npcap-devguide.xml

<sect1 id="npcap-devguide">
  <title>Developing software with Npcap</title>
  <sect1info>
    <abstract>
      <para> Writing software that captures or injects network traffic is easy
        with Npcap.  This guide describes the Npcap SDK, WinPcap compatibility,
        and the Npcap API.</para>
    </abstract>
  </sect1info>

  <sect2 id="npcap-development">
    <title>Using the Npcap SDK</title>

    <para>
      To build software that uses Npcap, use the latest version of the Npcap Software Development Kit (SDK).
      The latest SDK can be downloaded on <ulink role="hidepdf" url="https://npcap.com/#download">Npcap.org</ulink>.
      Updates to the SDK are much less frequent than updates to the Npcap binaries.
    </para>
  </sect2>

  <sect2 id="npcap-examples">
    <title>Examples</title>

    <para>
      Examples of applications using Npcap are available <ulink role="hidepdf" url="https://github.com/nmap/npcap/tree/master/Examples">in the Examples directory</ulink> in the source distribution.
    Several of these examples are explored in more depth in the <xref linkend="npcap-tutorial" />.
    </para>

    <para>
      Npcap developer Yang Luo has also provided an example:
      <ulink role="hidepdf" url="https://github.com/hsluoyz/UserBridge/">UserBridge</ulink>,
      which is a tool to redirect all packets from one interface to another.
    </para>
  </sect2>

  <sect2 id="npcap-devguide-updating">
    <title>Updating WinPcap software to Npcap</title>
    <para>
      For the most part, Npcap is completely compatible with software written
      for WinPcap. Minor changes need to be made to <xref
        linkend="npcap-feature-native-dll">DLL loading order</xref> and in some
      cases <xref linkend="npcap-feature-native-servicename">driver service
        name</xref>. 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.
    </para>

    <para>Apart from the libpcap API, WinPcap exported a few functions used by
      <ulink url="https://www.winpcap.org/windump/">WinDump</ulink> 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: <code>getservent</code>, <code>endservent</code>, and
      <code>eproto_db</code>.</para>

    <para>One other function exported by WinPcap, <code>wsockinit</code>, is
      available via the Npcap API as <code>pcap_wsockinit</code>. It calls
      <code>WSAStartup</code> for Windows Sockets version 1.1 and ensures that
      <code>WSACleanup</code> is called when the process ends.</para>

  </sect2>

  <sect2 id="npcap-detect">
    <title>How to detect what version Npcap/WinPcap you are using?</title>

    <para>
      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.
    </para>

    <sect3 id="npcap-detect-version">
      <title>Npcap version</title>

      <para> 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
        <quote>file version</quote> was advanced to <quote>5.0.0.000</quote> at
        that point. The major version will always be <quote>5</quote> 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
        <quote>5.0.92.612</quote> is Npcap 0.92, built on June 12th.</para>
    </sect3>

    <sect3 id="npcap-detect-install-time">
      <title>Install-time detection</title>

      <para>
        You can check the existence of <filename>C:\Program Files\Npcap\NPFInstall.exe</filename> to
        detect Npcap's existence. If Npcap exists, you can check the file version of
        <filename>C:\Program Files\Npcap\NPFInstall.exe</filename> to detect Npcap e-version. The
        e-version also gives you the version. The NSIS code is shown below. <varname>$inst_ver</varname>
        is an e-version string like <quote>5.0.7.424</quote>
      </para>
      <screen>
GetDllVersion "C:\Program Files\Npcap\NPFInstall.exe" $R0 $R1
IntOp $R2 $R0 / 0x00010000
IntOp $R3 $R0 &amp; 0x0000FFFF
IntOp $R4 $R1 / 0x00010000
IntOp $R5 $R1 &amp; 0x0000FFFF
StrCpy $inst_ver "$R2.$R3.$R4.$R5"</screen>

      <para>
        You can check the installation options of an already installed Npcap by reading the registry
        key: <filename>HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\npcap\Parameters</filename>.
        The entries like <filename>AdminOnly</filename>,
        <filename>LoopbackSupport</filename>, <filename>DltNull</filename>,<filename>Dot11Support</filename>,
        <filename>WinPcapCompatible</filename>, etc. 
        are <code>REG_DWORD</code> type. A 0x00000001 value
        indicates the installation option is <emphasis>CHECKED</emphasis>.
      </para>
      <para>Note: Prior to Npcap 0.93, these values were stored in the
        <filename>Services\npcap</filename> key directly.</para>
    </sect3>

    <sect3 id="npcap-detect-run-time">
      <title>Run-time detection</title>

      <para>
        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 <filename>wpcap.dll</filename> 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
        <function>pcap_lib_version</function> to get the Npcap/WinPcap version
        string.
      </para>

