Make tcpdump find wpcap.dll on Windows if it's not in the system library.

See https://npcap.com/guide/npcap-devguide.html#npcap-feature-native-dll-implicitly
for details on what's being done.

Fix #1226.

Author: guyharris

CMakeLists.txt

@@ -1418,6 +1418,9 @@ add_executable(tcpdump ${TCPDUMP_SOURCE_LIST_C})
 if(NOT C_ADDITIONAL_FLAGS STREQUAL "")
     set_target_properties(tcpdump PROPERTIES COMPILE_FLAGS ${C_ADDITIONAL_FLAGS})
 endif()
+if(NOT "${PCAP_LINK_FLAGS}" STREQUAL "")
+    set_target_properties(tcpdump PROPERTIES LINK_FLAGS ${PCAP_LINK_FLAGS})
+endif()
 target_link_libraries(tcpdump netdissect ${TCPDUMP_LINK_LIBRARIES})
 
 ######################################

cmake/Modules/FindPCAP.cmake

@@ -61,6 +61,42 @@ if(WIN32)
   if(PCAP_FOUND)
     set(PCAP_LIBRARIES ${PCAP_LIBRARY})
     set(PCAP_INCLUDE_DIRS ${PCAP_INCLUDE_DIR})
+
+    #
+    # We need to look for wpcap.dll in \Windows\System32\Npcap first,
+    # as either:
+    #
+    #  1) WinPcap isn't installed and Npcap isn't installed in "WinPcap
+    #     API-compatible Mode", so there's no wpcap.dll in
+    #     \Windows\System32, only in \Windows\System32\Npcap;
+    #
+    #  2) WinPcap is installed and Npcap isn't installed in "WinPcap
+    #     API-compatible Mode", so the wpcap.dll in \Windows\System32
+    #     is a WinPcap DLL, but we'd prefer an Npcap DLL (we should
+    #     work with either one if we're configured against WinPcap,
+    #     and we'll probably require Npcap if we're configured againt
+    #     it), and that's in \Windows\System32\Npcap;
+    #
+    #  3) Npcap is installed in "WinPcap API-compatible Mode", so both
+    #     \Windows\System32 and \Windows\System32\Npcap have an Npcap
+    #     wpcap.dll.
+    #
+    # Unfortunately, Windows has no notion of an rpath, so we can't
+    # set the rpath to include \Windows\System32\Npcap at link time;
+    # what we need to do is to link wpcap as a delay-load DLL and
+    # add \Windows\System32\Npcap to the DLL search path early in
+    # main() with a call to SetDllDirectory().
+    #
+    # We add /delayload:wpcap.dll to the linker options here.
+    #
+    # See https://npcap.com/guide/npcap-devguide.html#npcap-feature-native-dll-implicitly
+    #
+    set(PCAP_LINK_FLAGS /delayload:wpcap.dll)
+
+    #
+    # Delay-loading libraries means we need to link with delayimp.lib.
+    #
+    set(PCAP_LIBRARIES ${PCAP_LIBRARIES} delayimp.lib)
   endif()
 else(WIN32)
   #

tcpdump.c

@@ -1437,6 +1437,42 @@ main(int argc, char **argv)
 	netdissect_options Ndo;
 	netdissect_options *ndo = &Ndo;
 
+#ifdef _WIN32
+	/*
+	 * We need to look for wpcap.dll in \Windows\System32\Npcap first,
+	 * as either:
+	 *
+	 *  1) WinPcap isn't installed and Npcap isn't installed in "WinPcap
+	 *     API-compatible Mode", so there's no wpcap.dll in
+	 *     \Windows\System32, only in \Windows\System32\Npcap;
+	 *
+	 *  2) WinPcap is installed and Npcap isn't installed in "WinPcap
+	 *     API-compatible Mode", so the wpcap.dll in \Windows\System32
+	 *     is a WinPcap DLL, but we'd prefer an Npcap DLL (we should
+	 *     work with either one if we're configured against WinPcap,
+	 *     and we'll probably require Npcap if we're configured againt
+	 *     it), and that's in \Windows\System32\Npcap;
+	 *
+	 *  3) Npcap is installed in "WinPcap API-compatible Mode", so both
+	 *     \Windows\System32 and \Windows\System32\Npcap have an Npcap
+	 *     wpcap.dll.
+	 *
+	 * Unfortunately, Windows has no notion of an rpath, so we can't
+	 * set the rpath to include \Windows\System32\Npcap at link time;
+	 * what we need to do is to link wpcap as a delay-load DLL and
+	 * add \Windows\System32\Npcap to the DLL search path early in
+	 * main() with a call to SetDllDirectory().
+	 *
+	 * The same applies to packet.dll.
+	 *
+	 * We add \Windows\System32\Npcap here.
+	 *
+	 * See https://npcap.com/guide/npcap-devguide.html#npcap-feature-native-dll-implicitly
+	 */
+	if (!SetDllDirectoryA("C:\\Windows\\System32\\Npcap"))
+		error("SetDllDirectory failed");
+#endif
+
 	/*
 	 * Initialize the netdissect code.
 	 */