Taming the Wild Netfilter

Using Netfilter and ipchains to increase security on home systems.
Concluding Remarks

Using Netfilter's stateful rules, you can actually increase the security of your home system with less effort by making judicious use of its connection tracking. Many more options are also available to you. This article scratches only the surface. I recommend you make use of Rusty's Unreliable Guides available on the Netfilter site (mentioned earlier).

For home users with simple needs, keep your firewall simple. I do not recommend most firewall tools and scripts because they layer unnecessary complexity into your firewall. If you don't understand a rule, don't implement it. The first three stateful rules (using the -m state rule) will keep you in good stead. If an attacker has already been in and compromised a system, the rules won't help. They also won't protect you against e-mail-based trojans, but they will protect against direct attacks. I suggest, if you don't use IRC, you log and drop outgoing IRC connections:

$IPT -j filter -I tcprules -p tcp --destination-port 6667 -j LOG --log-prefix "IRC attempt "
$IPT -j filter -I tcprules 2 -p tcp --destination-port 6667 -j DROP

Also, if you don't need anyone entering your network, don't open any ports (as we did in our second example). This article did not discuss how to segregate your network properly to isolate internet-accessible systems from trusted internal systems. If you require this level of complexity, and your risk assessment asks for it, it might be time to call for knowledgeable help.

David A. Bandel (dbandel@pananix.com) is a Linux/UNIX consultant currently living in the Republic of Panama. He is coauthor of Que Special Edition: Using Caldera OpenLinux.



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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

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.

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.
ROCSYS technologies ltd

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.