0 weight 1 nexthop via 192.168.2.1 dev eth1 weight 1

# Limpa cache
ip route flush cached

Use o comando “ip route show table main” para ver como ficou…
# ip route show table main

192.168.1.1/24 dev eth0 proto kernel scope link src 192.168.1.100
192.168.2.1/24 dev eth1 proto kernel scope link src 192.168.2.100
192.168.3.1/24 dev eth2 proto kernel scope link src 192.168.3.1
default
nexthop via 192.168.1.100 dev eth0 weight 1
nexthop via 192.168.2.100 dev eth1 weight 1

Adicione no cron o comando para limpar o cache dos dns a cada 10 min
# vim /etc/crontab
00-59/10 * * * * root ip route flush cached

Aqui fiz o siguinte script para verificar se algum dos link cai, assim refazendo a rota e deixadondo para o que estiver online.

# vim /root/uplink.sh

#! /bin/sh
# - - - - - - - - - - - - - - - #
# Script por Rudimar Remontti   #
# www.remontti.com.br           #
# - - - - - - - - - - - - - - - #
echo -e '\e[33;1mVerificando Links: \e[m'

# - - - - - - - - - - - #
# Configuracoes         #
# - - - - - - - - - - - #
GTW_1=192.168.1.1
ITF_1=eth0
GTW_2=192.168.2.1
ITF_2=eth1
RT_BRT_1=200.176.3.142 # Ip do terra.com.br ele ira pingar p/ verificar se responde
RT_BRT_2=200.176.3.142 # Ip do terra.com.br...
# - - - - - - - - - - - #

echo
echo -e "\e[30;1mLink 1: $GTW_1\e[m"
echo -e "\e[30;1mLink 2: $GTW_2\e[m"
echo 

# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - #
PING_1=`ping $RT_BRT_1 -I $ITF_1 -c 1 |grep packets |cut -c 24`
PING_2=`ping $RT_BRT_2 -I $ITF_2 -c 1 |grep packets |cut -c 24`
# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - #

UP=`echo "($PING_1 + $PING_2)" | bc`
if [ $UP = 2 ]; then 

        route del default
        ip route add default scope global nexthop via $GTW_1 dev $ITF_1 weight 1 nexthop via $GTW_2 dev $ITF_2 weight 1
        ip route flush cached

        echo -n 'Link 1 e Link 2  Status:     '
        echo -e '\e[32;1m[ ON ] \e[m'
        echo

elif [ $UP = 0 ]; then

        route del default
        ip route add default scope global nexthop via $GTW_1 dev $ITF_1 weight 1 nexthop via $GTW_2 dev $ITF_2 weight 1
        ip route flush cached

        echo
        echo -n Link 1:
        echo -e '\e[32;1m  [  OKAY  ] \e[m'
        echo -n Link 2:
        echo -e '\e[32;1m  [  OKAY  ] \e[m'
        echo

elif [ $UP = 1 ]; then
        echo
        if [ $PING_1 = 1 ]; then
                echo -n Link 1:
                echo -e '\e[32;1m  [  OKAY  ] \e[m'
        else
                route del default
                ip route add default scope global nexthop via $GTW_2 dev $ITF_2 weight 1
                ip route flush cached
                data=`date`
                echo "$data -> Link 1 OFF" >> /root/links.log
                echo -n Link 1:
                echo -e '\e[31;1m  [  OFF   ] \e[m'
        fi
        if [ $PING_2 = 1 ]; then
                echo -n Link 2:
                echo -e '\e[32;1m  [  OKAY  ] \e[m'
        else
                route del default
                ip route add default scope global nexthop via $GTW_1 dev $ITF_1 weight 1
                ip route flush cached
                data=`date`
                echo "$data -> Link 2 OFF" >> /root/links.log
                echo -n Link 2:
                echo -e '\e[31;1m  [  OFF   ] \e[m'

        fi
        echo
        if [ $PING_1 = 0 ]; then
                echo -e '\e[30;1mRotas reconfigurada, gatway através do Link 2.\e[m'
                ROTE=`ip route show |grep default |cut -c 9-100`
                echo -e "\e[30;1m$ROTE\e[m"

        elif [ $PING_2 = 0 ]; then
                echo -e '\e[30;1mRotas reconfigurada, gatway através do Link 1.\e[m'
                ROTE=`ip route show |grep default |cut -c 9-100`
                echo -e "\e[30;1m$ROTE\e[m"
        fi
else
        echo ERRO!
fi
# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - #
echo
ip route show |grep nexthop
echo

# chmod 755 /root/uplink.sh

Adicionamos ao cron (5min)
# vim /etc/crontab
00-59/5 * * * * root /root/uplink.sh

Carregue seu firewall na inicialização do sistema, utilize o iptraf para ver o trafego saindo pelas duas interfaces.
Se vc tem dois link com cargas diferente altere o weight.

Abraço

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