From 9a366544c297d5c558800f9ffc301e2cb5a6a672 Mon Sep 17 00:00:00 2001
From: Etienne Dechamps <etienne@edechamps.fr>
Date: Sat, 4 Oct 2014 15:01:11 +0100
Subject: [PATCH] Make sure to discover MTU with relays.

Currently, when tinc sends UDP SPTPS datagrams through a relay, it
doesn't automatically start discovering PMTU with the relay. This means
that unless something else triggers PMTU discovery, tinc will keep using
TCP when sending packets through the relay.

This patches fixes the issue by explicitly establishing UDP tunnels with
relays.
---
 src/net_packet.c | 37 +++++++++++++++++++++++++------------
 1 file changed, 25 insertions(+), 12 deletions(-)

diff --git a/src/net_packet.c b/src/net_packet.c
index a1b174e9..076b37f4 100644
--- a/src/net_packet.c
+++ b/src/net_packet.c
@@ -497,19 +497,27 @@ void receive_tcppacket(connection_t *c, const char *buffer, int len) {
 	receive_packet(c->node, &outpkt);
 }
 
+static bool try_sptps(node_t *n) {
+	if(n->status.validkey)
+		return true;
+
+	logger(DEBUG_TRAFFIC, LOG_INFO, "No valid key known yet for %s (%s)", n->name, n->hostname);
+
+	if(!n->status.waitingforkey)
+		send_req_key(n);
+	else if(n->last_req_key + 10 < now.tv_sec) {
+		logger(DEBUG_ALWAYS, LOG_DEBUG, "No key from %s after 10 seconds, restarting SPTPS", n->name);
+		sptps_stop(&n->sptps);
+		n->status.waitingforkey = false;
+		send_req_key(n);
+	}
+
+	return false;
+}
+
 static void send_sptps_packet(node_t *n, vpn_packet_t *origpkt) {
-	if(!n->status.validkey) {
-		logger(DEBUG_TRAFFIC, LOG_INFO, "No valid key known yet for %s (%s)", n->name, n->hostname);
-		if(!n->status.waitingforkey)
-			send_req_key(n);
-		else if(n->last_req_key + 10 < now.tv_sec) {
-			logger(DEBUG_ALWAYS, LOG_DEBUG, "No key from %s after 10 seconds, restarting SPTPS", n->name);
-			sptps_stop(&n->sptps);
-			n->status.waitingforkey = false;
-			send_req_key(n);
-		}
+	if (!try_sptps(n))
 		return;
-	}
 
 	uint8_t type = 0;
 	int offset = 0;
@@ -755,12 +763,17 @@ static bool send_sptps_data_priv(node_t *to, node_t *from, int type, const char
 	node_t *relay = (to->via != myself && (type == PKT_PROBE || (len - SPTPS_DATAGRAM_OVERHEAD) <= to->via->minmtu)) ? to->via : to->nexthop;
 	bool direct = from == myself && to == relay;
 	bool relay_supported = (relay->options >> 24) >= 4;
+	bool tcponly = (myself->options | relay->options) & OPTION_TCPONLY;
+
+	/* We don't really need the relay's key, but we need to establish a UDP tunnel with it and discover its MTU. */
+	if (!direct && relay_supported && !tcponly)
+		try_sptps(relay);
 
 	/* Send it via TCP if it is a handshake packet, TCPOnly is in use, this is a relay packet that the other node cannot understand, or this packet is larger than the MTU.
 	   TODO: When relaying, the original sender does not know the end-to-end PMTU (it only knows the PMTU of the first hop).
 	         This can lead to scenarios where large packets are sent over UDP to relay, but then relay has no choice but fall back to TCP. */
 
-	if(type == SPTPS_HANDSHAKE || ((myself->options | relay->options) & OPTION_TCPONLY) || (!direct && !relay_supported) || (type != PKT_PROBE && (len - SPTPS_DATAGRAM_OVERHEAD) > relay->minmtu)) {
+	if(type == SPTPS_HANDSHAKE || tcponly || (!direct && !relay_supported) || (type != PKT_PROBE && (len - SPTPS_DATAGRAM_OVERHEAD) > relay->minmtu)) {
 		char buf[len * 4 / 3 + 5];
 		b64encode(data, buf, len);
 		/* If no valid key is known yet, send the packets using ANS_KEY requests,
-- 
2.20.1