Linux Kernel in a Nutshell

Name

NETFILTER — Network packet filtering

Netfilter is a framework for filtering and mangling network packets that pass through your Linux box.

The most common use of packet filtering is to run your Linux box as a firewall protecting a local network from the Internet. The type of firewall provided by this kernel support is called a packet filter, which means that it can reject individual network packets based on type, source, destination etc. The other kind of firewall, a proxy-based one, is more secure but more intrusive and more bothersome to set up; it inspects the network traffic much more closely, modifies it, and has knowledge about the higher level protocols, which a packet filter lacks. Moreover, proxy-based firewalls often require changes to the programs running on the local clients. Proxy-based firewalls don't need support by the kernel, but they are often combined with a packet filter, which works only if you say yes here.

You should also say yes here if you intend to use your Linux box as the gateway to the Internet for a local network of machines without globally valid IP addresses. This is called masquerading. If one of the computers on your local network wants to send something to the outside, your box can "masquerade" as that computer, i.e., it forwards the traffic to the intended outside destination, but modifies the packets to make it look like they came from the firewall box itself. Masquerading works both ways: if the outside host replies, the Linux box will silently ...