Adds support to use existing TAP interfaces on POSIX systems. (#153)

* Fixes some doxygen comments.

* Adds the possibility to use an existing TUN/TAP interface for POSIX systems.

* Be sure to not set interface UP/DOWN if not root and if using existing TUN/TAP interface.

* Adds some notes in configuration file regarding use of fscp.name parameter on POSIX systems. Also do not destroy tap interface on macOS/BSD if fscp.name is specified.

* Adds precisions in freelan.cfg file about using existing TAP interface. Minor tweaks in POSIX asiotap with existing interface.

apps/freelan/config/freelan.cfg

@@ -370,9 +370,14 @@ public_endpoint=0.0.0.0
 # If no name or an empty name is provided, the first available adapter will be
 # used.
 #
-# On UNIX, it is the name of the tap adapter to create. Depending on your
-# system, some names might be restricted, and something in the form of tapX
-# (where X is a positive number) is recommended.
+# On POSIX systems, it is the name of the tap adapter to create or to use.
+# Depending on your system, some names might be restricted, and something in the
+# form of tapX (where X is a positive number) is recommended.
+#
+# If you use an existing interface and run as non-root, the interface name MUST
+# exist, belong to the user, be UP and already has IP configuration or use DHCP.
+# Additionnaly on Linux, user MUST have read-write permissions on /dev/net/tap
+# or /dev/net/tun (depending on which mode is used).
 #
 # If no name or an empty name is provided, a tap adapter will be created with
 # an available name.

libs/asiotap/include/asiotap/posix/posix_tap_adapter.hpp

@@ -203,6 +203,11 @@ namespace asiotap
 			void destroy_device(boost::system::error_code& ec);
 
 			posix_route_manager m_route_manager;
+
+			/**
+			 * \brief If we use an existing tun/tap adapter.
+			 */
+			bool m_existing_tap;
 	};
 }
 

libs/asiotap/src/posix/posix_tap_adapter.cpp

@@ -57,6 +57,7 @@
 
 #include <linux/if_tun.h>
 #include <sys/sysmacros.h>
+
 /**
  * \struct in6_ifreq
  * \brief Replacement structure since the include of linux/ipv6.h introduces conflicts.
@@ -234,20 +235,22 @@ namespace asiotap
 	{
 		ec = boost::system::error_code();
 
+		m_existing_tap = !_name.empty();
+
 #if defined(LINUX)
 		const std::string dev_name = (layer() == tap_adapter_layer::ethernet) ? "/dev/net/tap" : "/dev/net/tun";
 
 		if (::access(dev_name.c_str(), F_OK) == -1)
 		{
 			if (errno != ENOENT)
 			{
-			// Unable to access the tap adapter yet it exists: this is an error.
+				// Unable to access the tap adapter yet it exists: this is an error.
 				ec = boost::system::error_code(errno, boost::system::system_category());
 
 				return;
 			}
 
-		// No tap found, create one.
+			// No tap found, create one.
 			if (::mknod(dev_name.c_str(), S_IFCHR | S_IRUSR | S_IWUSR, ::makedev(10, 200)) == -1)
 			{
 				ec = boost::system::error_code(errno, boost::system::system_category());
@@ -309,7 +312,7 @@ namespace asiotap
 
 			netifr.ifr_qlen = 100; // 100 is the default value
 
-			if (::ioctl(socket.native_handle(), SIOCSIFTXQLEN, (void *)&netifr) < 0)
+			if (getuid() == 0 && ::ioctl(socket.native_handle(), SIOCSIFTXQLEN, (void *)&netifr) < 0)
 			{
 				ec = boost::system::error_code(errno, boost::system::system_category());
 
@@ -484,6 +487,12 @@ namespace asiotap
 
 	void posix_tap_adapter::destroy_device(boost::system::error_code& ec)
 	{
+		// do not attempt to destroy interface if non-root
+		if(getuid() != 0)
+		{
+			return;
+		}
+
 #if defined(MACINTOSH) || defined(BSD)
 		descriptor_handler socket = open_socket(AF_INET, ec);
 
@@ -514,12 +523,18 @@ namespace asiotap
 
 		strncpy(netifr.ifr_name, name().c_str(), IFNAMSIZ);
 
-		// Set the interface UP
+		// Get the interface flags
 		if (::ioctl(socket.native_handle(), SIOCGIFFLAGS, static_cast<void*>(&netifr)) < 0)
 		{
 			throw boost::system::system_error(errno, boost::system::system_category());
 		}
 
+		// as non-root, assume that existing TAP is correctly configured
+		if (getuid() != 0 && m_existing_tap)
+		{
+			return;
+		}
+
 		if (connected)
 		{
 #ifdef MACINTOSH
@@ -539,6 +554,7 @@ namespace asiotap
 #endif
 		}
 
+		// Set the interface UP
 		if (::ioctl(socket.native_handle(), SIOCSIFFLAGS, static_cast<void*>(&netifr)) < 0)
 		{
 			throw boost::system::system_error(errno, boost::system::system_category());
@@ -612,6 +628,12 @@ namespace asiotap
 
 	void posix_tap_adapter::configure(const configuration_type& configuration)
 	{
+		// as non-root, assume that existing TAP is correctly configured
+		if(getuid() != 0 && m_existing_tap)
+		{
+			return;
+		}
+
 		if (configuration.ipv4.network_address)
 		{
 			if (layer() == tap_adapter_layer::ethernet)