Ubuntu - Networking - Configuration - Promiscuous Mode

Promiscuous mode is a security policy, which allows a network adapter to see all traffic traversing the network.

By default, a network adapter usually only receives frames that are meant for it.

ip link set eth1 promisc on

ifconfig eth0 promisc

Put the following in /etc/sysconfig/network-scripts/ifcfg-eth1

BOOTPROTO=static
DEVICE=eth1
ONBOOT=yes
TYPE=Ethernet
PROMISC=yes
USERCTL=no
PEERDNS=no
HWADDR=xx:xx:xx:xx:xx:xx

NOTE: Change eth1 and the hardware address (HWADDR=xx:xx:xx:xx:xx:xx) to reflect your system.