Keep track of the largest UDP packet size received from a node.

src/graph.c

@@ -238,6 +238,7 @@ static void check_reachability(void) {
 
 			n->status.udp_confirmed = false;
 			n->maxmtu = MTU;
+			n->maxrecentlen = 0;
 			n->minmtu = 0;
 			n->mtuprobes = 0;
 

src/net_packet.c

@@ -95,6 +95,7 @@ static void udp_probe_timeout_handler(void *data) {
 
 	logger(DEBUG_TRAFFIC, LOG_INFO, "Too much time has elapsed since last UDP ping response from %s (%s), stopping UDP communication", n->name, n->hostname);
 	n->status.udp_confirmed = false;
+	n->maxrecentlen = 0;
 	n->mtuprobes = 0;
 	n->minmtu = 0;
 	n->maxmtu = MTU;
@@ -387,6 +388,9 @@ static bool receive_udppacket(node_t *n, vpn_packet_t *inpkt) {
 		origlen -= MTU/64 + 20;
 	}
 
+	if(inpkt->len > n->maxrecentlen)
+		n->maxrecentlen = inpkt->len;
+
 	inpkt->priority = 0;
 
 	if(!DATA(inpkt)[12] && !DATA(inpkt)[13])
@@ -962,6 +966,7 @@ static void try_mtu(node_t *n) {
 		return;
 
 	if(udp_discovery && !n->status.udp_confirmed) {
+		n->maxrecentlen = 0;
 		n->mtuprobes = 0;
 		n->minmtu = 0;
 		n->maxmtu = MTU;

src/node.c

@@ -179,6 +179,7 @@ void update_node_udp(node_t *n, const sockaddr_t *sa) {
 	/* invalidate UDP information - note that this is a security feature as well to make sure
 	   we can't be tricked into flooding any random address with UDP packets */
 	n->status.udp_confirmed = false;
+	n->maxrecentlen = 0;
 	n->mtuprobes = 0;
 	n->minmtu = 0;
 	n->maxmtu = MTU;

src/node.h

@@ -93,6 +93,8 @@ typedef struct node_t {
 
 	struct timeval mtu_ping_sent;           /* Last time a MTU probe was sent */
 
+	length_t maxrecentlen;			/* Maximum size of recently received packets */
+
 	length_t mtu;                           /* Maximum size of packets to send to this node */
 	length_t minmtu;                        /* Probed minimum MTU */
 	length_t maxmtu;                        /* Probed maximum MTU */