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SubjectRe: Netfiltering - NF_IP_LOCAL_OUT - how it works???
Hi Vishwas, sorry for the late reply.  Most netfilter developers have
been to the netfilter developer workshop, I guess.

you should ask this question on the netfilter-devel mailinglist, where
it is more on-topic than on lkml.

On Thu, Aug 14, 2003 at 03:06:26PM -0700, Vishwas Raman wrote:

> While initializing the module, I register a NF_IP_LOCAL_OUT hook for the
> outgoing packet and change skb->dst->output to my_ip_output() instead of
> ip_output() in that hook function. After loading the module, I see
> control being transferred to my_ip_output() for all outgoing packets
> which in turn calls ip_output() and everything seems to work well.
> The exit function of the module also unregisters the hook that I am using.
> The problem is that after I unload the module, which in turn unregisters
> the hook, I have a kernel panic happening each time I use TCP.
> The panic occurs at the following point, ip_build_and_send_pkt() in
> ip_output.c where it is trying to call
> output_maybe_reroute);
> I thought once the unregistering of the hook is done, it no longer looks
> for that hook function. I have no idea why it is failing. May be I am
> doing something grossly wrong with netfiltering. Anyone who is familiar
> with netfiltering and has registered and unregistered hooks before might
> be able to guide me regarding this.

I think either you are doing something wrong while unregistering from
the netfilter hook - or you are running into a race condition. It might
happen, that you assign the skb->dst->output function of a packet to
your function, and then you remove the module before that packet is
actually sent.

> -Vishwas.

- Harald Welte <>
Programming is like sex: One mistake and you have to support it your lifetime
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