On Fri, Nov 15, 2002 at 09:39:40PM +0100, J.Smith wrote:
> I am having some troubles with displaying an AIX 4.3.3 'tcpdump' format fddi
> capture in Ethereal.
The troubles were exactly what I suspected they were. I've checked in a
fix, which strips off the 3 padding bytes; I've attached a patch that
contains the fix.
> Although Ethereal can read the captures, the data does
> not get displayed correctly, and gets interpreted as 'void frame'. I am not
> sure if this is because the 'AIX-tcpdump' file-format is different from the
> 'standard-tcpdump' file-format, or if Ethereal is just not able to interpret
> tcpdump FDDI captures correctly yet.
It's not really either one. The file format differences are:
1) they don't use DLT_ values in the file header to indicate
link-layer types (we work around that);
2) they store time stamps in seconds/nanoseconds rather than
seconds/microseconds form (we work around that);
but the problem is that
3) in the packet payload (which I don't consider part of the
file format, in the strict sense) they put in padding.
Ethereal has no trouble with normal tcpdump FDDI captures.
Index: wiretap/libpcap.c
===================================================================
RCS file: /usr/local/cvsroot/ethereal/wiretap/libpcap.c,v
retrieving revision 1.83
diff -c -r1.83 wiretap/libpcap.c
*** wiretap/libpcap.c 22 Oct 2002 09:11:13 -0000 1.83
--- wiretap/libpcap.c 16 Nov 2002 20:18:59 -0000
***************
*** 843,848 ****
--- 843,849 ----
guint packet_size;
guint orig_size;
int bytes_read;
+ char fddi_padding[3];
bytes_read = libpcap_read_header(wth, err, &hdr, FALSE);
if (bytes_read == -1) {
***************
*** 855,860 ****
--- 856,883 ----
wth->data_offset += bytes_read;
packet_size = hdr.hdr.incl_len;
orig_size = hdr.hdr.orig_len;
+
+ /*
+ * AIX appears to put 3 bytes of padding in front of FDDI
+ * frames; strip that crap off.
+ */
+ if (wth->file_type == WTAP_FILE_PCAP_AIX &&
+ (wth->file_encap == WTAP_ENCAP_FDDI ||
+ wth->file_encap == WTAP_ENCAP_FDDI_BITSWAPPED)) {
+ /*
+ * The packet size is really a record size and includes
+ * the padding.
+ */
+ packet_size -= 3;
+ orig_size -= 3;
+ wth->data_offset += 3;
+
+ /*
+ * Read the padding.
+ */
+ if (!libpcap_read_rec_data(wth->fh, fddi_padding, 3, err))
+ return FALSE; /* Read error */
+ }
*data_offset = wth->data_offset;
