From: Nathan Stratton Treadway <nathanst@ontko.com>
Date: Sat, 12 Sep 2015 14:33:52 +0000 (+0200)
Subject: Fix invalid checksum generation.
X-Git-Tag: release-1.1pre12~97
X-Git-Url: https://tinc-vpn.org/git/browse?p=tinc;a=commitdiff_plain;h=ae89a25695411149a7499189c9771762ad1f1726

Fix invalid checksum generation.

Use equation 3 given in RFC 1624 and the UpdateTTL() example function given
RFC 1141.

# Conflicts:
#	src/route.c
---

diff --git a/src/route.c b/src/route.c
index 27851469..70c5806b 100644
--- a/src/route.c
+++ b/src/route.c
@@ -164,7 +164,7 @@ static void clamp_mss(const node_t *source, const node_t *via, vpn_packet_t *pac
 		/* Found it */
 		uint16_t oldmss = DATA(packet)[start + 22 + i] << 8 | DATA(packet)[start + 23 + i];
 		uint16_t newmss = mtu - start - 20;
-		uint16_t csum = DATA(packet)[start + 16] << 8 | DATA(packet)[start + 17];
+		uint32_t csum = DATA(packet)[start + 16] << 8 | DATA(packet)[start + 17];
 
 		if(oldmss <= newmss)
 			break;
@@ -175,11 +175,13 @@ static void clamp_mss(const node_t *source, const node_t *via, vpn_packet_t *pac
 		DATA(packet)[start + 22 + i] = newmss >> 8;
 		DATA(packet)[start + 23 + i] = newmss & 0xff;
 		csum ^= 0xffff;
-		csum -= oldmss;
+		csum += oldmss ^ 0xffff;
 		csum += newmss;
+		csum = (csum & 0xffff) + (csum >> 16);
+		csum += csum >> 16;
 		csum ^= 0xffff;
 		DATA(packet)[start + 16] = csum >> 8;
-		DATA(packet)[start + 17] = csum & 0xff;
+		DATA(packet)[start + 17] = csum;
 		break;
 	}
 }