extcap - The external capture interface
NAME DESCRIPTION GRAMMAR ELEMENTS EXAMPLES TABLE FIELD SECURITY CONSIDERATIONS SEE ALSO NOTESNAME
extcap − The external capture interface
DESCRIPTION
The extcap (external capture) 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 application.
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/wireshark or extcap/stratoshark directories. Please note that scripts need to be executable without prefacing a script interpreter before the call.
WINDOWS USERS: 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.
When Wireshark launches an extcap, it automatically adds its installation path (normally C:\Program Files\Wireshark\) to the DLL search path so that the extcap library dependencies can be found (it is not designed to be launched by hand). This is done on purpose. There should only be extcap programs (executables, Python scripts, ...) in the extcap folder to reduce the startup time and not have Wireshark trying to execute other file types.
GRAMMAR ELEMENTS
Grammar elements:
arg (options)
argument for CLI calling
number
Reference # of argument for other values, display order
call
Literal argument to call (−−call=...)
display
Displayed name
default
Default value, in proper form for type
range
Range of valid values for UI checking (min,max) in proper form
type
Argument type for UI filtering for raw, or UI type for selector:
integer
unsigned
long (may include scientific / special notation)
double
string (display a textbox)
selector (display selector table, all values as strings)
editselector (selector table which can be overridden, all
values as strings)
boolean (display checkbox)
booleanflag (display checkbox)
radio (display group of radio buttons with provided values,
all values as strings)
fileselect (display a dialog to select a file from the
filesystem, value as string)
multicheck (display a textbox for selecting multiple
options, values as strings)
table (display a table that is populated by the user,
selections can be configured, values as commandlines
arguments)
password (display a textbox with masked text)
timestamp (display a calendar)
value (options)
Values for
argument selection
arg Argument # this value applies to
EXAMPLES
Example 1:
arg
{number=0}{call=−−channel}{display=Wi−Fi
Channel}{type=integer}{required=true}
arg {number=1}{call=−−chanflags}{display=Channel
Flags}{type=radio}
arg
{number=2}{call=−−interface}{display=Interface}{type=selector}
value {arg=0}{range=1,11}
value {arg=1}{value=ht40p}{display=HT40+}
value {arg=1}{value=ht40m}{display=HT40−}
value {arg=1}{value=ht20}{display=HT20}
value {arg=2}{value=wlan0}{display=wlan0}
Example 2:
arg
{number=0}{call=−−usbdevice}{USB
Device}{type=selector}
value {arg=0}{call=/dev/sysfs/usb/foo/123}{display=Ubertooth
One sn 1234}
value {arg=0}{call=/dev/sysfs/usb/foo/456}{display=Ubertooth
One sn 8901}
Example 3:
arg
{number=0}{call=−−usbdevice}{USB
Device}{type=selector}
arg {number=1}{call=−−server}{display=IP address
for log
server}{type=string}{validation=(?:\d{1,3}\.){3}\d{1,3}}
flag {failure=Permission denied opening Ubertooth
device}
Example 4:
arg
{number=0}{call=−−username}{display=Username}{type=string}
arg
{number=1}{call=−−password}{display=Password}{type=password}
Example 5: timestamp
arg
{number=0}{call=−−start}{display=Start
Time}{type=timestamp}
arg {number=1}{call=−−end}{display=End
Time}{type=timestamp}
Example 6: multicheck
arg
{number=0}{call=−−device}{display=Device}{type=multicheck}
value {arg=0}{value=USBDEV}{display=USB
devices}{enabled=false}
value
{arg=0}{value=/dev/sysfs/usb/foo/123}{display=Ubertooth One
sn 1234}{parent=USBDEV}
value
{arg=0}{value=/dev/sysfs/usb/foo/456}{display=Ubertooth One
sn 8901}{parent=USBDEV}
value {arg=0}{value=PCIDEV}{display=PCI
devices}{enabled=false}
value {arg=0}{value=/sys/devices/pci123}{display=Device
1}{parent=PCIDEV}
value {arg=0}{value=/sys/devices/pci456}{display=Device
2}{parent=PCIDEV}
TABLE FIELD
The "table" field is a bit different, in that its values can have additional configuration options. The values can either be entered manually by the user, or a list of available values can be provided using the same API as "multicheck". Unlike most APIs, the value is provided using a commandline−like API, separated by spaces instead of comas. For instance, a configured "myparam" attribute where the user selected three values (1, 2 and 3) would receive:
−−myparam "1 2 3"
If a "prefix" is configured to be "−−p", it would receive the following:
−−myparam "−−p 1 −−p 2 −−p 3"
The values selected in a table field may have specific configuration options, if "configurable=true" for the "multicheck" field. In this case, the extcap will be called again when the user presses the configuration wheel that matches a value in the table, with the additional "−−extcap−config−option−name <option_name> −−extcap−config−option−value <option_value>" with no prefix. An extcap program should respond with an additional set of arguments, which will be opened in a popup. Once those are configured, the extcap might get the following, assuming for instance that an additional "param1" is requested when value is 1, and "param2" when value is 2.
−−myparam "−−p 1 −−param1 1 −−p 2 −−param2 true −−p 3"
SECURITY CONSIDERATIONS
• If you’re running Wireshark as root, we can’t save you.
• Dumpcap retains suid/setgid and group execute permissions for users in the âwiresharkâ group only.
• Third−party capture programs run with whatever privileges they’re installed with.
• If an attacker can write to a system binary directory, it’s game over.
• You can find your local extcap directory in About › Folders.
SEE ALSO
wireshark(1), tshark(1), dumpcap(1), androiddump(1), sshdig(1), sshdump(1), randpktdump(1)
NOTES
Extcap is feature of Wireshark. The latest version of Wireshark can be found at <https://www.wireshark.org>.
HTML versions of
the Wireshark project man pages are available at
<https://www.wireshark.org/docs/man−pages>.