How to prevent IP spoofing on Ubuntu.

IP spoofing is the creation of network packets with fake IP
addresses in order to log in from another IP address.

1. To prevent open the file "/etc/host.conf":
sudo vi /etc/host.conf

2. Replace the following lines:
The "order" line is only used by old versions of the C library.
​order hosts,bind
​multi on

With following lines:
The "order" line is only used by old versions of the C library.
​order bind,hosts
​nospoof on

That's it.