Date: 08 Feb 1999 00:16:13 +0100 From: Dag-Erling Smorgrav <des@flood.ping.uio.no> To: wpaul@FreeBSD.ORG Cc: nsouch@FreeBSD.ORG, hackers@FreeBSD.ORG Subject: Regarding tcpdump and plip Message-ID: <86n22pg5z6.fsf@niobe.ewox.org> In-Reply-To: <xzpsocit7ca.fsf@flood.ping.uio.no>
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[moving over to -hackers...]
As I mentioned in earlier correspondance on -current, tcpdump is
broken wrt plip connections. The reason for this is that, as I read
the code in src/contrib/libpcap/gencode.c, libpcap expect DLT_NULL-
type interfaces to produce a four-byte header with the link type in
the first two bytes, whereas the PLIP driver produces a two-byte
header consisting solely of the link type.
The obvious fix is to patch src/sys/dev/ppbus/if_plip.c to announce
and use a four-byte header as libpcap expects it to do; I have
attached an untested patch which does this. I'm not sure it's the
*correct* fix, however, and I have a hunch that it's only a partial
fix since it seems that the plip code does not construct a header for
incoming packets - if I am correct, then even with the attached patch
tcpdump will only show outgoing packets correctly. I'll toy around
with it a little to see if I can confirm or deny my hunch, and/or fix
it to work in all cases.
On a related note, I was rather surprised, when browsing through the
ppbus code, to discover that it is based on an older version of the
parallell port code than what was then (and is still) in -CURRENT.
What caught my attention was that src/sys/dev/ppbus/nlpt.c still uses
the old probe code, instead of the new one I wrote about a year ago,
which was integrated in revision 1.66 of src/sys/i386/isa/lpt.c on
1998/02/20. There may be other differences as well, but I did not look
too closely.
Also, the plip code still causes regular panics. I haven't caught a
dump yet because my dump device was misconfigured, but I've had at
least two plip-related panics on an otherwise perfectly stable system
in the last two weeks. I'll send a backtrace as soon as I have one.
DES
--
Dag-Erling Smorgrav - des@flood.ping.uio.no
Index: if_plip.c
===================================================================
RCS file: /home/ncvs/src/sys/dev/ppbus/if_plip.c,v
retrieving revision 1.9
diff -u -r1.9 if_plip.c
--- if_plip.c 1999/01/30 15:35:39 1.9
+++ if_plip.c 1999/02/07 22:31:42
@@ -123,7 +123,7 @@
#define CLPIP_SHAKE 0x80 /* This bit toggles between nibble reception */
#define MLPIPHDRLEN CLPIPHDRLEN
-#define LPIPHDRLEN 2 /* We send 0x08, 0x00 in front of packet */
+#define LPIPHDRLEN 4 /* We send 0x08, 0x00, 0x00, 0x00 in front of packet */
#define LPIP_SHAKE 0x40 /* This bit toggles between nibble reception */
#if !defined(MLPIPHDRLEN) || LPIPHDRLEN > MLPIPHDRLEN
#define MLPIPHDRLEN LPIPHDRLEN
@@ -743,11 +743,11 @@
* try to free it or keep a pointer to it).
*/
struct mbuf m0;
- u_short hdr = 0x800;
+ u_char hdr[4] = { 0x08, 0x00, 0x00, 0x00 };
m0.m_next = m;
- m0.m_len = 2;
- m0.m_data = (char *)&hdr;
+ m0.m_len = 4;
+ m0.m_data = (char *)hdr;
bpf_mtap(ifp, &m0);
}
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