The Users' Guide covers the basics of installing and removing
Npcap, interactions with WinPcap, frequently asked questions,
and how to report bugs.
Because Npcap is a packet capture architecture, not merely a software
library, some aspects of installation and configuration may fall to the end
user. This Users' Guide covers the basics of installing, configuring, and
removing Npcap, as well as how to report bugs.
Npcap is distributed as a signed executable installer, downloadable from
versions are backwards-compatible, and users of the free non-commercial
version are encouraged to upgrade regularly for security and stability
fixes. Software distributors may have separate requirements for supported
Npcap versions. Please refer to
the Npcap License for
The Npcap installer and uninstaller are easy to use in
“Graphical Mode” (direct run) and “Silent Mode” (run with
/S parameter, available only with Npcap OEM).
The installer accepts several command-line options that correspond to the
options presented in the graphical interface (GUI). The options can be
set by command-line flags taking the form
The values for these options must be one of:
Graphical installer options
The following options are presented as checkboxes in the
installer, but can be set or locked via command-line flags. Unless
otherwise noted, the default for these options is
Support loopback traffic ("Npcap Loopback Adapter"
will be created). This option allows Npcap to
capture and inject traffic that the system sends to itself.
Windows does not ordinarily involve NDIS filter drivers in
loopback traffic, so a WFP callout driver is used in conjuntion
with a virtual network adapter that is a clone of the Microsoft
KM-TEST loopback adapter.
The default for this option is
Restrict Npcap driver's access to Administrators
only. When this option is chosen, the devices
created by the Npcap driver for capture and injection on each
network adapter will be created with a restrictive ACL that
only allows access to the device by the SYSTEM and built-in
Administrators. Because this level of access requires UAC
elevation, a helper binary,
is used to request elevation for each process that opens a
Support raw 802.11 traffic (and monitor mode) for
wireless adapters. This option installs a second
Lightweight Filter Driver that uses the Native WiFi API to
capture raw 802.11 WiFi frames on devices that are put into
network monitor mode. Captured frames are given a Radiotap
header. Not all hardware or network drivers support the Native
Install Npcap in WinPcap API-compatible
Mode. Npcap uses the same API and DLL names as
WinPcap, so to avoid unintentionally removing working WinPcap
installations, it places its DLLs in a different directory than
WinPcap. This option also installs the DLLs to the system
directory, so software written for WinPcap will work
seamlessly, though the new features of Npcap will not be
available. This requires removal of any old WinPcap
Command-line installation options
Some advanced or deprecated options are only available on the
command-line. Options marked
subject to removal in future versions.
/S (Silent install, Npcap OEM only)
Installs Npcap without showing any graphical windows or
prompts. Silent install is available only for Npcap OEM.
The default for this option is
yes, so the
installer will not set a system restore point. Windows may
independently create a restore point because of the driver
installation independent from this option. To ensure a
restore point is made, specify
Control termination of
processes using Npcap during upgrades or WinPcap when
/winpcap_mode=yes is chosen. See
the section called “Uninstaller options”
for more detailed discussion.
/D (destination directory)
The destination directory for installation can be overridden by
/D option, with a few restrictions. First, it will
only affect where Npcap keeps its installation logs and helper utilities.
The driver and DLLs will always be installed into the appropriate
directories below %SYSTEMROOT%\System32\. Second, the
/D must be the last option in the command, and the path
must not contain quotes. For example, to change the installation directory
C:\Path With Spaces\, the invocation would be:
<version>.exe /D=C:\Path With Spaces
Automatically start the Npcap driver at boot
time. This option defaults to
yes, because Windows expects NDIS filter
drivers to be available at boot time. If you choose to disable
this, Windows may not start networking for up to 90 seconds
Use DLT_NULL as the loopback interface's link layer
protocol instead of DLT_EN10MB. This option
yes because it is more efficient
to use the NULL data link type than to create a fake Ethernet
header for each captured packet. Certain older software using
the undocumented Packet API may be unable to handle DLT_NULL,
but we have not been made aware of any.
