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EXTCAP: DEVELOPER GUIDE
=======================
The extcap interface is a versatile plugin interface that allows external binaries
to act as capture interfaces directly in wireshark. It is used in scenarios, where
the source of the capture is not a traditional capture model
(live capture from an interface, from a pipe, from a file, etc). The typical
example is connecting esoteric hardware of some kind to the main wireshark app.
Without extcap, a capture can always be achieved by directly writing to a capture file:
the-esoteric-binary --the-strange-flag --interface=stream1 --file dumpfile.pcap &
wireshark dumpfile.pcap
but the extcap interface allows for such a connection to be easily established and
configured using the wireshark GUI.
The extcap subsystem is made of multiple extcap binaries that are automatically
called by the GUI in a row. In the following chapters we will refer to them as
"the extcaps".
Extcaps may be any binary or script within the extcap directory. Please note, that scripts
need to be executable without prefacing a script interpreter before the call.
WINDOWS USER: Because of restrictions directly calling the script may not always work.
In such a case, a batch file may be provided, which then in turn executes the script. Please
refer to doc/extcap_example.py for more information.
THE CAPTURE PROCESS
===================
The actual capture is run after a setup process that can be made manually by the
user or automatically by the GUI. All the steps performed are done for every extcap.
Let's go through those steps.
STEP1: the extcap is queried for its interfaces.
extcapbin --extcap-interfaces
This call must print the existing interfaces for this extcap and must return 0.
The output must conform to the grammar specified for extcap, and it is specified
in the doc/extcap.4 generated man page (in the build dir).
Example:
$ extcapbin --extcap-interfaces
extcap {version=1.0}
interface {value=example1}{display=Example interface 1 for extcap}
interface {value=example2}{display=Example interface 2 for extcap}
The version for the extcap sentence (which may exist as often as wanted, but only the
last one will be used) will be used for displaying the version information of the extcap
interface in the about dialog of Wireshark (Qt only).
The value for each interface will be used in subsequent calls as the interface
name IFACE.
STEP2: the extcap is queried for valid DLTs (Data Link Types) for all the
interfaces returned by the step 1.
extcapbin --extcap-dlts --extcap-interface IFACE
This call must print the valid DLTs for the interface specified. This call is
made for all the interfaces and must return 0.
Example:
$ extcapbin --extcap-interface IFACE --extcap-dlts
dlt {number=147}{name=USER1}{display=Demo Implementation for Extcap}
A binary or script, which neither provides an interface list or a DLT list will
not show up in the extcap interfaces list.
STEP3: the extcap is queried for the configuration options for an interface.
extcapbin --extcap-interface IFACE --extcap-config
Each interface can have custom options that are valid for this interface only.
Those config options are specified on the command line when running the actual
capture. To allow an end-user to specify certain options, such options may be
provided using the extcap config argument.
To share which options are available for an interface, the extcap responds to
the command --config, that shows all the available options (aka additional command
line options).
Those options are automatically presented via a dialog to the user for the individual
interface.
Example:
$ extcapbin --extcap-interface IFACE --extcap-config
arg {number=0}{call=--delay}{display=Time delay}{tooltip=Time delay between packages}{type=integer}{range=1,15}{required=true}
arg {number=1}{call=--message}{display=Message}{tooltip=Package message content}{type=string}
arg {number=2}{call=--verify}{display=Verify}{tooltip=Verify package content}{type=boolflag}
arg {number=3}{call=--remote}{display=Remote Channel}{tooltip=Remote Channel Selector}{type=selector}
arg {number=4}{call=--server}{display=IP address for log server}{type=string}{validation=\\b(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\b}
value {arg=3}{value=if1}{display=Remote1}{default=true}
value {arg=3}{value=if2}{display=Remote2}{default=false}
Now the user can click on the options and change them. They are sent to the
extcap when the capture is launched.
There are two kind of options available:
* file, integer, string, boolean, boolflag - are value based options and each expect a single value
via the command-line call
* selector, checkbox - are selections and can be presented multiple times in the command line. Both
expect subsequent "value" items in the config list, with the corresponding argument selected via arg
STEP4: the capture. Once the interfaces are listed and configuration is customized
by the user, the capture is run.
extcapbin --extcap-interface IFACE [params] --capture [--extcap-capture-filter CFILTER] --fifo FIFO
To run the capture, the extcap must implement the --capture, --extcap-capture-filter
and --fifo option.
