To build software that uses Npcap, use the latest version of the Npcap Software Development Kit (SDK). The latest SDK can be downloaded on Npcap.org. Updates to the SDK are much less frequent than updates to the Npcap binaries.
Examples of applications using Npcap are available in the Examples directory in the source distribution. Several of these examples are explored in more depth in the the section called “Npcap Development Tutorial”.
Npcap developer Yang Luo has also provided an example: UserBridge, which is a tool to redirect all packets from one interface to another.
For the most part, Npcap is completely compatible with software written for WinPcap. Minor changes need to be made to the section called “DLL loading” and in some cases the section called “Service name”. However, there have been many improvements to the libpcap API between the last release of WinPcap and the current release of Npcap. Reviewing the changes may help improve performance, reliability, and maintainability of software that uses Npcap.
Apart from the libpcap API, WinPcap exported a few functions used by
WinDump that were
related to porting a Unix-style tool to Windows but unrelated to packet
capture. Those functions were not documented in the WinPcap
documentation, have never been included in libpcap, and are therefore not
in the Npcap API: getservent, endservent, and
eproto_db.
One other function exported by WinPcap, wsockinit, is
available via the Npcap API as pcap_wsockinit. It calls
WSAStartup for Windows Sockets version 1.1 and ensures that
WSACleanup is called when the process ends.
Sometimes, our user software needs to detect the existence of Npcap/WinPcap at install-time or run-time. Although Npcap's GUI installer has the ability to handle this, you may want to handle it by yourself in some conditions, like you run Npcap installer in silent-mode. The run-time detection is even more useful. Your software probably has some functions that rely on Npcap's particular features (like loopback capture). You need to know if you are running on top of Npcap or the legacy WinPcap to control whether to switch your functions on. Fortunately, Npcap provides you some methods to detect Npcap/WinPcap at install-time and run-time.
Npcap has a version number that is independent of WinPcap. The last release of WinPcap was version 4.1.3, but Npcap started over counting versions from 0.00. In order to make it clear to the installers and other software that Npcap is newer and more advanced, the executable “file version” was advanced to “5.0.0.000” at that point. The major version will always be “5” to distinguish Npcap from WinPcap. The minor version is Npcap's major version; the revision is Npcap's minor version; and the build number is an encoding of the build date. So a file version of “5.0.92.612” is Npcap 0.92, built on June 12th.
You can check the existence of C:\Program Files\Npcap\NPFInstall.exe to
detect Npcap's existence. If Npcap exists, you can check the file version of
C:\Program Files\Npcap\NPFInstall.exe to detect Npcap e-version. The
e-version also gives you the version. The NSIS code is shown below. $inst_ver
is an e-version string like “5.0.7.424”
GetDllVersion "C:\Program Files\Npcap\NPFInstall.exe" $R0 $R1 IntOp $R2 $R0 / 0x00010000 IntOp $R3 $R0 & 0x0000FFFF IntOp $R4 $R1 / 0x00010000 IntOp $R5 $R1 & 0x0000FFFF StrCpy $inst_ver "$R2.$R3.$R4.$R5"
You can check the installation options of an already installed Npcap by reading the registry
key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\npcap\Parameters.
The entries like AdminOnly,
LoopbackSupport, DltNull,Dot11Support,
VlanSupport, WinPcapCompatible, etc.
are REG_DWORD type. A 0x00000001 value
indicates the installation option is CHECKED.
Note: Prior to Npcap 0.93, these values were stored in the
Services\npcap key directly.
Npcap and WinPcap can be installed together on a system. Which capture
library is used by the user software relies on the DLL loading path. If
Npcap's wpcap.dll is loaded first, then you are using
Npcap, vice versa. However, it's difficult and fragile to check the DLL
loading path by yourself. Fortunately, you can use
pcap_lib_version to get the Npcap/WinPcap version
string.
char *pcap_version = pcap_lib_version();
printf("%s", pcap_version);
// Npcap output: "Npcap version 0.92, based on libpcap version 1.8.1"
// WinPcap output: "WinPcap version 4.1.3"
Npcap requires the npcap service to be running. If
installed in “WinPcap Compatible Mode”, the
npf service can be started instead. Given that
npcap service is always installed
in both modes, a good practice is just trying the npcap service first.
If it fails, then try the npf service. This is also what most of our users
do in their software based on our investigation. A code sample from Nmap is
here.
Prerequisite: Uncheck the Install Npcap in WinPcap API-compatible Mode option at
install-time (which is by default).
Npcap installs its DLLs into C:\Windows\System32\Npcap\
instead of WinPcap's C:\Windows\System32\. Because of how Windows'
DLL search path works,
your application will use WinPcap first by default when Npcap and WinPcap coexist,
as C:\Windows\System32\ is prior to C:\Windows\System32\Npcap\.
