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man pages section 4: Device and Network Interfaces

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Updated: Wednesday, July 27, 2022

extcap (4)


extcap - The extcap interface


Please see following description for synopsis


EXTCAP(4)                                                            EXTCAP(4)

       extcap - The extcap interface

       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 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 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.

       When Wireshark launches an extcap, it automatically adds its
       installation path (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 (executable, python scripts, ...) in the extcap
       folder to reduce the startup time and not have Wireshark trying to
       execute other file types.

       Grammar elements:

       arg (options)

           argument for CLI calling


           Reference # of argument for other values, display order


           Literal argument to call (--call=...)


           Displayed name


           Default value, in proper form for type


           Range of valid values for UI checking (min,max) in proper form


           Argument type for UI filtering for raw, or UI type for selector:

               long (may include scientific / special notation)
               selector (display selector table, all values as strings)
               boolean (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)
               password (display a textbox with masked text)
               timestamp (display a calendar)

       value (options)

               Values for argument selection
               arg     Argument # this value applies to

       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

       Example 5: arg {number=0}{call=--start}{display=Start
       Time}{type=timestamp} arg {number=1}{call=--end}{display=End

       o   Users running wireshark as root, we can't save you

       o   Dumpcap retains suid/setgid and group+x permissions to allow users
           in wireshark group only

       o   Third-party capture programs run w/ whatever privs they're
           installed with

       o   If an attacker can write to a system binary directory, we're game
           over anyhow

       o   Reference the folders tab in the wireshark->about information, to
           see from which directory extcap is being run

       See attributes(7) for descriptions of the following attributes:

       |ATTRIBUTE TYPE |           ATTRIBUTE VALUE             |
       |Availability   | diagnostic/wireshark/wireshark-common |
       |Stability      | Uncommitted                           |

       wireshark(1), tshark(1), dumpcap(1), androiddump(1), sshdump(1),

       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

       Source code for open source software components in Oracle Solaris can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-

       This software was built from source available at
       https://github.com/oracle/solaris-userland. The original community
       source was downloaded from  http://www.wireshark.org/download/src/all-

       Further information about this software can be found on the open source
       community website at http://www.wireshark.org/.