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$Id$
This document is an attempt, to bring some light to the things done, when
packet capturing is performed. There might be things missing, and others
maybe wrong :-( The following will concentrate a bit on the win32 gtk
port of ethereal.
XXX: when ongoing file reorganisation will be completed, the following
two lists maybe won't be needed any longer!
libpcap related source files:
-----------------------------
pcap-util.c
pcap-util.h
pcap-util-int.h
pcap-util-unix.c
capture-wpcap.c
capture-wpcap.h
Capture related source files:
-----------------------------
capture.c
capture.h
capture-stop-conditions.c
capture-stop-conditions.h
conditions.c (XXX: is conditions generic or capture specific?)
conditions.h
Capture driver
--------------
Etheral doesn't have direct access to the capture hardware. Instead of this,
it uses the Libpcap/Winpcap library to capture data from network cards.
On Win32, in capture-wpcap.c the function g_module_open("wpcap") is called
to load the wpcap.dll. This dll includes all functions needed for
packet capturing.
Capture File
------------
There are some kind of targets to put the capture data into:
-temporary file
-user specified "single" capture file
-user specified "ringbuffer" capture file
Which kind of file is used depends on the user settings. In principle there
is no difference in handling these files, so if not otherwise notified,
it will be called the capture file.
The capture file is stored, using the wiretap library.
Start capture
-------------
A capture is started, by specifying at the command line to start the capture,
or trigger the ok button in the "Capture Options" dialog box. The capture start
is actually done by calling the do_capture() function in capture.c.
Normal capturing
----------------
In normal mode, Ethereal will open the target capture file, prepare pcap things,
init stop conditions, init the capture statistic dialog and start a loop which
is running until the flag ld.go is FALSE.
Inside this loop,
-gtk main things are updated
-pcap_dispatch(capture_pcap_cb) is called
-the capture stop conditions are checked (ld.go is set to FALSE to finish)
-update the capture statistic dialog
While this loop is running, the pcap_dispatch() will call capture_pcap_cb()
for every packet captured. Inside this, the packet data is converted into
wtap (wiretap) format and saved to file. Beside saving, it is trying to
do some basic dissecting (for the statistic window), by calling the appropriate
capture_xxx function.
When the stop button in the menu/toolbar or at the statistics dialog is pressed,
it will simply set the ld.go flag to FALSE, and the loop will finish.
Sync mode
---------
When the "update list of packets in real time" option is used for capturing,
another Ethereal instance is spawned and the two instances linked together
using a pipe. To open the pipe, a "command line" is build and a second
Ethereal instance (another process) is spawn with it. As the old instance
doesn't need it's end of the pipe, the write direction is closed immediately.
The new instance will open the capture statistic dialog, capture the data
in the normal way described above and send the captured data through the pipe
to the old instance. It will not open a main screen to show packets.
When the capture is finished (using the close button in the dialog),
the new instance closes it's side of the pipe and exits completely.
In the old instance, the cap_pipe_input_cb() function is called "cyclically"
(unix:waiting for pipe, win32:timer,1000ms) to read data from the pipe and show it
on the main screen. While this capture is in progress, no other capture file
can be opened. The closing of the pipe indicates the old instance that
the capturing is finished.
Updating
--------
The actual packet capturing inside the libpcap is done using it's own task.
Catching and processing the packet data from the libpcap is done using the
pcap_dispatch() function.
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