Consegui resolver esse problema editando /etc/udev/rules.d/70-persistent-net.rules
. Ao tornar a eth0 minha NIC WAN, o Linux usou automaticamente seu gateway como o gateway padrão.
/etc/udev/rules.d/70-persittent-net.rules :
# This file was automatically generated by the /lib/udev/write_net_rules
# program, run by the persistent-net-generator.rules rules file.
#
# You can modify it, as long as you keep each rule on a single
# line, and change only the value of the NAME= key.
# PCI device 0x10ec:/sys/devices/pci0000:00/0000:00:1c.4/0000:03:00.0 (r8169)
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="FF:FF:FF:FF:FF:F0", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth0"
# PCI device 0x10ec:/sys/devices/pci0000:00/0000:00:1c.5/0000:04:00.0/0000:05:01.0 (r8169)
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="FF:FF:FF:FF:FF:F1", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth1"
# PCI device 0x10ec:0x8169 (r8169)
SUBSYSTEM=="net", ACTION=="add", DRIVERS=="?*", ATTR{address}=="FF:FF:FF:FF:FF:F2", ATTR{dev_id}=="0x0", ATTR{type}=="1", KERNEL=="eth*", NAME="eth2"