So when Npcap and WinPcap coexist, an application that want to use Npcap instead
of WinPcap must make C:\Windows\System32\Npcap\ precedent to the
C:\Windows\System32\ in the DLL search path. Here are two ways
to modify this search path to make your application load Npcap's DLLs first,
based on how your application links Npcap/WinPcap's library
(wpcap.dll).
Implicit linking means that either you specified wpcap.lib
in your Project Properties -> Configuration Properties
-> Linker -> Input -> Additional Dependencies in Visual Studio,
or specified #pragma comment(linker, "wpcap.lib") in your code.
You need to do the following two steps:
Specify wpcap.dll as a delay-loaded DLL: In
Visual Studio, open the Project Properties window. Go to:
Configuration Properties -> Linker -> Input
-> Delay Loaded Dlls. Enter wpcap.dll
in that option.
Before calling any wpcap.dll functions,
call SetDllDirectory to add C:\Windows\System32\Npcap\
to DLL search path.
Here is an example called WinDump, a simple packet capture tool using Npcap/WinPcap. And this commit makes it able to use Npcap first when Npcap and WinPcap coexist.
Explicit linking means that you explicitly called LoadLibrary
to load wpcap.dll and called GetProcAddress to get the
function pointers.
You need to do the following one step:
Before calling LoadLibrary to load wpcap.dll,
call SetDllDirectory to add C:\Windows\System32\Npcap\
to DLL search path.
The function init_npcap_dll_path is provided in the following example:
WinDump
Because Npcap is a NDIS 6 LWF filter driver it is designed to run at system boot, so software will generally not need to start it, unlike WinPcap which was often installed in a demand-start configuration.
Npcap uses service name “npcap” instead of WinPcap's “npf” with “WinPcap Compatible Mode” OFF. So applications using net start npf for starting service must change to this: run net start npcap first, if it fails, then try net start npf.
Npcap 0.9983 and newer support loopback traffic capture and injection without requiring a particular installation option.
Npcap's loopback adapter device is reported by
pcap_findalldevs() as
“\Device\NPF_Loopback”. This name is always available even
if “Legacy loopback support” was chosen at install time,
which puts the name of the legacy loopback adapter in the
LoopbackAdapter REG_SZ value of the
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\npcap\Parameters.
Registry key.
Traffic captured and injected on the loopback adapter uses the
DLT_NULL data link type, which consists of a 4-byte
header in host byte order that is either 2 for IPv4 packets or 24 for
IPv6 packets.
The MTU of “Npcap Loopback Adapter” is hard-coded to 65536 by Npcap. Software using Npcap should get this value automatically and no special handling is needed. This value is arbitrary and does not imply a limitation on the Windows loopback stack, so it may be possible to capture packets with a size larger than the adapter's MTU.
Don't try to make OID requests to “Npcap Loopback Adapter” except
OID_GEN_MAXIMUM_TOTAL_SIZE (MTU). Those requests will still succeed like
other adapters do, but they only make sense for NDIS adapters and Npcap doesn't even use the
NDIS way to handle the loopback traffic. The only handled OID request by Npcap is
OID_GEN_MAXIMUM_TOTAL_SIZE. If you query its value, you will always get
65550 (65536 + 14). If you try to set its value, the operation will always fail.
If you use IP Helper API to get adapter list, you will get an interface named
like “Loopback Pseudo-Interface 1”. This interface is a DUMMY interface by Microsoft
and can't be seen in NDIS layer. And it also takes the 127.0.0.1/::1 IP address. A good practice
for software is replacing the AdapterName of the
“Loopback Pseudo-Interface 1” entry with
“NPF_Loopback”, as Nmap does in its enhancements to
libdnet.
“Legacy loopback support” installs a copy of the
Microsft KM-TEST loopback adapter named “Npcap Loopback
Adapter” for software that expects to find the loopback adapter
via ordinary Windows API calls. The features and operation are no
different from standard loopback support, but the
name of the adapter will be written to the
LoopbackAdapter Registry value.
Prerequisite: Check the Support raw 802.11 traffic (and monitor mode) for wireless adapters option at install-time.
Install the latest version Npcap with the
Support raw 802.11 traffic (and monitor mode) for wireless
adapters option checked in the installation wizard. With this
option checked, Npcap will see packets with Radiotap +
802.11 headers for wireless adapters. Otherwise, Npcap will
see packets with fake Ethernet headers for wireless
adapters.
Run WlanHelper.exe with
Administrator privilege. If you use
-i, follow the interactive prompts to choose your
wireless adapter and select “Network Monitor” mode.
WlanHelper.exe also supports parameters to be used
in an API manner, run WlanHelper.exe -h for
details.