      <screen>
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"</screen>

      <para>Npcap requires the <varname>npcap</varname> service to be running. If
        installed in <quote>WinPcap Compatible Mode</quote>, the
        <varname>npf</varname> service can be started instead.  Given that
        <varname>npcap</varname> service is always installed
        in both modes, a good practice is just trying the <varname>npcap</varname> service first.
        If it fails, then try the <varname>npf</varname> service. This is also what most of our users
        do in their software based on our investigation. A code sample from Nmap is
        <ulink role="hidepdf" url="https://github.com/nmap/nmap/blob/8c8e4a08c6c6b7abd2343e5921aafb6077bdb257/mswin32/winfix.cc#L322-L328">here</ulink>.
      </para>
    </sect3>
  </sect2>

  <sect2 id="npcap-feature-native">
    <title>For software that want to use Npcap first when Npcap and WinPcap coexist</title>

    <para>
      Prerequisite: Uncheck the <option>Install Npcap in WinPcap API-compatible Mode</option> option at
      install-time (which is by default).
    </para>

    <sect3 id="npcap-feature-native-dll">
      <title>DLL loading</title>

      <para>Npcap installs its DLLs into <filename>C:\Windows\System32\Npcap\</filename>
        instead of WinPcap's <filename>C:\Windows\System32\</filename>. Because of how Windows'
        <ulink role="hidepdf" url="https://msdn.microsoft.com/en-us/library/windows/desktop/ms686203(v=vs.85).aspx">DLL search path</ulink> works,
        your application will use WinPcap first by default when Npcap and WinPcap coexist,
        as <filename>C:\Windows\System32\</filename> is prior to <filename>C:\Windows\System32\Npcap\</filename>.
        So when Npcap and WinPcap coexist, an application that want to use Npcap instead
        of WinPcap must make <filename>C:\Windows\System32\Npcap\</filename> precedent to the
        <filename>C:\Windows\System32\</filename> 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
        (<filename>wpcap.dll</filename>).</para>

      <sect4 id="npcap-feature-native-dll-implicitly">
        <title>If the application <emphasis>implicitly</emphasis> links <filename>wpcap.dll</filename></title>

        <para>Implicit linking means that either you specified <filename>wpcap.lib</filename>
          in your <option>Project Properties</option> -> <option>Configuration Properties</option>
          -> <option>Linker</option> -> <option>Input</option> -> <option>Additional Dependencies</option> in Visual Studio,
          or specified <code>#pragma comment(linker, "wpcap.lib")</code> in your code.</para>

        <para>You need to do the following two steps:</para>

        <itemizedlist>
          <listitem><para>Specify <filename>wpcap.dll</filename> as a delay-loaded DLL: In
              Visual Studio, open the <option>Project Properties</option> window. Go to:
              <option>Configuration Properties</option> -> <option>Linker</option> -> <option>Input</option>
              -> <option>Delay Loaded Dlls</option>. Enter <filename>wpcap.dll</filename>
              in that option.</para></listitem>

          <listitem><para>Before calling any <filename>wpcap.dll</filename> functions,
              call <function>SetDllDirectory</function> to add <filename>C:\Windows\System32\Npcap\</filename>
              to DLL search path.</para></listitem>
        </itemizedlist>

        <!-- TODO: Should we take ownership of WinDump if it is used as an example? -->
        <para><ulink role="hidepdf" url="https://github.com/hsluoyz/WinDump/">Here</ulink>
          is an example called WinDump, a simple packet capture tool using Npcap/WinPcap.
          And <ulink role="hidepdf" url="https://github.com/hsluoyz/WinDump/commit/dffe2eaa520fc3b449ec0a90dcfa24f96359bbfa">this commit</ulink>
          makes it able to use Npcap first when Npcap and WinPcap coexist.</para>
      </sect4>

      <sect4 id="npcap-feature-native-dll-explicitly">
        <title>If the application <emphasis>explicitly</emphasis> links <filename>wpcap.dll</filename></title>

        <para>Explicit linking means that you explicitly called <function>LoadLibrary</function>
          to load <filename>wpcap.dll</filename> and called <function>GetProcAddress</function> to get the
          function pointers.</para>

        <para>You need to do the following one step:</para>

        <itemizedlist>
          <listitem><para>Before calling <function>LoadLibrary</function> to load <filename>wpcap.dll</filename>,
              call <function>SetDllDirectory</function> to add <filename>C:\Windows\System32\Npcap\</filename>
              to DLL search path.</para></listitem>
        </itemizedlist>

        <para>The function <function>init_npcap_dll_path</function> is provided in the following example:
          <ulink role="hidepdf" url="https://github.com/hsluoyz/WinDump/commit/dffe2eaa520fc3b449ec0a90dcfa24f96359bbfa">WinDump</ulink></para>
      </sect4>
    </sect3>