They are automatically added by wireshark that opens the fifo for reading. All
the other options are automatically added to run the capture. The extcap interface
is used like all other interfaces (meaning that capture on multiple interfaces, as
well as stopping and restarting the capture is supported).
ARGUMENTS
==========
The extcap interface provides the possibility for generating a GUI dialog to
set and adapt settings for the extcap binary.
All options must provide a number, by which they are identified. No NUMBER may be
provided twice. Also all options must present the elements CALL and DISPLAY, where
call provides the arguments name on the command-line and display the name in the GUI.
Additionally TOOLTIP may be provided, which will give the user an explanation within
the GUI, about what to enter into this field.
These options do have types, for which the following types are being supported:
* INTEGER, UNSIGNED, LONG, DOUBLE - this provides a field for entering a numeric value
of the given data type. A DEFAULT value may be provided, as well as a RANGE
arg {number=0}{call=--delay}{display=Time delay}{tooltip=Time delay between packages}{type=integer}{range=1,15}{default=0}
* STRING - Let the user provide a string to the capture
arg {number=1}{call=--server}{display=IP Address}{tooltip=IP Address for log server}{type=string}{validation=\\b(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\b}
- validation allows to provide a regular expression string, which is used to check
the user input for validity beyond normal data type or range checks. Back-slashes
must be escaped (as in \\b for \b)
* PASSWORD - Let the user provide a masked string to the capture. Password strings are
not saved, when the extcap configuration is being saved
arg {number=0}{call=--password}{display=The user password}{tooltip=The password for the connection}{type=password}
* BOOLEAN,BOOLFLAG - this provides the possibility to set a true/false value.
BOOLFLAG values will only appear in the command-line if set to true, otherwise they
will not be added to the command-line call for the extcap interface
arg {number=2}{call=--verify}{display=Verify}{tooltip=Verify package content}{type=boolflag}
* LOGFILE - Let the user provide a filepath to the capture. If FILE_MUSTEXIST is being provided,
the GUI checks if the file exists
arg {number=3}{call=--logfile}{display=Logfile}{tooltip=A file for log messages}{type=fileselect}{file_mustexist=false}
* SELECTOR, RADIO, MULTICHECK - an optionfield, where the user may choose one or more options from.
If PARENT is provided for the value items, the option fields for MULTICHECK and SELECTOR are being
presented in a tree-like structure. SELECTOR and RADIO values must present a default value, which will
be the value provided to the extcap binary for this argument
arg {number=3}{call=--remote}{display=Remote Channel}{tooltip=Remote Channel Selector}{type=selector}
value {arg=3}{value=if1}{display=Remote1}{default=true}
value {arg=3}{value=if2}{display=Remote2}{default=false}
VALIDATION OF ARGUMENTS
=======================
Arguments may be set with "{required=true}" which enforces a value being provided, before a capture can be started
using the extcap options dialog. This is not being checked, if the extcap is started via a simple double-click. The
necessary fields are marked for the customer, to ensure a visibility for the end customer of the required argument.
Additionally text and number arguments may also be checked using a regular expression, which is provided using
the validation attribute (see example above). The syntax for such a check is the same as for Qt RegExp classes.
This feature is only active in the Qt version of Wireshark.
DEVELOPMENT
===========
To have extcap support, extcap must be enabled. Moreover the specific extcap must
be compiled. Examples for autotools and cmake compiling the extcap plugin androiddump
are provided within wireshark.
autotools: ./configure --with-extcap --enable-androiddump
cmake: cmake -DENABLE_EXTCAP=ON -DBUILD_androiddump ..
Additionally there is an example python script available in doc/extcap_example.py.
When developing a new extcap, it must be created under extcap/ directory and
added to the makefiles. Once compiled it will be under the extcap/ directory
in the build dir.
If the current extcaps configuration is copied, the new extcap will need specific
configuration flags like --enable-<newextcap> or -DBUILD_<newextcap>
Since this is a plugin, the developer must ensure that the extcap dir will be
loaded. To see which dir is loaded, they must open HELP->ABOUT->FOLDER and look
for "Extcap Path". If there is a system path (like /usr/local/lib/wireshark/extcap/)
the extcaps in the build dir are not loaded. To load them, wireshark must be
loaded with WIRESHARK_RUN_FROM_BUILD_DIRECTORY=1. Now the "Extcap path" should
point to the extcap dir under your build environment.
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