1

I need to forward an interface and I wonder why is disable by default? There is any security issue when I enable it? Is there any side effect when I enable it?

2

ip_forward is only useful one multihomed systems (more than one network interface) and makes routing of network packets between those interfaces possible. By enabling ip_forward your box essentially becomes a router (gateway).

Routing is an optional network function and as such disabled by default.

As for the downside/possible risks: when one of those interfaces is facing a public subnet (WAN/internet or possible hostile LAN), routing makes attacks possible. That's why you need to address those potential security issues with a firewall.

There is an excellent answer at What is kernel ip forwarding? with more information on the subject.

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