epan/dissectors/packet-ieee8023.c

   1 /* packet-ieee8023.c
   2  * Routine for dissecting Ethernet packets with a length field (as opposed
   3  * to a type field).
   4  *
   5  * The name "ieee8023" is historical, dating back to when IEEE Std 802.3
   6  * had only a length field and expected all packets to have an 802.2
   7  * header following the MAC header.  Since IEEE 802.3y-1997, 802.3
   8  * supports either a type field or a length field, so it's no longer
   9  * correct to refer to "802.3" vs. "D/I/X" or vs. "Ethernet II" frames.
  10  *
  11  * Wireshark - Network traffic analyzer
  12  * By Gerald Combs <gerald@wireshark.org>
  13  * Copyright 1998 Gerald Combs
  14  *
  15  * SPDX-License-Identifier: GPL-2.0-or-later
  16  */
  17
  18 #include "config.h"
  19
  20 #include <epan/packet.h>
  21 #include <epan/exceptions.h>
  22 #include <epan/expert.h>
  23 #include <epan/show_exception.h>
  24 #include "packet-ieee8023.h"
  25 #include "packet-eth.h"
  26
  27 void proto_reg_handoff_ieee802_3(void);
  28
  29 static dissector_handle_t ipx_handle;
  30 static dissector_handle_t llc_handle;
  31 static dissector_handle_t ccsds_handle;
  32
  33 void
  34 dissect_802_3(volatile int length, bool is_802_2, tvbuff_t *tvb,
  35               int offset_after_length, packet_info *pinfo, proto_tree *tree,
  36               proto_tree *fh_tree, int length_id, int trailer_id, expert_field* ei_len,
  37               int fcs_len)
  38 {
  39   proto_item *length_it;
  40   tvbuff_t   *volatile next_tvb = NULL;
  41   tvbuff_t   *trailer_tvb = NULL;
  42   const char *saved_proto;
  43   int         captured_length, reported_length;
  44
  45   length_it = proto_tree_add_uint(fh_tree, length_id, tvb,
  46                                   offset_after_length - 2, 2, length);
  47
  48   /* Get the length of the payload.
  49      If the FCS length is positive, remove the FCS.
  50      (If it's zero, there's no FCS; if it's negative, we don't know whether
  51      there's an FCS, so we'll guess based on the length of the trailer.) */
  52   reported_length = tvb_reported_length_remaining(tvb, offset_after_length);
  53   if (fcs_len > 0) {
  54     if (reported_length >= fcs_len)
  55       reported_length -= fcs_len;
  56   }
  57
  58   /* Make sure the length in the 802.3 header doesn't go past the end of
  59      the payload. */
  60   if (length > reported_length) {
  61     length = reported_length;
  62     expert_add_info(pinfo, length_it, ei_len);
  63   }
  64
  65   /* Give the next dissector only 'length' number of bytes. */
  66   captured_length = tvb_captured_length_remaining(tvb, offset_after_length);
  67   if (captured_length > length)
  68     captured_length = length;
  69   next_tvb = tvb_new_subset_length_caplen(tvb, offset_after_length, captured_length, length);
  70
  71   /* Dissect the payload either as IPX or as an LLC frame.
  72      Catch non-fatal exceptions, so that if the reported length
  73      of "next_tvb" was reduced by some dissector before an
  74      exception was thrown, we can still put in an item for
  75      the trailer. */
  76   saved_proto = pinfo->current_proto;
  77   TRY {
  78     if (is_802_2)
  79       call_dissector(llc_handle, next_tvb, pinfo, tree);
  80     else {
  81       /* Check if first three bits of payload are 0x7.
  82          If so, then payload is IPX.  If not, then it's CCSDS.
  83          Refer to packet-eth.c for setting of is_802_2 variable. */
  84       if (tvb_get_bits8(next_tvb, 0, 3) == 7)
  85         call_dissector(ipx_handle, next_tvb, pinfo, tree);
  86       else
  87         call_dissector(ccsds_handle, next_tvb, pinfo, tree);
  88     }
  89   }
  90   CATCH_NONFATAL_ERRORS {
  91     /* Somebody threw an exception that means that there was a problem
  92        dissecting the payload; that means that a dissector was found,
  93        so we don't need to dissect the payload as data or update the
  94        protocol or info columns.
  95
  96        Just show the exception and then drive on to show the trailer,
  97        after noting that a dissector was found and restoring the
  98        protocol value that was in effect before we called the subdissector. */
  99     show_exception(next_tvb, pinfo, tree, EXCEPT_CODE, GET_MESSAGE);
 100   }
 101   ENDTRY;
 102
 103   /* Restore the protocol value, so that any exception thrown by
 104      tvb_new_subset_remaining() refers to the protocol for which
 105      this is a trailer. */
 106   pinfo->current_proto = saved_proto;
 107
 108   /* Construct a tvbuff for the trailer; if the trailer is past the
 109      end of the captured data, this will throw a BoundsError, which
 110      is what we want, as it'll report that the packet was cut short. */
 111   trailer_tvb = tvb_new_subset_remaining(tvb, offset_after_length + length);
 112
 113   add_ethernet_trailer(pinfo, tree, fh_tree, trailer_id, tvb, trailer_tvb, fcs_len, offset_after_length);
 114 }
 115
 116 void
 117 proto_reg_handoff_ieee802_3(void)
 118 {
 119   /*
 120    * Get handles for the subdissectors.
 121    */
 122   ipx_handle = find_dissector("ipx");
 123   llc_handle = find_dissector("llc");
 124   ccsds_handle = find_dissector("ccsds");
 125 }
 126
 127 /*
 128  * Editor modelines  -  https://www.wireshark.org/tools/modelines.html
 129  *
 130  * Local Variables:
 131  * c-basic-offset: 2
 132  * tab-width: 8
 133  * indent-tabs-mode: nil
 134  * End:
 135  *
 136  * ex: set shiftwidth=2 tabstop=8 expandtab:
 137  * :indentSize=2:tabSize=8:noTabs=true:
 138  */