question concerning skb_headroom in fragments

From: Richard Jones (richard@earthmen.com)
Date: Tue Nov 28 2000 - 03:55:35 EST


I've written a module that steals packets at the net_bh (via proto
handler) and hard_start_xmit points. I massage this data before
injecting it back into the network. At the moment I'm testing this
with ping. All works fine until packet sizes reach the MTU (1500, I'm
on ethernet). At this point it appears that the fragmented echo reply
packets are only getting a skb_headroom of 2 , where previously this
was 16 when unfragmented, the echo reply fragments also appear to have
grown larger than MTU (1514). I realise there are other mechanisms of
achieving what I'm after, however they are both too high up the stack
and lack efficient kernel<->userland transfers. Anyways I've been
chasing this for a few days and wondered if anyone had some hints
about what might be going wrong or at least where to look.

Thanks,

Richard.

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