Vuurmuur NFQUEUE support

Vuurmuur supported the QUEUE target for a while already, even though it needed a little bit of a hack to handle the state. This is because the iptables ruleset Vuurmuur creates is quite simple: after a few general protection rules it starts by accepting traffic with the state established. Since there is no way to say ‘queue established traffic that was queued before’ in iptables I decided to use traffic marking to distinguish between traffic to be queued or accepted. But there was a problem with this approach. I didn’t want to cripple the marking of traffic for other purposes, such as traffic shaping and routing, so I decided to use mark-ranges to either queue or accept:

/sbin/iptables -t filter -A FORWARD -m mark –mark 0x0/0xff000000 -m state –state ESTABLISHED -j ACCEPT

/sbin/iptables -t filter -A FORWARD -m mark –mark 0x1000000/0xff000000 -m state –state ESTABLISHED -j QUEUE

The reason I am explaining all this is that while this works fine for the QUEUE target with a single queue, it won’t work as well for NFQUEUE. One of the new things with NFQUEUE is that there are up to 65536 different queues. The above solution won’t scale for numbers like this.

I could just settle for supporting a few queues so the marking would still work. However, this conflicts with my design goals for Vuurmuur. It is the goal to put as little artificial limitations in the usage of Vuurmuur as possible. A second goal is that Vuurmuur should be able to run on standard distribution kernels. So I’m not interested in adding obscure features that require patching of the kernel and the iptables userland tools. For this purpose, I consider Debian Stable to be the standard.

For the NFQUEUE support I decided to repeat the mark trick using the newer connmark target and match. The advantage of connmark is that you only have to set it once for a connection, as opposed to for every packet with mark. Additionally connmark works separately from mark, so marking the traffic for other purposes still works. Because connmark wasn’t supported by many distributions at the time the QUEUE support was added, I didn’t use it before. At this time only the NFQUEUE support uses connmark, so if your system doesn’t support it nothing will change.

The Vuurmuur NFQUEUE support is now available in the Vuurmuur SVN repository.

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