This Manual describes the programming interface and the source code of
Npcap. It provides detailed descriptions of the functions and structures
exported to programmers, along with complete documentation of the Npcap
internals. Several tutorials and examples are provided as well.
Npcap is an architecture for packet capture and network analysis for
Windows operating systems, consisting of a software library and a network
Most networking applications access the network through widely-used
operating system primitives such as sockets. It is easy to access data on
the network with this approach since the operating system copes with the
low level details (protocol handling, packet reassembly, etc.) and
provides a familiar interface that is similar to the one used to read and
Sometimes, however, the “easy way” is not up to the task,
since some applications require direct access to packets on the network.
That is, they need access to the “raw” data on the network
without the interposition of protocol processing by the operating
The purpose of Npcap is to give this kind of access to Windows
applications. It provides facilities to:
- capture raw packets, both the ones destined to the machine where
it's running and the ones exchanged by other hosts (on shared media)
- filter the packets according to user-specified rules before
dispatching them to the application
- transmit raw packets to the network
- gather statistical information on the network traffic
This set of capabilities is obtained by means of a device driver,
which is installed inside the networking portion of the Windows kernel,
plus a couple of DLLs.
All of these features are exported through a powerful programming
interface, easily usable by applications. The main goal of this manual is
to document this interface, with the help of several examples.
What kind of programs use Npcap?
The Npcap programming interface can be used by many types of
network tools for analysis, troubleshooting, security and monitoring.
In particular, classical tools that rely on Npcap are:
- network and protocol analyzers
- network monitors
- traffic loggers
- traffic generators
- user-level bridges and routers
- network intrusion detection systems (NIDS)
- network scanners
- security tools
Npcap receives and sends the packets independently from the host
protocols, like TCP/IP. This means that it isn't able to block, filter or
manipulate the traffic generated by other programs on the same machine: it
simply “sniffs” the packets that transit on the wire. Therefore, it does not
provide the appropriate support for applications like traffic shapers, QoS
schedulers and personal firewalls.
Npcap is an update of WinPcap
to the NDIS 6 Light-Weight Filter (LWF) API.
It supports Windows Vista, 7, 8 and 10. It is developed
by the Nmap Project
as a continuation of the project started by Yang Luo
under Google Summer of Code 2013 and
It also received many helpful tests from Wireshark
Feature comparison with WinPcap
Npcap carries on the WinPcap legacy, but is not without its own
innovations. Here are some of the most exciting improvements and new
features that Npcap adds:
NDIS 6 Support: Npcap makes use of the new NDIS Lightweight Filter driver in
Windows Vista and later. This driver type is faster and has less overhead
than the legacy NDIS 5 Intermediate Driver
type used by WinPcap.
Latest libpcap API Support: Npcap provides support
for the latest libpcap API
by accepting libpcap as a Git submodule.
The latest libpcap 1.8 has integrated more fascinating features and functions than the
deprecated libpcap 1.0.0 shipped by WinPcap.
Moreover, since Linux already has a good support for latest libpcap API, using
Npcap on Windows facilitates your software to base on the same API on both Windows and Linux.
“Admin-only Mode” Support: Npcap supports to restrict its
use to Administrators for safety purpose. If Npcap is installed with
the option “Restrict Npcap driver's access to Administrators only” checked,
when a non-Admin user tries to start a user software (Nmap, Wireshark, etc),
the User Account Control (UAC)
dialog will prompt asking for Administrator privilege. Only when the end
user chooses Yes, the driver can be accessed. This is similar to UNIX
where you need root access to capture packets.
Loopback Packet Capture: Npcap is able to
see Windows loopback packets using the
Windows Filtering Platform (WFP). After installation, Npcap will create an
adapter named “Npcap
Loopback Adapter” for you. If you are a Wireshark user, choose this adapter
to capture, you will see all loopback traffic the same way as other
non-loopback adapters. Try it by typing in commands like ping 127.0.0.1
(IPv4) or ping ::1 (IPv6).
