From: Florian Westphal <[email protected]>
Date: Thu, 7 May 2015 23:04:24 +0200
> There was interest in keeping geometry of original fragments on forward.
>
> This (re)enables this feature.
>
> on router with mtu 1500 on all interfaces and netfilter conntrack enabled:
...
> Caveat:
> This disables the optimization made in commit
> 3cc4949269e01f39443d0 ("ipv4: use skb coalescing in defragmentation") for
> everyone as soon as nf_defrag_ipv4 modules are loaded (conntrack defrag
> hooks earlier than ipv4 stacks own defragmentation for local delivery),
> and there is no way to easily determine if we will forward the skb at that
> stage.
>
> ip_fragment checks the size of the frag skbs vs. the outgoing device mtu
> before using them so if device mtu is smaller than the frag skb length
> the device mtu will be used instead for refragmentation.
>
> Cc: Eric Dumazet <[email protected]>
> Signed-off-by: Florian Westphal <[email protected]>
Indeed, I agree that we should only modify the packet's geomtry if we
know it's to be locally delivered.
But paying the cost just because a netfilter module is loaded, that's
really heavy handed and shows really bad engineering on our part.
When I hear "happens when netfilter modules are loaded", it translates
into my head as "all the time". And for you it should too, because
effectively that's how the world operates.
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