forked from osmocom/wireshark
compile a document about heuristic dissectors, following:
http://www.wireshark.org/lists/wireshark-dev/200808/msg00234.html svn path=/trunk/; revision=26146
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Ulf Lamping
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7530143278
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@ -44,6 +44,7 @@ You'll find additional dissector related information in the following README
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files:
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- README.binarytrees - fast access to large data collections
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- README.heuristic - what are heuristic dissectors and how to write them
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- README.malloc - how to obtain "memory leak free" memory
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- README.plugins - how to "pluginize" a dissector
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- README.request_response_tracking - how to track req./resp. times and such
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@ -0,0 +1,200 @@
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$Revision: 25920 $
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$Date: 2008-08-04 22:41:43 +0200 (Mo, 04 Aug 2008) $
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$Author: ulfl $
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This file is a HOWTO for Wireshark developers. It describes how Wireshark
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heuristic protocol dissectors works and how to write them.
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This file is compiled to give in depth information on Wireshark.
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It is by no means all inclusive and complete. Please feel free to send
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remarks and patches to the developer mailing list.
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Prerequisites
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-------------
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As this file is an addition to README.developer, it is essential to read
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and understand that document first.
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Why heuristic dissectors?
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-------------------------
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When Wireshark "receives" a packet, it has to find the right dissector to
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start decoding the packet data. Often this can be done by known conventions,
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e.g. the Ethernet type 0x800 means "IP on top of Ethernet" - an easy and
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reliable match for Wireshark.
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Unfortunately, these conventions are not always available, or (accidentially
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or knowingly) some protocols don't care about those conventions and "reuse"
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existing "magic numbers / tokens".
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For example TCP defines port 80 only for the use of HTTP traffic. But, this
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convention doesn't prevent anyone from using TCP port 80 for some different
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protocol, or on the other hand using HTTP on a port number different than 80.
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To solve this problem, Wireshark introduced the so called heuristic dissector
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mechanism to try to deal with these problems.
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How Wireshark uses heuristic dissectors?
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----------------------------------------
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While Wireshark starts, heuristic dissectors (HD) register themselves slightly
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different than "normal" dissectors, e.g. a HD can ask for any TCP packet, as
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it *may* contain interesting packet data for this dissector. In reality more
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than one HD will exist for e.g. TCP packet data.
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So if Wireshark has to decode TCP packet data, it will first try to find a
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dissector registered directly for the TCP port used in that packet. If it
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finds such a registered dissector it will just hand over the packet data to it.
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In case there is no such "normal" dissector, WS will hand over the packet data
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to the first matching HD. Now the HD will look into the data and decide if that
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data looks like the dissector "is interested in". The return value signals WS
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if the HD processed the data (so WS can stop working on that packet) or the
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heuristic didn't matched (so WS tries the next HD until one matches - or the
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data simply can't be processed).
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XXX - mention "use heuristic sub dissectors first"
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How do these heuristics work?
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-----------------------------
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Difficult to give a general answer here. The usual heuristic works as follows:
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A HD looks into the first few packet bytes and searches for common patterns that
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are specific to the protocol in question. Most protocols starts with a
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specific header, so a specific pattern may look like (synthetic example):
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1) first byte must be 0x42
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2) second byte is a type field and only can contain values between 0x20 - 0x33
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3) third byte is a flag field, where the lower 4 bits always contain the value 0
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4) fourth and fifth bytes contains a 16 length field, where the value can't be
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longer than 10000 bytes
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So the heuristic dissector will check incoming packet data for all of the
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4 above conditions, and only if all of the four conditions are true there is a
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good chance that the packet really contains the expected protocol - and the
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dissector continues to decode the packet data. If one condition fails, it's
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very certainly not the protocol in question and the dissector returns to WS
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immediately "this is not my protocol" - maybe some other heuristic dissector
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is interested!
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Obviously, this is *not* 100% bullet proof, but the best WS can offer to its
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users here - and improving the heuristic is always possible if it turns out
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that it's not good enough to distinguish between two given protocols.
