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NPcap

NPcap is an update of WinPcap to NDIS 6 Light-Weight Filter (LWF) technique. It is sponsored but not officially supported by the Nmap Project and Google Summer of Code (2013 and 2015).

Features

  1. NDIS 6 Support: NPcap makes use of new LWF driver in Windows 7 and later (the legacy driver is used on XP and Vista), it's faster than the old NDIS 5 Intermediate technique, One reason is that packet data stucture has changed (from NDIS_PACKET to NET_BUFFER_LIST) since Vista and NDIS 5 needs to handle extra packet structure conversion.
  2. "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.
  3. Coexist With WinPcap: NPcap can coexist with WinPcap and share the DLL binary interface with WinPcap. So the applications unaware of NPcap SHOULD be able to use NPcap automatically if WinPcap is unavailable. The applications who knows NPcap's existence can choose to use NPcap or WinPcap first.

Architecture

NPcap tries to keep the original WinPcap architecture as much as possible. As the table shows, you will find it very similar with WinPcap.

File                 Src Directory            Description
wpcap.dll            wpcap                    the same with WinPcap
packet.dll           packetWin7\Dll           changed driver name, add `Admin-only mode` here
npcap.sys            packetWin7\npf           port from NDIS 5 to NDIS 6, the name is changed from npf.sys to npcap.sys
NPFInstall.exe       packetWin7\NPFInstall    a lwf driver installation tool we added to NPcap
NPcapHelper.exe      packetWin7\Helper        the helper program for `Admin-only mode`, will run under Administrator rights

Build

  • wpcap.dll, packet.dll, NPFInstall.exe and NPcapHelper.exe need to be built using Visual Studio 2008 with Windows SDK 8.1.
  • npcap.sys needs to be built using Visual Studio 2013 with Windows Driver Kit 8.1.

Packaging

NPcap uses NSIS script to package itself. The script location is: installer\NPcap-for-nmap.nsi. Compiling this script will generate the installer named npcap-nmap-%VERSION%.exe. The prebuilt installer is in my SVN repository, which can be used to test without building it.

Before compiling the script, make sure you copied the files need packaging into right folders:

XP: (the same with WinPcap)
  x86:
    installer\npf.sys
    installer\rpcapd.exe
    installer\wpcap.dll
    installer\nt5\x86\Packet.dll
  x64:
    installer\x64\npf.sys
    installer\x64\wpcap.dll
    installer\nt5\x64\Packet.dll
    
Vista: (the same with WinPcap)
  x86:
    installer\npf.sys
    installer\rpcapd.exe
    installer\wpcap.dll
    installer\vista\x86\Packet.dll
  x64:
    installer\x64\npf.sys
    installer\x64\wpcap.dll
    installer\vista\x64\Packet.dll
    
Win7 and later (with "Admin-only mode" OFF):
  x86:
    installer\win7_above\x86\npcap.cat
    installer\win7_above\x86\npcap.inf
    installer\win7_above\x86\npcap.sys
    installer\win7_above\x86\NPFInstall.exe
    installer\win7_above\x86\Packet.dll
    installer\wpcap.dll
  x64:
    installer\win7_above\x64\npcap.cat
    installer\win7_above\x64\npcap.inf
    installer\win7_above\x64\npcap.sys
    installer\win7_above\x64\NPFInstall.exe
    installer\win7_above\x64\Packet.dll
    installer\x64\wpcap.dll
    
Win7 and later (with "Admin-only mode" ON):
  x86:
    installer\win7_above\x86\admin_only\npcap.cat
    installer\win7_above\x86\admin_only\npcap.inf
    installer\win7_above\x86\admin_only\npcap.sys
    installer\win7_above\x86\admin_only\NPcapHelper.exe
    installer\win7_above\x86\NPFInstall.exe
    installer\win7_above\x86\Packet.dll
    installer\wpcap.dll
  x64:
    installer\win7_above\x64\admin_only\npcap.cat
    installer\win7_above\x64\admin_only\npcap.inf
    installer\win7_above\x64\admin_only\npcap.sys
    installer\win7_above\x64\admin_only\NPcapHelper.exe
    installer\win7_above\x64\NPFInstall.exe
    installer\win7_above\x64\Packet.dll
    installer\x64\wpcap.dll

Run

  1. Run and install the NPcap installer: npcap-nmap-%VERSION%.exe.
  2. Use Nmap or Wireshark to test NPcap.

Known Issues

  1. The trunk Nmap doesn't support NPcap yet, we will make it available in the next Nmap release. A prebuilt executable that supports NPcap can be found in: https://svn.nmap.org/nmap-exp/yang/NPcap-LWF/nmap.exe.
  2. Wireshark doesn't support NPcap yet, the community is working on it.

License

NPcap is published under The MIT License (MIT).

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