From: Guus Sliepen <guus@tinc-vpn.org>
Date: Tue, 6 May 2014 10:39:59 +0000 (+0200)
Subject: Nexthop calculation should always use the shortest path.
X-Git-Tag: release-1.0.24~9
X-Git-Url: https://tinc-vpn.org/git/browse?p=tinc;a=commitdiff_plain;h=8d64561dbc0542e7e1185591b8ecde3e56e1bfca;hp=6685f2c8afc4775c3656dccc5a37286c01c0e854

Nexthop calculation should always use the shortest path.

When tinc runs the graph algorithms and updates the nexthop and via pointers,
it uses a breadth-first search, but it can sometimes revisit nodes that have
already been visited if the previous path is marked as being indirect, and
there is a longer path that is "direct". The via pointer should be updated in
this case, because this points to the closest hop to the destination that can
be reached directly. However, the nexthop pointer should not be updated.

This fixes a bug where there could potentially be a routing loop if a node in
the graph has an edge with the indirect flag set, and some other edge without
that flag, the indirect edge is part of the minimum spanning tree, and a
broadcast packet is being sent.
---

diff --git a/src/graph.c b/src/graph.c
index edea91bf..9592f980 100644
--- a/src/graph.c
+++ b/src/graph.c
@@ -212,9 +212,13 @@ static void sssp_bfs(void) {
 			   && (!e->to->status.indirect || indirect))
 				continue;
 
+			// Only update nexthop the first time we visit this node.
+
+			if(!e->to->status.visited)
+				e->to->nexthop = (n->nexthop == myself) ? e->to : n->nexthop;
+
 			e->to->status.visited = true;
 			e->to->status.indirect = indirect;
-			e->to->nexthop = (n->nexthop == myself) ? e->to : n->nexthop;
 			e->to->prevedge = e;
 			e->to->via = indirect ? n->via : e->to;
 			e->to->options = e->options;