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To: Philippe Regnauld <regnauld@tetard.frmug.fr.net>
cc: hackers@freebsd.org (hackers)
Subject: Re: tcpdump barfs on interface lp0 
In-reply-to: Your message of "Sat, 20 Jan 1996 18:50:57 PST."
             <199601210250.DAA00319@tetard.frmug.fr.net> 
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Date: Mon, 22 Jan 1996 12:34:20 PST
From: Bill Fenner <fenner@parc.xerox.com>
Message-Id: <96Jan22.123428pst.177479@crevenia.parc.xerox.com>
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--===_0_Mon_Jan_22_12:33:42_PST_1996
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This is probably because the lpt driver incorrectly uses the DLT_NULL 
encapsulation; it prepends 2 bytes but tcpdump expects 4.  Try this patch.

  Bill


--===_0_Mon_Jan_22_12:33:42_PST_1996
Content-Type: application/octet-stream
Content-Description: lpt.c.diff

*** lpt.c.orig	Mon Jan 22 12:26:45 1996
--- lpt.c	Mon Jan 22 12:30:12 1996
***************
*** 1344,1360 ****
  	if (ifp->if_bpf) {
  	    /*
  	     * We need to prepend the packet type as
! 	     * a two byte field.  Cons up a dummy header
  	     * to pacify bpf.  This is safe because bpf
  	     * will only read from the mbuf (i.e., it won't
  	     * try to free it or keep a pointer to it).
  	     */
  	    struct mbuf m0;
! 	    u_short hdr = 0x800;
  
  	    m0.m_next = m;
! 	    m0.m_len = 2;
! 	    m0.m_data = (char *)&hdr;
  
  	    bpf_mtap(ifp->if_bpf, &m0);
  	}
--- 1344,1364 ----
  	if (ifp->if_bpf) {
  	    /*
  	     * We need to prepend the packet type as
! 	     * a two byte field.  The DLT_NULL encapsulation
! 	     * assumes a four byte header.  Cons up a dummy header
  	     * to pacify bpf.  This is safe because bpf
  	     * will only read from the mbuf (i.e., it won't
  	     * try to free it or keep a pointer to it).
  	     */
  	    struct mbuf m0;
! 	    struct {
! 		u_short type;
! 		u_short padding;
! 	    } nullhdr = { 0x800 };
  
  	    m0.m_next = m;
! 	    m0.m_len = 4;
! 	    m0.m_data = (char *)&nullhdr;
  
  	    bpf_mtap(ifp->if_bpf, &m0);
  	}

--===_0_Mon_Jan_22_12:33:42_PST_1996--