Re: Netfiltering - NF_IP_LOCAL_OUT - how it works???

[Vishwas Raman]

Harald Welte wrote:
> 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
> >
> >    NF_HOOK(PF_INET, NF_IP_LOCAL_OUT, skb, NULL, rt->u.dst.dev,
> >                    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.

Actually I did solve the problem. All I had to do was reset 
skb->dst->output() to ip_output() in my_ip_output() which is defined in 
my module. The problem was that even after my module was unloaded the 
destination cache was still pointing to my_ip_output() which was 
non-existent...

Thanks,

-Vishwas.




> > -Vishwas.



--
--
Vishwas Raman
Software Engineer, Eternal Systems, Inc,
5290 Overpass Rd, Bldg D, Santa Barbara. CA 93111
Email: vishwas@xxxxxxxxxxxxxxxxxxx
Tel:   (805) 696-9051 x246
Fax:   (805) 696-9083
URL:   http://www.eternal-systems.com/

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