Taming the Wild Netfilter

Using Netfilter and ipchains to increase security on home systems.

For those of you who have taken the plunge and upgraded from kernel 2.2.X (or even 2.0.X) to 2.4.X, congratulations. If, like a number of folks, you're running some form of firewall using either ipchains or ipfwadm, your scripts may work fine. But sooner or later you're probably going to want to upgrade.

In the 2.4.X kernels, Rusty Russell, the Linux packet-filter guru, and his crew of coders have implemented Netfilter into the kernel. Netfilter is the replacement for ipchains or ipfwadm. Fortunately, Netfilter permits you to keep using ipchains or ipfwadm until you can come to grips with iptables by adding a compatibility layer via a kernel module that permits these older packet filters to run. But Netfilter has so many exciting new additions, you'll want to convert those rules as soon as possible. One word of caution, though, if you load the ipchains or ipfwadm modules, you can't load ip_tables (and vice versa). So it's all or nothing. After reading this article, however, making the change should be easy.

For those new to packet filtering, ignore the ipchains translations and use the iptables examples. While not all ipchains commands and options will be translated to iptables, this text should provide a good idea about how to construct a packet-filter firewall by translating ipchains commands into iptables commands.

The reason you'll want to upgrade to Netfilter is because it, unlike ipchains or ipfwadm, is stateful. What this means is it can track connections and permit incoming responses to outgoing requests without creating gaping holes in the firewall. The connection tracking opens a specific, temporary hole for responses and only from the contacted server. We'll see how this works later. The drawback is that with connection tracking in use, Netfilter will need to use a little more memory because the connections are tracked in RAM. So your 4MB 386-16 may no longer be up to the job, depending on your filtering requirements.

Background

The actual Netfilter implementation is broken into two parts, the kernel portion known as Netfilter and the userland tool that interfaces with Netfilter and creates the rulesets, iptables. Both are required to implement your packet-filtering firewall.

First, we'll concentrate on the kernel portion. Netfilter includes support for IPv4 and IPv6. It does not, however, filter any other protocols, so your firewall should not run IPX, AppleTalk or any other protocol that might be used to circumvent iptables rules. Similarly, you must not enable the kernel fast-switching option. This item is one of the last ones in the network options section of the kernel's configuration menu. The code permits fast switching at a low level in the IP stack. The Netfilter code resides at a much higher level, so fast switching effectively bypasses Netfilter completely.

Kernel Configuration

In order to get started using Netfilter, you'll need to have your kernel compiled for Netfilter support. Most distributions include this support by default, so a quick test is in order. If you can insert the module ip_tables, then you won't need to worry about this section. As root, run the command

modprobe ip_tables

Then run

lsmod | grep ip_tables
If ip_tables show up, you're in good shape. If not, don't worry, rebuilding a kernel is extremely easy. This text won't cover the complete kernel rebuild process, but many resources are available to help you through this step. If you find you need to rebuild your kernel, the Sidebar will provide you some guidance on what to include for a complete Netfilter-enabled kernel.

Enabling Netfilter in Your Kernel

Netfilter Modules

If you've built and installed all the modules, all modules will auto-install when a rule is entered except ip_tables, ip_nat_ftp and ip_conntrack_ftp. These can be loaded either manually or as part of your iptables startup script.

A full build and installation of Netfilter produces a large number of modules, but most firewalls will only use a few. The modules that aren't loaded aren't taking up memory, so don't worry about what you don't use.

Getting and Installing iptables

Your distribution may have installed iptables, and it almost certainly did if your kernel has Netfilter support. But if you want the very latest, you'll probably have to get it from the Netfilter site. Netfilter is available at netfilter.filewatcher.org. Download and compile it according to the instructions in the INSTALL file. The following instructions assume the kernel sources are in /usr/src/linux. If not, adjust the following instructions appropriately. If you need to run

make pending-patches KERNEL_DIR=/usr/src/linux

or

make patch-o-matic KERNEL_DIR=/usr/src/linux
then you'll need to recompile your kernel before continuing. Otherwise, you can ignore these two commands. In general, the patch-o-matic is for users with special needs and is of interest to the average user.

After running

make KERNEL_DIR=/usr/src/linux

run

make install KERNEL_DIR=/usr/src/linux
You're now ready to use iptables.

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NetFilter for non-IP packets

Anonymous's picture

Hi, Iam new to netfilter. I want to know whether i can use netfilter API to send/receive (and possibly replace) ethernet packets irrespective of the protocol used in the ethernet payload(without writing my own device driver) or netfilter just provides api to work only at IP level.

Limiting simultaneous incoming connections by IP?

Ivan Baldo's picture

It seems dificult to find this information.
How does one limit the amount of connections by IP?
For example, one may want to limit the amount of connections from an IP to 10, so from a single IP you only allow up to 10 connections.
It is useful to avoid a single IP eating all the connections to an email server or webserver or whatever service, to avoid simple to do DoS.
Using the programs for this isn't that nice, since one would like to limit by DROP and not REJECT in some cases, and also some programs don't even have the option to limit the connections allowed by each IP.
Thanks for your article!

i am using netfilter hook to

Anonymous's picture

i am using netfilter hook to capture a simple packet which contains an http request then it is been sent to a particular node by dev_queue_xmit function by changing its source ip and hardware address but am unable to transmit i have used nf_stolen option please help

Will You Tell Me How To ..

NEWBIE's picture

Enhance & secure web-server, mail server(qmail) & internet speed (in LAN with 100 workstations).I have got only one HP Proliant ML150 Server(Xeon/256MbRam/34*3SCSI Discs)..I am using Fedora Core3.

How can we configure Squid for offline mode ?

Re: Taming the Wild Netfilter

Anonymous's picture

Hi,
I have implemented, IP tunnelling in kernel by hooking to Netfilters. The implementation goes as a module. I am sending the packet out by calling dev_queue_xmit by filling the required fields in the skb and returning NF_STOLEN. I just want to know, if I capture a packet at pre/post routing hook and mangle the IP addresses(which could result in local/forward delivery) and return NF_ACCEPT to corresponding NF hook, would the packet be routed. This is required so that, if any module registered to NF after this tunneling module, can also get the packet and also I don't need to fill in the fields in the skb and call dev_queue_xmit.
BTW, I am Suresh.
thanks
regards
Suresh

Re: Taming the Wild Netfilter

Anonymous's picture

Does anybody know how to get snort to work with netfilter?

Re: Taming the Wild Netfilter

Anonymous's picture

Snort is an IDS..Netfilter is a firewall..what do you mean getting Snort to work with netfilter?

Re: Taming the Wild Netfilter

Anonymous's picture

Snort can be run on Netfilter. Look into snort_inline, It takes packets using ip_queue.o module to user space where snort is run.
I suggest you rewrite snort engine in kernel space and reduce the packet moving time from kernel space to user space and thus increase the performance.
Ravi
ravivsn@rocsys.com
ROCSYS technologies ltd
Hyderabad

Re: Taming the Wild Netfilter

Anonymous's picture

THANK YOU DAVID!!!!!! For a VERY long time now I have been trying to figure out why, when I made my INPUT chain's policy DROP/REJECT, my connection would fall off too. After months of searching, including a few wild goose chases, the answer was your discussion about allowing ESTABLISHED,RELATED connections on the INPUT chain. I now have a default policy of deny, whereas before I had to keep track of what was being broadcast and lock it down. THANKS AGAIN!!

You're welcome.... Eat more

DAVID's picture

You're welcome....
Eat more fruits.

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