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 396e35a..8f20ec7 100644 (file)
--- a/src/graph.c
+++ b/src/graph.c
@@ -176,9 +176,13 @@ static void sssp_bfs(void) {
                           && (e->to->distance != n->distance + 1 || e->weight >= e->to->prevedge->weight))
                                continue;
 
+                       // Only update nexthop if it doesn't increase the path length
+
+                       if(!e->to->status.visited || (e->to->distance == n->distance + 1 && e->weight >= e->to->prevedge->weight))
+                               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;