Use the Npcap API from your user software as usual. For example, launch Wireshark and capture on the wireless adapter, viewingall 802.11 packets (data + control + management).
If you need to return to “Managed Mode”, run WlanHelper.exe again, following the prompts or selecting the appropriate command-line options to switch off the “Monitor Mode”.
You can use WlanHelper.exe tool to
switch on the “Monitor Mode” in order to see
802.11 control and management packets. You
can also use the pcap_set_rfmon function within your
code, as Wireshark does.
Switching on the “Monitor Mode” will
disconnect your wireless network from the AP, you can switch back to
“Managed Mode” using the same
WlanHelper.exe tool.
The WlanHelper.exe tool is
installed to “%SYSTEMROOT%\System32\Npcap” after installing Npcap.
“Managed Mode” (for Linux) = “Extensible Station Mode” (aka “ExtSTA”, for Windows)
“Monitor Mode” (for Linux) = “Network Monitor Mode” (aka “NetMon”, for Windows)
“Master Mode” (for Linux) = “Extensible Access Point” (aka “ExtAP”, for Windows)
WlanHelper is used to set/get the operation mode (like “Monitor
Mode”) for a wireless adapter on Windows. WlanHelper tries to
follow the grammar of iwconfig, a wireless
management tool for Linux. So if you rename
WlanHelper.exe to iwconfig.exe,
your command lines for WlanHelper will be exactly the same with the
iwconfig tool.
Note: WlanHelper must run under Administrator privilege.
Run netsh wlan show interfaces, get the Name or GUID for the interface.
Run WlanHelper -h to see the man page.
Example 1. WlanHelper Man
C:\> WlanHelper.exe
WlanHelper for Npcap 0.91 ( https://npcap.com )
Usage: WlanHelper [Commands]
or: WlanHelper {Interface Name or GUID} [Options]
OPTIONS:
mode : Get interface operation mode
mode <managed|monitor|master|..> : Set interface operation mode
modes : Get all operation modes supported by the interface, comma-separated
channel : Get interface channel
channel <1-14> : Set interface channel (only works in monitor mode)
freq : Get interface frequency
freq <VALUE> : Set interface frequency (only works in monitor mode)
modu : Get interface modulation
modu <dsss|fhss|irbaseband|ofdm|hrdsss|erp|ht|vht|ihv (VALUE)|..> : Set interface modulation
modus : Get all modulations supported by the interface, comma-separated
COMMANDS:
-i : Enter the interactive mode
-h : Print this help summary page
OPERATION MODES:
managed : The Extensible Station (ExtSTA) operation mode
monitor : The Network Monitor (NetMon) operation mode
master : The Extensible Access Point (ExtAP) operation mode (supported from Windows 7 and later)
wfd_device : The Wi-Fi Direct Device operation mode (supported from Windows 8 and later)
wfd_owner : The Wi-Fi Direct Group Owner operation mode (supported from Windows 8 and later)
wfd_client : The Wi-Fi Direct Client operation mode (supported from Windows 8 and later)
802.11 MODULATIONS (https://en.wikipedia.org/wiki/IEEE_802.11):
802.11-1997 : dsss, fhss
802.11a : ofdm
802.11b : dsss
802.11g : ofdm
802.11n : mimo-ofdm
802.11ac : mimo-ofdm
EXAMPLES:
WlanHelper Wi-Fi mode
WlanHelper 42dfd47a-2764-43ac-b58e-3df569c447da channel 11
WlanHelper 42dfd47a-2764-43ac-b58e-3df569c447da freq 2
WlanHelper "Wireless Network Connection" mode monitor
SEE THE MAN PAGE (https://github.com/nmap/npcap) FOR MORE OPTIONS AND EXAMPLES
An example:
Example 2. WlanHelper API Usage
C:\>netsh wlan show interfacesThere is 1 interface on the system: Name :Wi-FiDescription : Qualcomm Atheros AR9485WB-EG Wireless Network Adapter GUID :42dfd47a-2764-43ac-b58e-3df569c447daPhysical address : a4:db:30:d9:3a:9a State : connected SSID : LUO-PC_Network BSSID : d8:15:0d:72:8c:18 Network type : Infrastructure Radio type : 802.11n Authentication : WPA2-Personal Cipher : CCMP Connection mode : Auto Connect Channel : 1 Receive rate (Mbps) : 150 Transmit rate (Mbps) : 150 Signal : 100% Profile : LUO-PC_Network Hosted network status : Not available C:\>WlanHelper.exemanaged C:\>wi-fimodeWlanHelper.exeSuccess C:\>wi-fimode monitorWlanHelper.exemonitor C:\>wi-fimodeWlanHelper.exeSuccess C:\>wi-fimode managedWlanHelper.exemanagedwi-fimode