/vlan_support (deprecated, ignored)
Support 802.1Q VLAN tag when capturing and sending
data (currently unsupported). This feature was
disabled in 2016 to prevent a crash and has not been
The uninstaller provided with Npcap also accepts some command-line options.
/S (Silent uninstall)
Uninstalls Npcap without showing any graphical windows or
prompts. Silent uninstall is available in all editions of Npcap,
not just Npcap OEM. If Npcap OEM installer in silent mode needs
to uninstall an older Npcap installation, it passes the
/S option to the existing uninstaller.
/Q (Quick uninstall)
Skips the confirmation page and finish page in the uninstall
wizard. This option does not have any meaning for silent
<yes|no> (do not kill processes)
Controls how the uninstaller handles processes that are still using
Npcap at the time of uninstall. The default value is
no, which allows the uninstaller to terminate
processes that would block Npcap from being uninstalled. If
/no_kill=yes is specified, then Npcap
uninstaller will fail if there are still applications using Npcap
driver or DLLs.
In the default case,
graphical uninstaller will give the user the choice to manually
close the offending programs, have the uninstaller terminate
them, or abort the uninstallation. In silent mode, Npcap
uninstaller will immediately terminate any command-line processes
that are using Npcap (like a Nmap process that is still
scanning), and wait for at most 15 seconds to gracefully
terminate any GUI processes that are using Npcap (like Wireshark
UI that is still capturing). “Gracefully” means
that if you are still capturing via Wireshark, Wireshark UI will
prompt the user about whether to save the current capture before
closing. The user will have 15 seconds to save his session.
Note: although Npcap uninstaller won't
terminate Wireshark UI processes immediately, the live capture
stops immediately. This is because Wireshark UI uses command-line
dumpcap.exe to capture, and
that command-line process will be terminated immediately.
If this option is provided on the
installer command line, it will be passed to
the Npcap uninstaller when doing an upgrade or
Disabled and enforced options for GUI Mode
We may disable or enforce certain options in the installer GUI to make them unselectable. This
usually means that those options can easily cause compatibility issues and are considered
not suitable for most users, or we think we need to enforce some rules for the Npcap API. Advanced users can still change their states via command-line
parameters, which is described in following sections.
Fortunately, if a distributor wants to start the Npcap installer GUI and disable or enforce
certain options for reasons like compatibility. It can also use the four value
mechanism by setting the command-line parameters to
For example, the following command will start an installer GUI with the
dlt_null disabled and unselected:
How to use Wireshark to capture raw 802.11 traffic in “Monitor Mode”
The latest Wireshark has already integrated the support for Npcap's “Monitor Mode” capture.
If you want to use Wireshark to capture raw 802.11 traffic in “Monitor Mode”, you need to
switch on the monitor mode inside the Wireshark UI instead of using the section called “WlanHelper”.
This is because Wireshark only recognizes the monitor mode set by itself. So when you turn
on monitor mode outside Wireshark (like in
WlanHelper), Wireshark will not know the adapter
has been in monitor mode, and will still try to capture in Ethernet mode, which will get no traffic.
So after all, the correct steps are:
Install latest version Wireshark and latest version Npcap with
Support raw 802.11 traffic option checked.
Launch Wireshark QT UI (GTK version is similar), go to “Capture options”.
Then toggle the checkbox in the “Monitor Mode” column of your wireless adapter's row.
Click the “Start” button. If you see a horizontal line instead of the checkbox,
then it probably means that your adapter doesn't support monitor mode. You can use the
WlanHelper tool to double-check this fact.
To decrypt encrypted 802.11 data
packets, you need to specify the decipher key in Wireshark, otherwise
you will only see 802.11 data packets.
Stop the capture in Wireshark UI when you finishes capturing, the monitor mode
will be turned off automatically by Npcap.
Network disconnects after installing Npcap: As Microsoft states
an optional NDIS light-weight filter (LWF) driver like Npcap could cause
90-second delay in network availability. Some solutions you could try
are: 1) wait for 90 seconds; 2) disable and re-enable the adapter icon in
ncpa.cpl; 3) reboot. If this doesn't work,
please file a bug report.
