broken ip checksum after frag reassemble of nfs READDIR?

Adam McDougall mcdouga9 at
Tue Apr 11 13:30:56 UTC 2006

On Wed, Apr 05, 2006 at 03:06:45PM +0200, Daniel Hartmeier wrote:

  On Wed, Apr 05, 2006 at 02:41:09PM +0200, Max Laier wrote:
  > The other big problem that just crossed my mind:  Reassembly in the bridge 
  > path!?  It doesn't look like the current bridge code on either OS is ready to 
  > deal with packets > MTU coming out of the filter.  The question here is 
  > probably how much IP processing we want to do in the bridge code?
  OpenBSD's bridge does, see bridge_fragment(). IIRC, we slightly adjusted
  ip_fragment() so it could be called from there, and not too much code
  had to be duplicated.
          if ((len - ETHER_HDR_LEN) > dst_if->if_mtu)
                  bridge_fragment(sc, dst_if, &eh, m);
          else {
                  bridge_ifenqueue(sc, dst_if, m);
          error = ip_fragment(m, ifp, ifp->if_mtu);
          if (error) {
                  m = NULL;
                  goto dropit;
          for (; m; m = m0) {
                  m0 = m->m_nextpkt;
                  m->m_nextpkt = NULL;
  		error = bridge_ifenqueue(sc, ifp, m);
  That's one more layer violation in bridge, but stateful filtering
  basically requires fragment reassembly, at least in general.

Would it be possible to get bridge reassembly and even a quick and 
dirty patch to fixup the checksum on every packet into FreeBSD soon?

I have 4 firewalls to deploy this summer, the simplest and smallest one
first which would benefit from these fixes but could probably get away
without them.  For my largest one I would prefer to use fragment
reassembly to improve the accuracy of my ruleset, but I can't risk a
jumbo packet wedging my firewalls, and of course bad checksum packets
are useless.  Using pf in routing mode is undesirable for my situations.

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