Add runtime detection of SO_REUSEPORT, use it instead of dup() if available.

lib/dns/dispatch.c

@@ -1677,7 +1677,7 @@ open_socket(isc_socketmgr_t *mgr, const isc_sockaddr_t *local,
 		result = isc_socket_open(sock);
 		if (result != ISC_R_SUCCESS)
 			return (result);
-	} else if (dup_socket != NULL) {
+	} else if (dup_socket != NULL && !isc_socket_hasreuseport()) {
 		result = isc_socket_dup(dup_socket, &sock);
 		if (result != ISC_R_SUCCESS)
 			return (result);

lib/isc/include/isc/socket.h

@@ -1010,6 +1010,12 @@ isc_socketmgr_maxudp(isc_socketmgr_t *mgr, int maxudp);
  * Test interface. Drop UDP packet > 'maxudp'.
  */
 
+bool
+isc_socket_hasreuseport(void);
+/*%<
+ * Return true if there is SO_REUSEPORT support
+ */
+
 #ifdef HAVE_LIBXML2
 int
 isc_socketmgr_renderxml(isc_socketmgr_t *mgr, xmlTextWriterPtr writer);

lib/isc/unix/socket.c

@@ -4584,13 +4584,21 @@ isc_socket_bind(isc_socket_t *sock0, const isc_sockaddr_t *sockaddr,
 		goto bind_socket;
 #endif
 	if ((options & ISC_SOCKET_REUSEADDRESS) != 0 &&
-	    isc_sockaddr_getport(sockaddr) != (in_port_t)0 &&
-	    setsockopt(sock->fd, SOL_SOCKET, SO_REUSEADDR, (void *)&on,
+	    isc_sockaddr_getport(sockaddr) != (in_port_t)0) {
+		if (setsockopt(sock->fd, SOL_SOCKET, SO_REUSEADDR, (void *)&on,
 		       sizeof(on)) < 0) {
-		UNEXPECTED_ERROR(__FILE__, __LINE__,
+			UNEXPECTED_ERROR(__FILE__, __LINE__,
 				 "setsockopt(%d) %s", sock->fd,
 				 isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,
 						ISC_MSG_FAILED, "failed"));
+		}
+		if (setsockopt(sock->fd, SOL_SOCKET, SO_REUSEPORT, (void *)&on,
+		       sizeof(on)) < 0) {
+			UNEXPECTED_ERROR(__FILE__, __LINE__,
+				 "setsockopt(%d) %s", sock->fd,
+				 isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,
+						ISC_MSG_FAILED, "failed"));
+		}
 		/* Press on... */
 	}
 #ifdef AF_UNIX
@@ -5449,6 +5457,38 @@ isc_socket_getfd(isc_socket_t *socket0) {
 	return ((short) sock->fd);
 }
 
+static isc_once_t	hasreuseport_once = ISC_ONCE_INIT;
+static bool		hasreuseport = false;
+
+static void
+init_hasreuseport() {
+#ifdef SO_REUSEPORT
+	int sock, yes = 1;
+	sock = socket(AF_INET, SOCK_DGRAM, 0);
+	if (sock < 0) {
+		close(sock);
+		return;
+	} else if (setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, (void *)&yes,
+                       sizeof(yes)) < 0) {
+		close(sock);
+		return;
+	} else if (setsockopt(sock, SOL_SOCKET, SO_REUSEPORT, (void *)&yes,
+                       sizeof(yes)) < 0) {
+		close(sock);
+		return;
+	}
+	hasreuseport = true;
+#endif
+}
+
+bool
+isc_socket_hasreuseport() {
+	RUNTIME_CHECK(isc_once_do(&hasreuseport_once, init_hasreuseport)
+		      == ISC_R_SUCCESS);
+	return (hasreuseport);
+}
+
+
 #if defined(HAVE_LIBXML2) || defined(HAVE_JSON)
 static const char *
 _socktype(isc_sockettype_t type)

lib/isc/win32/libisc.def.in

@@ -78,6 +78,7 @@ isc_socket_getpeername
 isc_socket_getsockname
 isc_socket_gettag
 isc_socket_gettype
+isc_socket_hasreuseport
 isc_socket_ipv6only
 isc_socket_listen
 isc_socket_open

lib/isc/win32/socket.c

@@ -3691,6 +3691,11 @@ isc_socket_socketevent(isc_mem_t *mctx, void *sender,
 	return (allocate_socketevent(mctx, sender, eventtype, action, arg));
 }
 
+bool
+isc_socket_hasreuseport() {
+        return (false);
+}
+
 #ifdef HAVE_LIBXML2
 
 static const char *