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Heuristic Code Example
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----------------------
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You can find a lot of code examples in the wireshark sources, e.g.:
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grep -l heur_dissector_add epan/dissectors/*.c
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returns (currently) 69 files.
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For the above example criteria, the following code example might do the work
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(combine this with the dissector skeleton in README.developer):
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XXX - please note: The following code examples were not tried in reality,
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please report problems to the dev-list!
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static gboolean dissect_PROTOABBREV(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
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{
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...
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/* 1) first byte must be 0x42 */
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if ( tvb_get_guint8(tvb, 0) != 0x42 )
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return (FALSE);
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/* 2) second byte is a type field and only can contain values between 0x20-0x33 */
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if ( tvb_get_guint8(tvb, 1) < 0x20 || tvb_get_guint8(tvb, 1) > 0x33 )
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return (FALSE);
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/* 3) third byte is a flag field, where the lower 4 bits always contain the value 0 */
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if ( tvb_get_guint8(tvb, 2) & 0x0f )
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return (FALSE);
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/* 4) fourth and fifth bytes contains a 16 length field, where the value can't be longer than 10000 bytes */
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/* Assumes network byte order */
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if ( tvb_get_ntohs(tvb, 3) > 10000 )
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return (FALSE);
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/* Assume it’s your packet and do dissection */
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...
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return (TRUE);
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}
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void
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proto_reg_handoff_PROTOABBREV(void)
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{
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static int PROTOABBREV_inited = FALSE;
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if ( !PROTOABBREV_inited )
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{
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/* register as heuristic dissector for both TCP and UDP */
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heur_dissector_add("tcp", dissect_PROTOABBREV, proto_PROTOABBREV);
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heur_dissector_add("udp", dissect_PROTOABBREV, proto_PROTOABBREV);
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}
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}
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Please note, that registering a heuristic dissector is only possible for a
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small variety of protocols. In most cases an heuristic is not needed, and
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adding the support would only add unused code to the dissector.
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TCP and UDP are prominent examples that support HDs, as there
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seems to be a tendency to reuse known port numbers for new protocols.
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XXX - what to grep for, if a protocol provides HD support or not?
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It's possible to write a dissector to be a dual heuristic/normal dissector.
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In that the case, dissect_PROTOABBREV should return an int with the number of
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bytes dissected by your protocol rather than simply returning TRUE. If
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heuristics fail, still just return 0.
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static int dissect_PROTOABBREV(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
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{
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...
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/* 1) first byte must be 0x42 */
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if ( tvb_get_guint8(tvb, 0) != 0x42 )
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return 0;
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/* 2) second byte is a type field and only can contain values between 0x20-0x33 */
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if ( tvb_get_guint8(tvb, 1) < 0x20 || tvb_get_guint8(tvb, 1) > 0x33 )
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return 0;
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/* 3) third byte is a flag field, where the lower 4 bits always contain the value 0 */
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if ( tvb_get_guint8(tvb, 2) & 0x0f )
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return 0;
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/* 4) fourth and fifth bytes contains a 16 length field, where the value can't be longer than 10000 bytes */
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/* Assumes network byte order */
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if ( tvb_get_ntohs(tvb, 3) > 10000 )
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return 0;
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/* Assume it’s your packet and do dissection */
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...
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return number_of_bytes_dissected;
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}
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void
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proto_reg_handoff_PROTOABBREV(void)
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{
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static int PROTOABBREV_inited = FALSE;
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dissector_handle_t PROTOABBREV_handle;
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if ( !PROTOABBREV_inited )
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{
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/* register as heuristic dissector for both TCP and UDP */
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heur_dissector_add("tcp", dissect_PROTOABBREV, proto_PROTOABBREV);
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heur_dissector_add("udp", dissect_PROTOABBREV, proto_PROTOABBREV);
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/* register as normal dissector for IP as well */
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PROTOABBREV_handle = new_create_dissector_handle(dissect_PROTOABBREV, proto_PROTOABBREV);
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dissector_add("ip.proto", IP_PROTO_PROTOABBREV, PROTOABBREV_handle);
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PROTOABBREV_inited = TRUE;
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}
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}
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