Ensure MTU probe replies are sent back the same way they came in.

Also sprinkle some comments over mtu_probe_h().

diff --git a/src/net_packet.c b/src/net_packet.c
index 902b081..e50135e 100644 (file)
--- a/src/net_packet.c
+++ b/src/net_packet.c
@@ -164,11 +164,26 @@ static void mtu_probe_h(node_t *n, vpn_packet_t *packet, length_t len) {
        logger(DEBUG_TRAFFIC, LOG_INFO, "Got MTU probe length %d from %s (%s)", packet->len, n->name, n->hostname);
 
        if(!packet->data[0]) {
+               /* It's a probe request, send back a reply */
+
                packet->data[0] = 1;
+
+               /* Temporarily set udp_confirmed, so that the reply is sent
+                  back exactly the way it came in. */
+
+               bool udp_confirmed = n->status.udp_confirmed;
+               n->status.udp_confirmed = true;
                send_udppacket(n, packet);
+               n->status.udp_confirmed = udp_confirmed;
        } else {
+               /* It's a valid reply: now we know bidirectional communication
+                  is possible using the address and socket that the reply
+                  packet used. */
+
                n->status.udp_confirmed = true;
 
+               /* If we haven't established the PMTU yet, restart the discovery process. */
+
                if(n->mtuprobes > 30) {
                        if(n->minmtu)
                                n->mtuprobes = 30;
@@ -176,6 +191,8 @@ static void mtu_probe_h(node_t *n, vpn_packet_t *packet, length_t len) {
                                n->mtuprobes = 1;
                }
 
+               /* If applicable, raise the minimum supported MTU */
+
                if(len > n->maxmtu)
                        len = n->maxmtu;
                if(n->minmtu < len)