Installation fails with error code
The cause is that you have “reached the maximum number of network filter
drivers”, see solution
Npcap Loopback Adapter is missing:
Npcap Loopback Adapter is actually a wrapper of Microsoft Loopback Adapter.
Such adapters won't show up in Wireshark if the
Basic Filtering Enging (BFE)
service was not running. To fix this issue, you should start this service at
manually and restart the Npcap service by running net stop npcap
and net start npcap. See details about this issue
Npcap only captures TCP handshake and teardown, but not data packets.
Some network adapters support offloading of tasks to free up CPU time for
performance reasons. When this happens, Npcap may not receive all of the
packets, or may receive them in a different form than is actually sent on the
wire. To avoid this issue, you may disable TCP Chimney, IP Checksum
Offloading, and Large Send Offloading in the network adapter properites on
Windows. See details about this issue in
#989 on our tracker.
Please report any bugs or issues about Npcap on
the Nmap Project's Issues tracker.
In your report, please provide your DiagReport output, user
software version (e.g. Nmap, Wireshark), steps to reproduce the problem, and other information
you think necessary. If your issue occurs only on a particular OS version (e.g. Win10
1511, 1607), please mention it in the report.
Npcap has provided a diagnostic utility called
It provides a lot of information including OS metadata, Npcap related files,
install options, registry values, services, etc. You can simply click the
C:\Program Files\Npcap\DiagReport.bat file to run
It will pop up a text report via Notepad (it's stored in:
Please always submit it to us if you encounter any issues.
For Vista users:
DiagReport is a script written for
and Vista doesn't have it installed by default. So if you are using Vista,
you need to install PowerShell 2.0 (KB968930) on your
system. Please download it here for x86
and here for x64.
Win7 and later systems have built-in PowerShell support and don't need
to do anything about it.
Npcap keeps track of the installation in a log file:
C:\Program Files\Npcap\install.log. Please submit it
together in your report if you encounter issues during the installation
(e.g. the installer halts).
Npcap keeps track of the driver installation (aka commands run by
NPFInstall.exe) in a log file:
C:\Program Files\Npcap\NPFInstall.log, please submit
it together in your report if you encounter issues during the driver
installation or problems with the “Npcap Loopback Adapter”.
There's another system-provided driver installation log in:
If you encounter errors during the driver/service installation, please copy
the Npcap-related lines out and send them together in
Dynamic link library (DLL) log
For problems with Npcap's regular operation, you may need to obtain a
debug log from
Packet.dll. To do this, you will
need a debug build of Npcap. If you are a Npcap developer, you can build
Packet.sln project with the
_DEBUG_TO_FILE macro defined. If you are an end user,
you can contact the Npcap development team for the latest Npcap debug
build. The debugging process will continue to append to the debug log
C:\Program Files\Npcap\Packet.log), so you may want
to delete it after an amount of time, or save your output to another
place before it gets too large.
If there is an issue with the Npcap driver, you can open an
Administrator command prompt, enter sc query
npcap to query the driver status and net start
npcap to start the driver (replace
<npf> if you
installed Npcap in “WinPcap Compatible Mode”). The command
output will inform you whether there's an error. If the driver is running
well, but the issue still exists, then you may need to check the driver's
log. Normal Npcap releases don't switch on the driver log function for
performance. Contact the Npcap development team to obtain a driver-debug
version of the Npcap installer. When you have got an appropriate
driver-debug version Npcap, you need to use DbgView
to read the Windows kernel log (which contains our driver log). You may
need to turn on DbgView before installing Npcap, if the error occurs when
the driver loads. When done, save the DbgView output to a file and submit
it in your report.
Blue screen of death (BSoD) dump
If you encountered BSoD when using Npcap, please attach the minidump
C:\Windows\Minidump\) to your report
together with the Npcap version. We may ask you to provide the full
C:\Windows\MEMORY.DMP) for further troubleshooting.