Configurar dos conexiones a internet

http://www.linux.org/docs/ldp/howto/Adv-Routing-HOWTO/lartc.rpdb.multiple-links.html
http://gentoo-wiki.com/Dual_internet_connections
http://gentoo-wiki.com/TIP_Dual-Homed_Gentoo_Server
http://www.shorewall.net/MultiISP.html
http://www.shorewall.net/Shorewall_Squid_Usage.html
http://www.quagga.net/
http://blog.taragana.com/index.php/archive/how-to-load-balancing-failover-with-dual-multi-wan-adsl-cable-connections-on-linux/
http://linux-ip.net/html/adv-multi-internet.html
http://www.indiangnu.org/index.php/archives/7

# post-up ip route add 10.0.0.0/24 dev eth4 src 10.0.0.253 table uplink2
# post-up ip route add default via 10.0.0.2 table uplink2
# post-up ip rule add from 10.0.0.253 table uplink2
# post-down ip rule del from 10.0.0.253 table uplink2

to enable shorewall, add the SNAT aliases and you can also do a minor optimization of the firewall while you are at it.The changes to shorewall.conf are as follows:
STARTUP_ENABLED=Yes
ADD_SNAT_ALIASES=Yes
FASTACCEPT=Yes