Wireshark-dev: Re: [Wireshark-dev] How to call a sub-dissector many times without knowing the e
From: wsgd <[email protected]>
Date: Sun, 01 Feb 2015 11:01:08 +0100
Hello Guy, Le 31/01/2015 22:56, Guy Harris a écrit :
On Jan 31, 2015, at 1:46 PM, wsgd <[email protected]> wrote:I want to call a sub-dissector many times. I know the total size of my data. I do NOT know the size to give to the sub-dissector. The sub-dissector will know (by itself) the size to dissect. The corresponding scheme from the parent dissector point of view : |-----------------------------------Packet data (total size is known)--------------------------------| |--Parent dissector--|--sub-dissector 1 (size is unknown)--|...|--sub-dissector N (size is unknown)--| The sub-dissector will be called by dissector_try_uint (or call_dissector or eventually dissector_try_heuristic). Question 1) Is it possible to call dissector_try_uint (or ...) with more data than needed by the sub-dissector ?Yes.Question 2) How to know how many bytes have been dissected by the sub-dissector ?Have the sub-dissector be a "new-style" dissector, which is expected to return the number of bytes of data that it dissected.
For now, I will care only about "new-style" dissector.
Looking at packet.h, call_dissector returns the return value of a new-style dissector : okFor "old-style" dissectors, which do not return a value, the dissector-calling mechanism will return tvb_captured_length(), so it appears as if the entire tvbuff handed to it was dissected. dissector_try_uint(), call_dissector(), etc. return the return value of a new-style dissector or the tvbuff captured length of an old-style dissector.
Looking at packet.h, dissector_try_uint returns a gboolean, not a size. So, instead of calling dissector_try_uint, I suppose I can do : - dissector_get_uint_handle - call_dissector I will try this. Thanks, Olivier
Note that a new-style dissector can return 0, which is assumed to mean "this isn't a packet for my protocol". This does *not* work for heuristic dissectors. Perhaps heuristic dissectors could be changed to return a bytes-dissected value rather than a Boolean, with 0 meaning "I dissected nothing because this isn't a packet for my protocol"; I think there were cases for non-heuristic dissectors where nothing is dissected but the dissection was valid, e.g. an RPC-like protocol where a reply has no data at the RPC layer but has data at the layer of the particular RPC protocol being used, and I think I discovered that when trying to make all dissectors new-style dissectors, but I can't remember what I found. For a heuristic dissector, however, the dissector has to look at *some* data in order to determine whether the packet is for its protocol or not, so presumably it could never return 0 if the packet is for its protocol. ___________________________________________________________________________ Sent via: Wireshark-dev mailing list <[email protected]> Archives: http://www.wireshark.org/lists/wireshark-dev Unsubscribe: https://wireshark.org/mailman/options/wireshark-dev mailto:[email protected]?subject=unsubscribe
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