Npcap - the Nmap Project's packet sniffing library for Windows

Friday, June 3, 2016

Npcap is an update of WinPcap to NDIS 6 Light-Weight Filter (LWF) technique. It supports Windows Vista, 7, 8 and 10 . It is sponsored by the Nmap Project and developed by Yang Luo under Google Summer of Code 2013 and 2015 . It also received many helpful tests from Wireshark and NetScanTools .

  1. NDIS 6 Support : Npcap makes use of new NDIS 6 Light-Weight Filter (LWF) API in Windows Vista and later (the legacy driver is used on XP). It's faster than the deprecated NDIS 5 API, which Microsoft could remove at any time.
  2. Extra Security : Npcap can be restricted so that only Administrators can sniff packets. If a non-Admin user tries to utilize Npcap through software such as Nmap or Wireshark, the user will have to pass a User Account Control (UAC) dialog to utilize the driver. This is conceptually similar to UNIX, where root access is generally required to capture packets.
  3. WinPcap Compatibility : If you choose WinPcap Compatible Mode at install-time, Npcap will use the WinPcap-style DLL directories c:\Windows\System32 and servcie name npf , allowing software built with WinPcap in mind to transparently use Npcap instead. If compatability mode is not selected, Npcap is installed in a different location C:\Windows\System32\Npcap with a different service name npcap so that both drivers can coexist on the same system. In this case, applications which only know about WinPcap will continue using that, while other applications can choose to use the newer and faster Npcap driver instead.
  4. Loopback Packet Capture : Npcap is able to sniff loopback packets (transmissions between services on the same machine) by 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 (IPv4) or ping ::1 (IPv6).
  5. Loopback Packet Injection : Npcap is also able to send loopback packets using the Winsock Kernel (WSK) technique. User-level software such as Nping can just send the packets out using Npcap Loopback Adapter just like any other adapter. Npcap then does the magic of removing the packet's Ethernet header and injecting the payload into the Windows TCP/IP stack.
  6. Raw 802.11 Packet Capture : 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 802.11 data packets with radiotap headers. Moreover, Npcap provides the WlanHelper.exe tool to help you switch to Monitor Mode on Windows. See more details about this feature in section For softwares that use Npcap raw 802.11 feature . See more details about radiotap here:


Run installer\Build.bat : build all DLLs and the driver. The DLLs need to be built using Visual Studio 2013 . And the driver needs to be built using Visual Studio 2015 with Windows SDK 10 10586 & Windows Driver Kit 10 10586 .

Run installer\Deploy.bat : copy the files from build directories to deployment directories and sign the files. Generate an installer named npcap-nmap-%VERSION%.exe using NSIS large strings build with the SysRestore plug-in (special build for Npcap) and sign the installer.

Generating debug symbols (optional)
Run installer\Deploy_Symbols.bat : copy the debug symbol files (.PDB) from build directories to deployment directories and package them into a zip file named npcap-nmap-<VERSION> using 7-Zip .

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