Loopback Packets Injection: Besides loopback packets
capturing, Npcap can also send out loopback packets using the
Winsock Kernel (WSK)
technique. A user software (e.g. Nmap) can just send packets
out using “Npcap Loopback Adapter” like other adapters.
“Npcap Loopback Adapter”
will automatically remove the packet's Ethernet header and
inject the payload into Windows TCP/IP stack.
Raw 802.11 Packets Capture Support: Npcap is able to see
802.11 packets instead of fake Ethernet packets on ordinary wireless
adapters. You need to select the
Support raw 802.11 traffic (and monitor
mode) for wireless adapters option in the installation wizard to enable
this feature. When your adapter is in “Monitor Mode”, Npcap will supply all
802.11 data + control + management packets with radiotap headers. When
your adapter is in “Managed Mode”, Npcap will only supply Ethernet
packets. Npcap directly supports to use Wireshark to capture in “Monitor Mode”.
Meantime, Npcap also provides the
tool to help you switch to “Monitor Mode” on your own. See more details
about this feature in section
“For software that uses Npcap raw 802.11 feature”.
See more details about radiotap here:
“WinPcap API-compatible Mode”
Support: “WinPcap API-compatible Mode” makes Npcap a
strict WinPcap replacement by using the same DLL location and service name as
WinPcap. This is useful for testing or migrating from software that only uses
WinPcap, but because Npcap is masquerading as WinPcap, software will not be
able to be aware of and use Npcap's newer features. It's notable that before
installing in this mode, any existing WinPcap installation will be
uninstalled and replaced.
The purpose of this manual is to provide a comprehensive and easy way
to browse the documentation of the Npcap architecture. You will find
three main sections: the section called “Npcap Users' Guide”,
the section called “Developing software with Npcap”,
and the section called “Npcap internals”.
the section called “Npcap Users' Guide” is for end users of Npcap, and
primarily concerns installation options, hardware compatibility, and bug
the section called “Developing software with Npcap” is for programmers who need to use
Npcap from an application: it contains information about functions and
data structures exported by the Npcap API, a manual for writing packet
filters, and information on how to include it in an application. A
tutorial with several code samples is provided as well; it can be used to
learn the basics of the Npcap API using a step-by-step approach, but it
also offers code snippets that demonstrate advanced features.
the section called “Npcap internals” is intended for Npcap developers
and maintainers, or for people who are curious about how this system
works: it provides a general description of the Npcap architecture and
explains how it works. Additionally, it documents the complete device
driver structure, the source code, the Packet.dll interface and the
low-level Npcap API. If you want to understand what happens inside Npcap
or if you need to extend it, this is the section you will want to
We call Npcap an architecture rather than
library because packet capture is a low level
mechanism that requires a strict interaction with the network adapter and
with the operating system, in particular with its networking
implementation, so a simple library is not sufficient.
For consistency with the literature, we will use the term
packet even though
frame is more accurate since the capture process
is done at the data-link layer and the data-link header is included in
the captured data.
Even though Npcap source code is publicly available for review, it is
not open source software and my not be redistributed without special
permission from the Nmap Project. The
allows end users to download, install, and use Npcap from our site for
free. Software providers (open source or otherwise) which want to use
Npcap functionality are welcome to point their users to npcap.org for
those users to download and install.
We fund the Npcap project by selling licenses to companies who wish
to redistribute Npcap within their products. The
Npcap OEM edition allows
companies to silently and seamlessly install Npcap during their product's
installation rather than asking users to download and install Npcap
themselves. The Npcap OEM commercial license also includes support,
updates and indemnification. This is similar to the commercial licenses
we offer for embedding Nmap in
commercial software. More details are available from the Npcap OEM page.
The latest Npcap release can always be found
on the Npcap
website as an executable installer and as a source code
Acknowledgements and copyright
Portions of this guide were adapted from the WinPcap documentation.
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