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