EXTCAP(4)

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version 4.7.4 · 17 versions
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💖 EXTCAP(4) 💖

okay bestie, so this is the manual page for extcap — the external capture interface — from the Wireshark fam. it's giving "esoteric hardware meets main character energy" and honestly? iconic. let's get into it.

✨ NAME ✨

extcap — The external capture interface. periodt.

🦋 DESCRIPTION 🦋

so the extcap (external capture) interface is this super versatile plugin interface that lets external binaries act as capture interfaces directly inside Wireshark. it's for those scenarios where the capture source is not your traditional live-from-an-interface, pipe, or file situation. the typical example? connecting some kind of esoteric hardware to the main Wireshark application. iykyk.

without extcap, you can always get a capture by directly writing to a capture file, like:

the-esoteric-binary --the-strange-flag --interface=stream1 --file dumpfile.pcap &
wireshark dumpfile.pcap

but the extcap interface lets you establish and configure that connection easily using the Wireshark GUI. lowkey a glow-up.

the extcap subsystem is made of multiple extcap binaries that are automatically called by the GUI in a row. in the following chapters we refer to them as "the extcaps". 🫶

extcaps may be any binary or script within the extcap/wireshark or extcap/stratoshark directories. please note: scripts need to be executable without prefacing a script interpreter before the call. no python script.py nonsense — it's gotta run on its own.

WINDOWS USERS 💀: because of restrictions, directly calling the script may not always work. in that case, a batch file may be provided that then in turn executes the script. please refer to doc/extcap_example.py for more info.

when Wireshark launches an extcap, it automatically adds its installation path (normally C:\Program Files\Wireshark\) to the DLL search path so 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 startup time and not have Wireshark trying to execute other file types. respect the vibe check. ✨

🎀 GRAMMAR ELEMENTS 🎀

here are the grammar elements, bestie — memorize them like your fave song lyrics:

  • 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)
    • folderselect (display a dialog to select a folder 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}

🧚 FILE AND FOLDER SELECT FIELDS 🧚

the "fileselect" and "folderselect" fields accept these additional attributes, bestie:

  • mustexist — if set to "true", the selected file or folder must already exist on disk (checked both in the GUI and, for the default value, on startup). defaults to "false", in which case the argument is treated as the target of a save operation and need not exist yet.
  • fileext — a file extension filter, in the same form accepted by Qt’s file dialogs (e.g. Capture Files (*.pcap *.pcapng)). only used by "fileselect".

Example:

arg {number=0}{call=--logdir}{display=Output Location}{type=folderselect}{mustexist=false}

🔥 TABLE FIELD 🔥

the "table" field is a bit different — 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 commas. 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 👑

okay this is the serious part, no cap — pay attention:

  • if you’re running Wireshark as root, we can’t save you. 💀
  • Dumpcap retains suid/setgid and group execute permissions for users in the “wireshark” 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 a 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.

and that's the whole page, bestie! 💅✨ this version is dated 2026-10-08 — so like, future vibes only. go forth and extcap responsibly. 🫶🎀

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