Re: [PATCH] Fix checksum bug for multicast/broadcast packets on postrouting hook

[Harald Welte <laforge@netfilter.org>]

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On Sun, Feb 15, 2004 at 01:09:23AM -0500, James Morris wrote:
> On Sat, 14 Feb 2004, Mika Penttilä wrote:
> 
> > James Morris wrote:
> > 
> > >The proposed solution below is to copy the skb rather than clone it, to 
> > >ensure that the original and looped back packets are independent.
> > >
> >
> > This is unneeded overhead for the common case. The right fix is to make 
> > sure the modifier (netfilter etc) makes the copy if needed. Actually, 
> > this is what skb_ip_make_writable() is doing.
> 
> The common case here will be only for locally generated multicast and 
> broadcast packets.
> 
> If the netfilter core code is modified instead, we will end up adding
> skb_ip_make_writable() to nf_hook_slow() which will be called for every 
> packet with an output device which uses hardware checksums.
> 
> Not sure which is worse, but here's a proposed patch which does this.
 
Why can't we somehow check inside the netfilter hook if the packet is
at least multicast/broadcast (or even better: also locally generated)?

> - James
> James Morris
-- 
- Harald Welte <laforge@netfilter.org>             http://www.netfilter.org/
============================================================================
  "Fragmentation is like classful addressing -- an interesting early
   architectural error that shows how much experimentation was going
   on while IP was being designed."                    -- Paul Vixie

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