    <sect3 id="npcap-feature-native-servicename">
      <title>Service name</title>

      <para>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.</para> 

      <para>Npcap uses service name <quote>npcap</quote> instead of WinPcap's <quote>npf</quote> with
        <quote>WinPcap Compatible Mode</quote> OFF. So applications using
        <command>net start npf</command> for starting service must change to this:
        run <command>net start npcap</command> first, if it fails, then try
        <command>net start npf</command>.</para>
    </sect3>
  </sect2>

  <sect2 id="npcap-feature-loopback">
    <title id="npcap-feature-loopback.title">For software that uses Npcap loopback feature</title>

    <para>
      Npcap 0.9983 and newer support loopback traffic capture and injection without requiring a particular installation option.
    </para>

    <para>
      Npcap's loopback adapter device is reported by
      <function>pcap_findalldevs()</function> as
      <quote>\Device\NPF_Loopback</quote>. This name is always available even
      if <quote>Legacy loopback support</quote> was chosen at install time,
      which puts the name of the legacy loopback adapter in the
      <filename>LoopbackAdapter</filename> REG_SZ value of the
      <filename>HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\npcap\Parameters</filename>.
      Registry key.
    </para>

    <para>
      Traffic captured and injected on the loopback adapter uses the
      <varname>DLT_NULL</varname> 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.
    </para>

    <para>
      The MTU of <quote>Npcap Loopback Adapter</quote> 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.
    </para>

    <para>
      Don't try to make OID requests to <quote>Npcap Loopback Adapter</quote> except
      <varname>OID_GEN_MAXIMUM_TOTAL_SIZE</varname> (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
      <varname>OID_GEN_MAXIMUM_TOTAL_SIZE</varname>. If you query its value, you will always get
      65550 (65536 + 14). If you try to set its value, the operation will always fail.
    </para>

    <para>If you use IP Helper API to get adapter list, you will get an interface named
      like <quote>Loopback Pseudo-Interface 1</quote>. 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 <varname>AdapterName</varname> of the
      <quote>Loopback Pseudo-Interface 1</quote> entry with
      <quote>NPF_Loopback</quote>, as Nmap does in its enhancements to
      libdnet.</para>

    <para><quote>Legacy loopback support</quote> installs a copy of the
      Microsft KM-TEST loopback adapter named <quote>Npcap Loopback
        Adapter</quote> 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
      <filename>LoopbackAdapter</filename> Registry value.
      </para>

  </sect2>

  <sect2 id="npcap-feature-dot11">
    <title id="npcap-feature-dot11.title">For software that uses Npcap raw 802.11 feature</title>

    <para>
      Prerequisite: Check the <option>Support raw 802.11 traffic (and monitor mode) for wireless adapters</option> option at install-time.
    </para>

    <sect3 id="npcap-feature-dot11-steps">
      <title>Steps</title>
      <itemizedlist>
        <listitem><para>Install the latest version Npcap with the
            <option>Support raw 802.11 traffic (and monitor mode) for wireless
              adapters</option> option checked in the installation wizard. With this
            option checked, Npcap will see packets with <emphasis>Radiotap +
              802.11</emphasis> headers for wireless adapters. Otherwise, Npcap will
            see packets with <emphasis>fake Ethernet</emphasis> headers for wireless
            adapters.</para></listitem>

        <listitem><para>Run <filename>WlanHelper.exe</filename> with
            <emphasis>Administrator privilege</emphasis>. If you use
            <option>-i</option>, follow the interactive prompts to choose your
            wireless adapter and select <quote>Network Monitor</quote> mode.
            <filename>WlanHelper.exe</filename> also supports parameters to be used
            in an API manner, run <command>WlanHelper.exe -h</command> for
            details.</para></listitem>

        <listitem><para>Use the Npcap API from your user software as usual. For
            example, launch Wireshark and capture on the wireless adapter, viewingall
            802.11 packets (<emphasis>data + control + management</emphasis>).
        </para></listitem>

        <listitem><para>If you need to return to <quote>Managed Mode</quote>, run
            <command>WlanHelper.exe</command> again, following the prompts or
            selecting the appropriate command-line options to switch off the
            <quote>Monitor Mode</quote>.</para></listitem>
      </itemizedlist>
    </sect3>

    <sect3 id="npcap-feature-dot11-tips">
      <title>Tips</title>
      <itemizedlist>
        <listitem><para>You can use <filename>WlanHelper.exe</filename> tool to
            switch on the <quote>Monitor Mode</quote> in order to see
            <emphasis>802.11 control and management</emphasis> packets. You
            can also use the <code>pcap_set_rfmon</code> function within your
            code, as Wireshark does.
        </para></listitem>

