epan/dissectors/packet-ieee8023.c

   1 /* packet-ieee8023.c
   2  * Routine for dissecting 802.3 (as opposed to D/I/X Ethernet) packets.
   3  *
   4  * $Id$
   5  *
   6  * Wireshark - Network traffic analyzer
   7  * By Gerald Combs <gerald@wireshark.org>
   8  * Copyright 1998 Gerald Combs
   9  *
  10  * This program is free software; you can redistribute it and/or
  11  * modify it under the terms of the GNU General Public License
  12  * as published by the Free Software Foundation; either version 2
  13  * of the License, or (at your option) any later version.
  14  *
  15  * This program is distributed in the hope that it will be useful,
  16  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  17  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  18  * GNU General Public License for more details.
  19  *
  20  * You should have received a copy of the GNU General Public License
  21  * along with this program; if not, write to the Free Software
  22  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  23  */
  24
  25 #include "config.h"
  26
  27 #include <glib.h>
  28 #include <epan/packet.h>
  29 #include <epan/exceptions.h>
  30 #include <epan/expert.h>
  31 #include <epan/show_exception.h>
  32 #include "packet-ieee8023.h"
  33 #include "packet-eth.h"
  34
  35 static dissector_handle_t ipx_handle;
  36 static dissector_handle_t llc_handle;
  37 static dissector_handle_t ccsds_handle;
  38
  39 void
  40 dissect_802_3(volatile int length, gboolean is_802_2, tvbuff_t *tvb,
  41               int offset_after_length, packet_info *pinfo, proto_tree *tree,
  42               proto_tree *fh_tree, int length_id, int trailer_id, expert_field* ei_len,
  43               int fcs_len)
  44 {
  45   proto_item            *length_it;
  46   tvbuff_t              *volatile next_tvb = NULL;
  47   tvbuff_t              *trailer_tvb = NULL;
  48   const char            *saved_proto;
  49   gint                  captured_length, reported_length;
  50   void                  *pd_save;
  51
  52   length_it = proto_tree_add_uint(fh_tree, length_id, tvb,
  53                                   offset_after_length - 2, 2, length);
  54
  55   /* Get the length of the payload.
  56      If the FCS length is positive, remove the FCS.
  57      (If it's zero, there's no FCS; if it's negative, we don't know whether
  58      there's an FCS, so we'll guess based on the length of the trailer.) */
  59   reported_length = tvb_reported_length_remaining(tvb, offset_after_length);
  60   if (fcs_len > 0) {
  61     if (reported_length >= fcs_len)
  62       reported_length -= fcs_len;
  63   }
  64
  65   /* Make sure the length in the 802.3 header doesn't go past the end of
  66      the payload. */
  67   if (length > reported_length) {
  68     length = reported_length;
  69     expert_add_info(pinfo, length_it, ei_len);
  70   }
  71
  72   /* Give the next dissector only 'length' number of bytes. */
  73   captured_length = tvb_length_remaining(tvb, offset_after_length);
  74   if (captured_length > length)
  75     captured_length = length;
  76   next_tvb = tvb_new_subset(tvb, offset_after_length, captured_length, length);
  77
  78   /* Dissect the payload either as IPX or as an LLC frame.
  79      Catch non-fatal exceptions, so that if the reported length
  80      of "next_tvb" was reduced by some dissector before an
  81      exception was thrown, we can still put in an item for
  82      the trailer. */
  83   saved_proto = pinfo->current_proto;
  84   pd_save = pinfo->private_data;
  85   TRY {
  86     if (is_802_2)
  87       call_dissector(llc_handle, next_tvb, pinfo, tree);
  88     else {
  89       /* Check if first three bits of payload are 0x7.
  90          If so, then payload is IPX.  If not, then it's CCSDS.
  91          Refer to packet-eth.c for setting of is_802_2 variable. */
  92       if (tvb_get_bits8(next_tvb, 0, 3) == 7)
  93         call_dissector(ipx_handle, next_tvb, pinfo, tree);
  94       else
  95         call_dissector(ccsds_handle, next_tvb, pinfo, tree);
  96     }
  97   }
  98   CATCH_NONFATAL_ERRORS {
  99     /* Somebody threw an exception that means that there was a problem
 100        dissecting the payload; that means that a dissector was found,
 101        so we don't need to dissect the payload as data or update the
 102        protocol or info columns.
 103
 104        Just show the exception and then drive on to show the trailer,
 105        after noting that a dissector was found and restoring the
 106        protocol value that was in effect before we called the subdissector. */
 107     pinfo->private_data = pd_save;
 108
 109     show_exception(next_tvb, pinfo, tree, EXCEPT_CODE, GET_MESSAGE);
 110   }
 111   ENDTRY;
 112
 113   /* Restore the protocol value, so that any exception thrown by
 114      tvb_new_subset_remaining() refers to the protocol for which
 115      this is a trailer, and restore the private_data structure in
 116      case one of the called dissectors modified it. */
 117   pinfo->private_data = pd_save;
 118   pinfo->current_proto = saved_proto;
 119
 120   /* Construct a tvbuff for the trailer; if the trailer is past the
 121      end of the captured data, this will throw a BoundsError, which
 122      is what we want, as it'll report that the packet was cut short. */
 123   trailer_tvb = tvb_new_subset_remaining(tvb, offset_after_length + length);
 124
 125   add_ethernet_trailer(pinfo, tree, fh_tree, trailer_id, tvb, trailer_tvb, fcs_len);
 126 }
 127
 128 void
 129 proto_reg_handoff_ieee802_3(void)
 130 {
 131   /*
 132    * Get handles for the subdissectors.
 133    */
 134   ipx_handle = find_dissector("ipx");
 135   llc_handle = find_dissector("llc");
 136   ccsds_handle = find_dissector("ccsds");
 137 }