forked from osmocom/wireshark
![Guy Harris](/assets/img/avatar_default.png)
We aren't using them now; stick to libpcap APIs (including Windows-only libpcap APIs). Change-Id: I812eaa31ba1e6e611418853105d3e00c9130a420 Reviewed-on: https://code.wireshark.org/review/37852 Petri-Dish: Guy Harris <gharris@sonic.net> Tested-by: Petri Dish Buildbot Reviewed-by: Guy Harris <gharris@sonic.net>
139 lines
4.8 KiB
Text
139 lines
4.8 KiB
Text
This document is an attempt, to bring some light to the things done, when
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packet capturing is performed. There might be things missing, and others
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maybe wrong :-( The following will concentrate a bit on the Windows
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port of Wireshark.
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XXX: when ongoing file reorganization will be completed, the following
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two lists maybe won't be needed any longer!
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libpcap related source files:
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-----------------------------
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capture-pcap-util.c
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capture-pcap-util.h
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capture-pcap-util-int.h
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capture-pcap-util-unix.c
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capture-wpcap.c
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capture-wpcap.h
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Capture related source files:
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-----------------------------
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capture.c
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capture.h
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capture_loop.c
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capture_loop.h
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capture_opts.c
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capture_sync.c
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capture_ui_utils.c
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capture_ui_utils.h
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Capture driver
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--------------
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Wireshark doesn't have direct access to the capture hardware. Instead of this,
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it uses the Libpcap/Winpcap library to capture data from network cards.
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On Win32, in capture-wpcap.c the function ws_module_open("wpcap.dll") is called
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to load the wpcap.dll. This dll includes all functions needed for
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packet capturing.
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Capture File
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------------
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There are some kinds of targets to put the capture data into:
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-temporary file
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-user specified "single" capture file
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-user specified "ringbuffer" capture file
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Which kind of file is used depends on the user settings. In principle there
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is no difference in handling these files, so if not otherwise notified,
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it will be called the capture file.
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The capture file is stored, using the wiretap library.
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Overview
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--------
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Capturing is done using a two task model: the currently running (parent)
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process will spawn a child process to do the real capture work, namely
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controlling libpcap. This two task model is used because it's necessary
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to split the capturing process (which should avoid packet drop) from the parent
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process which might need significant time to display the data.
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When a capture is started, the parent builds a "command line" and creates a
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new child process with it. A pipe from the child to the parent is created
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which is used to transfer control messages.
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The child will init libpcap and send the parent a "new capture file is used"
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control message through the pipe.
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The child cyclically takes the packet data from libpcap and saves it to disk.
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From time to time it will send the parent a "new packets" control message.
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If the parent process receives this "new packets" message and the option
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"Update list of packets in real time" is used, it will read the packet data
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from the file, dissect and display it.
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If the user wants to stop the capture, this can be done in two ways: by
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menu/toolbar of the parent process or the Stop button of the child processes
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dialog box (which obviously cannot be used it this dialog is hidden).
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The Stop button will stop the capture itself, close the control pipe and then
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closes itself. The parent will detect this and stop its part of the capture.
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If the menu/toolbar is used, the parent will send a break signal to the child
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which will lead to the same sequence as described above.
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Win32 only: as the windows implementation of signals simply doesn't work,
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another pipe from the parent to the child is used to send a "close capture"
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message instead of a signal.
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Start capture
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-------------
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A capture is started, by specifying to start the capture at the command line,
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trigger the OK button in the "Capture Options" dialog box and some more. The
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capture start is actually done by calling the capture_start() function in
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capture.c.
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Capture child (Loop)
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--------------------
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The capture child will open the target capture file, prepare pcap things,
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init stop conditions, init the capture statistic dialog (if not hidden) and
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start a loop which is running until the flag ld.go is FALSE.
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Inside this loop,
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-Qt main things are updated
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-pcap_dispatch(capture_pcap_cb) is called
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-the capture stop conditions are checked (ld.go is set to FALSE to finish)
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-update the capture statistic dialog (if not hidden)
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While this loop is running, the pcap_dispatch() will call capture_pcap_cb()
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for every packet captured. Inside this, the packet data is converted into
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wtap (wiretap) format and saved to file. Beside saving, it is trying to
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do some basic dissecting (for the statistic window), by calling the
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appropriate capture_xxx function.
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When the user triggered a capture stop or one of the capture stop conditions
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matched, the ld.go flag is set to FALSE, and the loop will stop shortly after.
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Capture parent
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--------------
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In the capture parent the cap_pipe_input_cb() function is called "cyclically"
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(unix:waiting for pipe, win32:timer,1000ms) to read data from the pipe and show
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it on the main screen. While the capture is in progress, no other capture file
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can be opened.
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Updating
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--------
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The actual packet capturing inside the libpcap is done using its own task.
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Catching and processing the packet data from the libpcap is done using the
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pcap_dispatch() function.
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