Date: Mon, 22 Jan 1996 12:34:20 PST From: Bill Fenner <fenner@parc.xerox.com> To: Philippe Regnauld <regnauld@tetard.frmug.fr.net> Cc: hackers@freebsd.org (hackers) Subject: Re: tcpdump barfs on interface lp0 Message-ID: <96Jan22.123428pst.177479@crevenia.parc.xerox.com> In-Reply-To: Your message of "Sat, 20 Jan 1996 18:50:57 PST." <199601210250.DAA00319@tetard.frmug.fr.net>
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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--
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