Description

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 forward the traffic to the correct local computer. This way, the computers on your local net are completely invisible to the outside world, even though they can reach the outside and can receive replies. It is even possible to run globally visible servers from within a masqueraded local network using a mechanism called port forwarding. Masquerading is also often called NAT (Network Address Translation). Other operating systems often call this term PAT (Port Address Translation).

Another use of Netfilter is in transparent proxying: if a machine on the local network tries to connect to an outside host, your Linux box can transparently forward the traffic to a local server, typically a caching proxy server.

Yet another use of Netfilter is building a bridging firewall. Using a bridge with Network packet filtering enabled makes iptables "see" the bridged traffic. For filtering on the lower network and Ethernet protocols over the bridge, use ebtables (located under bridge Netfilter configuration).

Various modules exist for Netfilter that replace the previous masquerading (ipmasqadm), packet filtering (ipchains), transparent proxying, and portforwarding mechanisms. Please see Documentation/Changes under iptables for the location of these packages.

Chances are that you should say yes here if you compile a kernel which will run as a router and no for regular hosts.