I have been researching the SSL Strip attack (a.k.a. arpspoof) attack. In this attack I need to edit the ip_forward file.
This attack works successfully, but I want to know why do I need to change /proc/sys/net/ipv4/ip_forward
to 1.
SSLstrip works as man in the middle, which in itself somewhat explains why you need to enable packet forwarding.
Your 'server' sits in between the client and the intended destination. As the client browses, the traffic is processed and modified by you, and then sent on to the destination. If you don't enable packet forwarding, the data would stop at your server, and the client would appear like it wasn't connected to the internet.
By default packet forwarding is off in Ubuntu, so by setting it to 1, you are enabling it. So, your title doesn't make a ton of sense.