[CRACK VH Dissector Pro
Amancio Mccrae
The parameters for tvb_new_subset_length_caplen() are in the header comment. As can be seen in the comments for the subsequent functions, tvb_new_subset_length() and tvb_new_subset_remaining() the backing_length and reported_length arguments can be set to -1 to consume the rest of the source buffer.
[quote]For your 2nd query, have you put a breakpoint in the dissector making the call_dissector() call to see what it's doing, e.g. find_handle() might be returning null.[/quote]As you can see from the code I posted, the handle can't be NULL And the code does go to ...(more)
re. documentation, it is what it is. Feel free to spend your time creating extensive documentation for free for others if you so wish. Most IDE's will show preceding comments in tooltips when hovering over a function in your code.
For the tvb_new_subset_xxx() functions, the backing_offset is as described in the comment excerpt you displayed. 0 is a positive value, and indicates the subset is to begin at the start of the source tvb.
I have previously experimentedwith creating Wireshark dissectors in Lua. The dissector I made back then was for a networkprotocol. Wireshark can also sniff USB traffic, so I thought it would be interesting to take a lookat that too.
The packets are called reports when using HID (described below). My PC is the host and the mouseis 1.7.1. The source name seems to vary between captures. I had 1.2.1 in the first screenshotand 1.7.1 in the second. The host will send an ack report for each report it receives from thedevice, back to the device. The filter I use in the screenshot will filter out the ack reports, asthey are just noise. USB is usually little endian, by the way.
Now that we know how the buttons and scroll wheel work, we can create the first dissector. We canpretty much copy/paste the boilerplate code from the previous network dissector series. Thedissector will then look like this:
It was fairly easy to figure out how the mouse buttons and scroll wheel works without looking at any formalspecifications. The mouse movement seems to be related to either the second and third byte, orthe fifth to eight bytes. So in order to figure out how to get the mouse position we have to lookup how the USB protocol actually works.
USB devices such as keyboards and mice use something called HID (Human Interface Devices). HID is kind ofa protocol on top of USB that provides a standardized way for keyboards and mice to communicate withthe host. My mouse uses HID rather than a proprietary Logitech driver when communicating with my PC.
If you are interested in reading more about HID can do that hereor here. In summary, when the mouse getsconnected to the PC it will send a report descriptor to it that tells the PC how the mouse willsend data. For instance, what does the first byte represent, what does the second represent, and soon. How to find the report descriptors can be read about here. Thereport descriptor for my mouse looks like this:
The unit of Report Size is bit. This usage page says that the button state is8 (Report Count) * 1 (Report Size) = 8 bits, which is one byte. One bitrepresents button state for one button. The next section looks like this:
I parse four new fields: vendor1, vendor2, x_offset and y_offset. vendor1 and vendor2 arenot very interesting. I think they are X and Y offset with 8-bit resolution. They might be there forbackwards compatibility. x_offset is the movement of the mouse in X direction compared to previousupdate. y_offset is the same, but in Y direction. I know they are offsets because the report descriptorsaid the positions were relative. There are also devices that can output absolute positions (touch screensfor instance), but my mouse gives relative position.
It contains an array of packet objects, meaning each packet/report is an object. There is a subobject called _source, that contains a sub object called layers, that contains the usb_mouseobject that I am interested in. Here is a Python script that parses the JSON file and plots the Xand Y positions:
The JSON file is open and deserialized into a list (packets). I use list comprehension to storeall the offsets in x_offsets and y_offsets. As mentioned before, the offsets are distances awayfrom the previous position. In order to get the actual position, we have to sum up the offsets withitertools.accumulate().
I am trying to change the way the mpls dissector dissects packets, but I want to do it without changing the source code. I was thinking of making a plugin, but I don't know if I will be able to hijack the default mpls dissector. Is this possible?
