Add the LocalDiscoveryAddress option.

When LocalDiscovery is enabled, tinc normally sends broadcast packets during
PMTU discovery to the broadcast address (255.255.255.255 or ff02::1). This
option lets tinc use a different address.

At the moment only one LocalDiscoveryAddress can be specified.

doc/tinc.conf.5.in

@@ -326,6 +326,9 @@ which normally would prevent the peers from learning each other's LAN address.
 .Pp
 Currently, local discovery is implemented by sending broadcast packets to the LAN during path MTU discovery.
 This feature may not work in all possible situations.
+.It Va LocalDiscoveryAddress Li = Ar address
+If this variable is specified, local discovery packets are sent to the given
+.Ar address .
 .It Va MACExpire Li = Ar seconds Pq 600
 This option controls the amount of time MAC addresses are kept before they are removed.
 This only has effect when

doc/tinc.texi

@@ -1056,6 +1056,10 @@ which normally would prevent the peers from learning each other's LAN address.
 Currently, local discovery is implemented by sending broadcast packets to the LAN during path MTU discovery.
 This feature may not work in all possible situations.
 
+@cindex LocalDiscoveryAddress
+@item LocalDiscoveryAddress <@var{address}>
+If this variable is specified, local discovery packets are sent to the given @var{address}.
+
 @cindex Mode
 @item Mode = <router|switch|hub> (router)
 This option selects the way packets are routed to other daemons.

src/net.h

@@ -125,6 +125,7 @@ extern int seconds_till_retry;
 extern int addressfamily;
 extern unsigned replaywin;
 extern bool localdiscovery;
+extern sockaddr_t localdiscovery_address;
 
 extern listen_socket_t listen_socket[MAXSOCKETS];
 extern int listen_sockets;

src/net_packet.c

@@ -56,6 +56,7 @@ static void send_udppacket(node_t *, vpn_packet_t *);
 
 unsigned replaywin = 16;
 bool localdiscovery = false;
+sockaddr_t localdiscovery_address;
 
 #define MAX_SEQNO 1073741824
 
@@ -580,12 +581,22 @@ static void choose_broadcast_address(const node_t *n, const sockaddr_t **sa, int
 	*sock = rand() % listen_sockets;
 
 	if(listen_socket[*sock].sa.sa.sa_family == AF_INET6) {
-		broadcast_ipv6.in6.sin6_port = n->prevedge->address.in.sin_port;
-		broadcast_ipv6.in6.sin6_scope_id = listen_socket[*sock].sa.in6.sin6_scope_id;
-		*sa = &broadcast_ipv6;
+		if(localdiscovery_address.sa.sa_family == AF_INET6) {
+			localdiscovery_address.in6.sin6_port = n->prevedge->address.in.sin_port;
+			*sa = &localdiscovery_address;
+		} else {
+			broadcast_ipv6.in6.sin6_port = n->prevedge->address.in.sin_port;
+			broadcast_ipv6.in6.sin6_scope_id = listen_socket[*sock].sa.in6.sin6_scope_id;
+			*sa = &broadcast_ipv6;
+		}
 	} else {
-		broadcast_ipv4.in.sin_port = n->prevedge->address.in.sin_port;
-		*sa = &broadcast_ipv4;
+		if(localdiscovery_address.sa.sa_family == AF_INET) {
+			localdiscovery_address.in.sin_port = n->prevedge->address.in.sin_port;
+			*sa = &localdiscovery_address;
+		} else {
+			broadcast_ipv4.in.sin_port = n->prevedge->address.in.sin_port;
+			*sa = &broadcast_ipv4;
+		}
 	}
 }
 

src/net_setup.c

@@ -444,6 +444,7 @@ bool setup_myself_reloadable(void) {
 	char *fmode = NULL;
 	char *bmode = NULL;
 	char *afname = NULL;
+	char *address = NULL;
 	char *space;
 	bool choice;
 
@@ -534,6 +535,16 @@ bool setup_myself_reloadable(void) {
 	get_config_bool(lookup_config(config_tree, "DirectOnly"), &directonly);
 	get_config_bool(lookup_config(config_tree, "LocalDiscovery"), &localdiscovery);
 
+	memset(&localdiscovery_address, 0, sizeof localdiscovery_address);
+	if(get_config_string(lookup_config(config_tree, "LocalDiscoveryAddress"), &address)) {
+		struct addrinfo *ai = str2addrinfo(address, myport, SOCK_DGRAM);
+		free(address);
+		if(!ai)
+			return false;
+		memcpy(&localdiscovery_address, ai->ai_addr, ai->ai_addrlen);
+	}
+
+
 	if(get_config_string(lookup_config(config_tree, "Mode"), &rmode)) {
 		if(!strcasecmp(rmode, "router"))
 			routing_mode = RMODE_ROUTER;