A packet filter is a piece of software which looks at the header of packets as they pass through, and decides the fate of the entire packet. It might decide to DROP the packet (i.e., discard the packet as if it had never received it), ACCEPT the packet (i.e., let the packet go through), or something more complicated.
Under Linux, packet filtering is built into the kernel (as a kernel module, or built right in), and there are a few trickier things we can do with packets, but the general principle of looking at the headers and deciding the fate of the packet is still there.
Control. Security. Watchfulness.
when you are using a Linux box to connect your internal network to another network (say, the Internet) you have an opportunity to allow certain types of traffic, and disallow others. For example, the header of a packet contains the destination address of the packet, so you can prevent packets going to a certain part of the outside network. As another example, I use Netscape to access the Dilbert archives. There are advertisements from doubleclick.net on the page, and Netscape wastes my time by cheerfully downloading them. Telling the packet filter not to allow any packets to or from the addresses owned by doubleclick.net solves that problem (there are better ways of doing this though: see Junkbuster).
when your Linux box is the only thing between the chaos of the Internet and your nice, orderly network, it's nice to know you can restrict what comes tromping in your door. For example, you might allow anything to go out from your network, but you might be worried about the well-known `Ping of Death' coming in from malicious outsiders. As another example, you might not want outsiders telnetting to your Linux box, even though all your accounts have passwords. Maybe you want (like most people) to be an observer on the Internet, and not a server (willing or otherwise). Simply don't let anyone connect in, by having the packet filter reject incoming packets used to set up connections.
sometimes a badly configured machine on the local network will decide to spew packets to the outside world. It's nice to tell the packet filter to let you know if anything abnormal occurs; maybe you can do something about it, or maybe you're just curious by nature.
Linux kernels have had packet filtering since the 1.1 series. The first generation, based on ipfw from BSD, was ported by Alan Cox in late 1994. This was enhanced by Jos Vos and others for Linux 2.0; the userspace tool `ipfwadm' controlled the kernel filtering rules. In mid-1998, for Linux 2.2, I reworked the kernel quite heavily, with the help of Michael Neuling, and introduced the userspace tool `ipchains'. Finally, the fourth-generation tool, `iptables', and another kernel rewrite occurred in mid-1999 for Linux 2.4. It is this iptables which this HOWTO concentrates on.
You need a kernel which has the netfilter infrastructure in it: netfilter is a general framework inside the Linux kernel which other things (such as the iptables module) can plug into. This means you need kernel 2.3.15 or beyond, and answer `Y' to CONFIG_NETFILTER in the kernel configuration.
iptables talks to the kernel and tells it what
packets to filter. Unless you are a programmer, or overly curious,
this is how you will control the packet filtering.
iptables tool inserts and deletes rules from the kernel's
packet filtering table. This means that whatever you set up, it will
be lost upon reboot; see
Making Rules Permanent for how to make sure they are restored the next time Linux
iptables is a replacement for
Using ipchains and ipfwadm for how to painlessly
avoid using iptables if you're using one of those tools.
Your current firewall setup is stored in the kernel, and thus will be lost on reboot. You can try the iptables-save and iptables-restore scripts to save them to, and restore them from a file.
The other way is to put the commands required to set up your rules in an initialization script. Make sure you do something intelligent if one of the commands should fail (usually `exec /sbin/sulogin').