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To: Archie Cobbs <archie@whistle.com>
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Subject: Re: BPF fix to if_loop.c
From: itojun@iijlab.net
Date: Wed, 31 May 2000 10:06:11 +0900
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>> 	sorry this was incorrect.  however, I think there should be check
>> 	if ifp really wants DLT_NULL encapsulation or not.
>Hmm.. maybe a better fix is to remove this BPF section altogether.. ?
>It's only going to result in showing the same packet twice, right?

	there are couple of different stories here.
	note that you can pass any ifp to if_simloop().

	a. when if_simloop(ifp = lo0), there must be call to bpf_mtap,
	   with DLT_NULL encapsulation.  if we don't do it here, nobody will.
	b. when if_simloop(ifp = non-lo0), there are two cases.
		b1. if it was called from layer 2 specific code like
		  if_ethersubr.c, if_simloop may want to inject the packet
		  with no encapsulation change.
		b2. if it was called from ip{6,}_mloopback, if_simloop() must
		  not call bpf_mtap.  with the current code, ifp_simloop
		  injects packet as is (with no layer 2 header) into, for
		  example to bpf for ethernet interface, passing junk to bpf
		  listeners.  this is the problem I experienced.

	in case of b1, your description should be correct (seeing same
	packet twice).  in case of b2, I'm still not sure how to resolve it.
	maybe I don't get the ultimate goal for this change from 4.4BSD.

itojun


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