ipv6: router reachability probing

RFC 4191 states in 3.5:

   When a host avoids using any non-reachable router X and instead sends
   a data packet to another router Y, and the host would have used
   router X if router X were reachable, then the host SHOULD probe each
   such router X's reachability by sending a single Neighbor
   Solicitation to that router's address.  A host MUST NOT probe a
   router's reachability in the absence of useful traffic that the host
   would have sent to the router if it were reachable.  In any case,
   these probes MUST be rate-limited to no more than one per minute per
   router.

Currently, when the neighbour corresponding to a router falls into
NUD_FAILED, it's never considered again. Introduce a new rt6_nud_state
value, RT6_NUD_FAIL_PROBE, which suggests the route should not be used but
should be probed with a single NS. The probe is ratelimited by the existing
code. To better distinguish meanings of the failure values, rename
RT6_NUD_FAIL_SOFT to RT6_NUD_FAIL_DO_RR.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/neighbour.c

@@ -1238,6 +1238,21 @@ out:
 }
 EXPORT_SYMBOL(neigh_update);
 
+/* Update the neigh to listen temporarily for probe responses, even if it is
+ * in a NUD_FAILED state. The caller has to hold neigh->lock for writing.
+ */
+void __neigh_set_probe_once(struct neighbour *neigh)
+{
+	neigh->updated = jiffies;
+	if (!(neigh->nud_state & NUD_FAILED))
+		return;
+	neigh->nud_state = NUD_PROBE;
+	atomic_set(&neigh->probes, NEIGH_VAR(neigh->parms, UCAST_PROBES));
+	neigh_add_timer(neigh,
+			jiffies + NEIGH_VAR(neigh->parms, RETRANS_TIME));
+}
+EXPORT_SYMBOL(__neigh_set_probe_once);
+
 struct neighbour *neigh_event_ns(struct neigh_table *tbl,
 				 u8 *lladdr, void *saddr,
 				 struct net_device *dev)