From: Guus Sliepen <guus@meshlink.io>
Date: Sun, 12 Jul 2020 20:45:37 +0000 (+0200)
Subject: When receiving an mDNS request, send a unicast response back.
X-Git-Url: http://git.meshlink.io/?p=meshlink;a=commitdiff_plain;h=8b44f34be360263e5f30fdd1559da66e1faa3b25

When receiving an mDNS request, send a unicast response back.
---

diff --git a/src/discovery.c b/src/discovery.c
index 428e4407..82d1edb5 100644
--- a/src/discovery.c
+++ b/src/discovery.c
@@ -167,7 +167,8 @@ static void send_mdns_packet(meshlink_handle_t *mesh, const discovery_address_t
 	char *fingerprint = meshlink_get_fingerprint(mesh, (meshlink_node_t *)mesh->self);
 	const char *keys[] = {MESHLINK_MDNS_NAME_KEY, MESHLINK_MDNS_FINGERPRINT_KEY};
 	const char *values[] = {mesh->name, fingerprint};
-	size_t size = prepare_packet(data, sizeof data, fingerprint, mesh->appname, "tcp", atoi(mesh->myport), 2, keys, values);
+	size_t size = prepare_packet(data, sizeof data, fingerprint, mesh->appname, "tcp", atoi(mesh->myport), 2, keys, values, false);
+	free(fingerprint);
 
 	switch(addr->address.sa.sa_family) {
 	case AF_INET:
@@ -233,13 +234,23 @@ static void mdns_io_handler(event_loop_t *loop, void *data, int flags) {
 	uint16_t port = 0;
 	const char *keys[2] = {MESHLINK_MDNS_NAME_KEY, MESHLINK_MDNS_FINGERPRINT_KEY};
 	char *values[2] = {NULL, NULL};
+	bool response;
 
-	if(parse_packet(buf, len, &name, mesh->appname, "tcp", &port, 2, keys, values)) {
+	if(parse_packet(buf, len, &name, mesh->appname, "tcp", &port, 2, keys, values, &response)) {
 		node_t *n = (node_t *)meshlink_get_node(mesh, values[0]);
 
 		if(n) {
 			logger(mesh, MESHLINK_INFO, "Node %s is part of the mesh network.\n", n->name);
 
+			if(!response && n != mesh->self) {
+				// Send a unicast response back
+				char *fingerprint = meshlink_get_fingerprint(mesh, (meshlink_node_t *)mesh->self);
+				const char *response_values[] = {mesh->name, fingerprint};
+				size_t size = prepare_packet(buf, sizeof(buf), fingerprint, mesh->appname, "tcp", atoi(mesh->myport), 2, keys, response_values, true);
+				sendto(io->fd, buf, size, MSG_DONTWAIT | MSG_NOSIGNAL, &sa.sa, sl);
+				free(fingerprint);
+			}
+
 			switch(sa.sa.sa_family) {
 			case AF_INET:
 				sa.in.sin_port = port;
@@ -504,6 +515,7 @@ static void netlink_parse(meshlink_handle_t *mesh, const void *data, size_t len)
 
 static void netlink_io_handler(event_loop_t *loop, void *data, int flags) {
 	(void)flags;
+	(void)data;
 	static time_t prev_update;
 	meshlink_handle_t *mesh = loop->data;
 
@@ -547,6 +559,7 @@ static void netlink_io_handler(event_loop_t *loop, void *data, int flags) {
 #elif defined(RTM_NEWADDR)
 static void pfroute_io_handler(event_loop_t *loop, void *data, int flags) {
 	(void)flags;
+	(void)data;
 	static time_t prev_update;
 	meshlink_handle_t *mesh = loop->data;
 
@@ -663,6 +676,8 @@ void discovery_stop(meshlink_handle_t *mesh) {
 
 	free(mesh->discovery.ifaces);
 	free(mesh->discovery.addresses);
+	mesh->discovery.ifaces = NULL;
+	mesh->discovery.addresses = NULL;
 	mesh->discovery.iface_count = 0;
 	mesh->discovery.address_count = 0;
 
diff --git a/src/mdns.c b/src/mdns.c
index b243b3ae..79de309f 100644
--- a/src/mdns.c
+++ b/src/mdns.c
@@ -247,14 +247,14 @@ static void buf_check_len_end(cbuf_t *buf, const uint8_t *ptr) {
 	}
 }
 
-size_t prepare_packet(void *vdata, size_t size, const char *name, const char *protocol, const char *transport, uint16_t port, int nkeys, const char **keys, const char **values) {
+size_t prepare_packet(void *vdata, size_t size, const char *name, const char *protocol, const char *transport, uint16_t port, int nkeys, const char **keys, const char **values, bool response) {
 	// Create the request/response packet right now
 	uint8_t *data = vdata;
 	buf_t buf = {data, size};
 
 	// Header
 	buf_add_uint16(&buf, 0); // TX ID
-	buf_add_uint16(&buf, 0); // flags
+	buf_add_uint16(&buf, response ? ntohs(1 << 15) : 0); // flags
 	buf_add_uint16(&buf, 1); // 1 question
 	buf_add_uint16(&buf, 1); // 1 answer RR
 	buf_add_uint16(&buf, 0); // 0 authority RRs
@@ -314,22 +314,24 @@ size_t prepare_packet(void *vdata, size_t size, const char *name, const char *pr
 	}
 }
 
-bool parse_packet(const void *vdata, size_t size, char **name, const char *protocol, const char *transport, uint16_t *port, int nkeys, const char **keys, char **values) {
+bool parse_packet(const void *vdata, size_t size, char **name, const char *protocol, const char *transport, uint16_t *port, int nkeys, const char **keys, char **values, bool *response) {
 	const uint8_t *data = vdata;
 	cbuf_t buf = {data, size};
 
 	// Header
 	buf_check_uint16(&buf, 0); // TX ID
-	buf_check_uint16(&buf, 0); // flags
+	uint16_t flags = buf_get_uint16(&buf); // flags
 	buf_check_uint16(&buf, 1); // 1 question
 	buf_check_uint16(&buf, 1); // 1 answer RR
 	buf_check_uint16(&buf, 0); // 0 authority RRs
 	buf_check_uint16(&buf, 2); // 1 checkitional RR
 
-	if(buf.len == -1) {
+	if(buf.len == -1 || flags & ~ntohs(1 << 15)) {
 		return false;
 	}
 
+	*response = flags & ntohs(1 << 15);
+
 	// Question section: _protocol._transport.local PTR IN
 	buf_check_ulabel(&buf, protocol);
 	buf_check_ulabel(&buf, transport);
diff --git a/src/mdns.h b/src/mdns.h
index a5384599..1bd8451d 100644
--- a/src/mdns.h
+++ b/src/mdns.h
@@ -6,6 +6,6 @@
 #include <stdint.h>
 #include <unistd.h>
 
-size_t prepare_packet(void *buf, size_t size, const char *name, const char *protocol, const char *transport, uint16_t port, int nkeys, const char **keys, const char **values);
-bool parse_packet(const void *buf, size_t size, char **name, const char *protocol, const char *transport, uint16_t *port, int nkeys, const char **keys, char **values);
+size_t prepare_packet(void *buf, size_t size, const char *name, const char *protocol, const char *transport, uint16_t port, int nkeys, const char **keys, const char **values, bool response);
+bool parse_packet(const void *buf, size_t size, char **name, const char *protocol, const char *transport, uint16_t *port, int nkeys, const char **keys, char **values, bool *response);