How to make a mirroring netfiler

["彭维" <pengwei2004@xxxxxxxxxxxxxxxxxx>]

I have mirrored the network traffic on a switch to a switch port which connects a
Linux Box(Redhat 9, one network interface, eth0). I want to anaylis the traffic,
so i set eht0 to promisc mode, open the FORWARD function(echo "1" >
/proc/sys/net/ipv4/ip_forward).
But i cann't see any data in FORWARD chain, how can i do.