This is generally possible with Lua. You can implement a Lua Chained Dissector that allows you to implement your own dissector either before or after the built-in dissector is called, which is entirely up to you as to how you implement your chained Lua dissector. Whether this is possible to do specifically with mpls though, I'm not entirely sure, because the mpls dissector registers for many different protocols, so depending on your needs, your chained dissector would need to do the same to ensure that it's called for the same protocols that carry mpls. Have a look at the epan/dissectors/packet-mpls.c:proto_reg_handoff_mpls() function to see all the places the mpls dissector registers, then intercept those you're concerned with.
in the question How can I decrypt SSL session with Lua dissector, a custom Lua dissector is added to the dissector table "usb.bulk". My usecase is similar: I need to dissect a proprietary protocol on top of the USB protocol. But the following instruction (as seen in that question) doesn't seem to do the trick:
Normally I would replace the dissector with my own, keeping a reference to the original one and call that explicitly if needed (like described here) - but I do not know which dissector(s) from that DissectorTable I need to replace. So I am stuck with the following questions:
Handling of USB captures is a bit complex in terms that the possibility to choose a dissector for the payload automatically often depends on whether the enumeration phase has been captured or not, because the "integer" dissector tables refer to values of fields of USB descriptors which are only transferred over the bus during the enumeration phase.
So in the particular case of that other Question, the 0xff (or 255) value inserted into the usb.bulk integer dissector table probably matches the value of bInterfaceClass from the interface descriptor. That table is, as its name suggests, only consulted for USB bulk transfers.
For payloads which follow some characteristic patterns, the choice of dissector is slightly easier, because it is possible to use heuristic to choose the proper dissector even if the enumeration phase is missing in the capture. A heuristic dissector also needs to be registered for a transport protocol, and all heuristic dissectors registered for a given transport are tried on all payload PDUs of that transport until one of them succeeds, with the following exception: if a heuristic dissector for protocol X succeeds on a PDU, it may declare that PDU to be part of a "conversation" of protocol X; doing so makes the transport protocol's dissector invoke the dissector for protocol X on further PDUs with identical address attributes (in our case, the USB address B.D.E - Bus.Device.Endpoint) directly rather than attempt all registered heuristic dissectors again. So for USB bulk transfers, you would register your protocol's heuristic dissector as one for transport usb.bulk. I'm simplifying here a bit because I don't know whether you actually need the details.
So the first things to do for you is to check what type of USB transfer your proprietary protocol is using, whether the pattern is unambiguous enough that it would be safe to base a heuristic on it. If you cannot rely on heuristic and the transfer used is bulk, find out what bInterfaceClass value your USB device is sending in its GET DESCRIPTOR Response CONFIGURATION answer during enumeration phase. For other transfer types, another field of another message may have been chosen as selector.
Thank you for your profound answer! It was the 'bInterfaceClass' field I was looking for. When I add my dissector to the dissector table with the value that my capture data contains (0xffff), it gets called properly! If I need to distinguish between different message types, I can ask the development team to provide more specific values for bInterfaceClass.
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Yes, this is possible to create a bluetooth profile or service dissector, but I am not sure you'll get a lot of information on creating wireshark dissectors on this forum. I'd recommend asking/checking out the wireshark wiki and forums, as well as generic guides for creating a dissector (like the one I found here)
yes, I understand. You may give me information about the Nordic Dissector. Is the BLE GATT/GAP decoding done by wireshark integrated dissector on top of the nordic dissector or is it done by the Nordic dissector itself?
However, this only handles time stamping, version checking of the nrf-sniffer, and other added metadata that is piped into wireshark. The actual dissection of bluetooth LE is performed by the generic btle dissector: -btle.c
Dissectors are meant to analyze some part of a packet's data. They are similar to their older brothers written in C. Note that Heuristic Dissectors and Post-Dissectors operate differently, and are described separately. This section only refers to pure Dissectors.
For case (3), your dissector should try to perform some sanity checking of an early field if possible. If the sanity check fails, then ignore this packet and wait for the next one. "Ignoring" the packet means returning the number 0 from your dissector.
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