        <listitem><para>Switching on the <quote>Monitor Mode</quote> will
            disconnect your wireless network from the AP, you can switch back to
            <quote>Managed Mode</quote> using the same
            <filename>WlanHelper.exe</filename> tool.</para></listitem>

        <listitem><para>The <filename>WlanHelper.exe</filename> tool is
            installed to <quote>%SYSTEMROOT%\System32\Npcap</quote> after installing Npcap.</para></listitem>
      </itemizedlist>
    </sect3>

    <sect3 id="npcap-feature-dot11-terminology">
      <title>Terminology</title>
      <para>
        <quote>Managed Mode</quote> (for Linux) = <quote>Extensible Station Mode</quote> (aka <quote>ExtSTA</quote>, for Windows)
      </para>

      <para>
        <quote>Monitor Mode</quote> (for Linux) = <quote>Network Monitor Mode</quote> (aka <quote>NetMon</quote>, for Windows)
      </para>

      <para>
        <quote>Master Mode</quote> (for Linux) = <quote>Extensible Access Point</quote> (aka <quote>ExtAP</quote>, for Windows)
      </para>
    </sect3>

    <sect3 id="npcap-feature-dot11-wlanhelper">
      <title>WlanHelper</title>
      <para>
        WlanHelper is used to set/get the operation mode (like <quote>Monitor
          Mode</quote>) for a wireless adapter on Windows.  WlanHelper tries to
        follow the grammar of <filename>iwconfig</filename>, a wireless
        management tool for Linux. So if you rename
        <filename>WlanHelper.exe</filename> to <filename>iwconfig.exe</filename>,
        your command lines for WlanHelper will be exactly the same with the
        iwconfig tool.
        <!-- TODO: check that this is still true, and more fully document WlanHelper -->
      </para>

      <sect4 id="npcap-feature-dot11-wlanhelper-usage">
        <title>WlanHelper's Usage</title>
        <para>
          Note: <command>WlanHelper</command> must run under <emphasis>Administrator privilege</emphasis>.
        </para>

        <sect5 id="npcap-feature-dot11-wlanhelper-usage-interactive">
          <title>Interactive way</title>
          <para>
            Run <command>WlanHelper</command> with the <option>-i</option> option.
          </para>
        </sect5>

        <sect5 id="npcap-feature-dot11-wlanhelper-usage-api">
          <title>Command-line API way</title>

          <itemizedlist>
            <listitem><para>Run <command>netsh wlan show interfaces</command>, get the <option>Name</option> or <option>GUID</option> for the interface.</para></listitem>
            <listitem><para>Run <command>WlanHelper -h</command> to see the man page.</para></listitem>
          </itemizedlist>
          <example id="npcap-ex-wlanhelper-man">
            <title>WlanHelper Man</title>
            <screen>
C:\> <userinput>WlanHelper.exe</userinput>
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 &lt;managed|monitor|master|..&gt;  : Set interface operation mode
modes                 : Get all operation modes supported by the interface, comma-separated
channel               : Get interface channel
channel &lt;1-14&gt;            : Set interface channel (only works in monitor mode)
freq                  : Get interface frequency
freq &lt;VALUE&gt;              : Set interface frequency (only works in monitor mode)
modu                  : Get interface modulation
modu &lt;dsss|fhss|irbaseband|ofdm|hrdsss|erp|ht|vht|ihv (VALUE)|..&gt; : 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</screen>
          </example>
          <para>
            An example:
          </para>
          <example id="npcap-ex-wlanhelper-api">
            <title>WlanHelper API Usage</title>
            <screen>
C:\> <userinput>netsh wlan show interfaces</userinput>

There is 1 interface on the system:

Name                   : <replaceable>Wi-Fi</replaceable>
Description            : Qualcomm Atheros AR9485WB-EG Wireless Network Adapter
GUID                   : <replaceable>42dfd47a-2764-43ac-b58e-3df569c447da</replaceable>
Physical 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:\> <userinput>WlanHelper.exe <replaceable>wi-fi</replaceable> mode</userinput>
managed
C:\> <userinput>WlanHelper.exe <replaceable>wi-fi</replaceable> mode monitor</userinput>
Success
C:\> <userinput>WlanHelper.exe <replaceable>wi-fi</replaceable> mode </userinput>
monitor
C:\> <userinput>WlanHelper.exe <replaceable>wi-fi</replaceable> mode managed</userinput>
Success
C:\> <userinput>WlanHelper.exe <replaceable>wi-fi</replaceable> mode</userinput>
managed</screen>
          </example>
        </sect5>
      </sect4>
    </sect3>

  </sect2>
</sect1>