ndisc: Accept routes from on-link prefixes form ra

It is possible that an ra leads to two routes having
the same prefix as well as the same prefix length.
One of them, however, refers to the on-link prefix,
and the other one to a route from the route information field.
(Moreover, they might have different route preferences.)
Hence, if both routes differ in the on-link property,
both are added, and the route from the route information
option receives a metric penalty.
Fixed #1163.

src/core/ndisc/nm-lndp-ndisc.c

@@ -211,6 +211,7 @@ receive_ra(struct ndp *ndp, struct ndp_msg *msg, gpointer user_data)
             const NMNDiscRoute route = {
                 .network = r_network,
                 .plen    = r_plen,
+                .on_link = TRUE,
                 .expiry_msec =
                     _nm_ndisc_lifetime_to_expiry(now_msec,
                                                  ndp_msg_opt_prefix_valid_time(msg, offset)),
@@ -249,6 +250,7 @@ receive_ra(struct ndp *ndp, struct ndp_msg *msg, gpointer user_data)
                 .network = network,
                 .gateway = gateway_addr,
                 .plen    = plen,
+                .on_link = FALSE,
                 .expiry_msec =
                     _nm_ndisc_lifetime_to_expiry(now_msec, ndp_msg_opt_route_lifetime(msg, offset)),
                 .preference = _route_preference_coerce(ndp_msg_opt_route_preference(msg, offset)),

src/core/ndisc/nm-ndisc.c

@@ -161,8 +161,9 @@ nm_ndisc_data_to_l3cd(NMDedupMultiIndex        *multi_idx,
             .table_any     = TRUE,
             .table_coerced = 0,
             .metric_any    = TRUE,
-            .metric        = 0,
-            .rt_pref       = ndisc_route->preference,
+            /* Non-on_link routes get a small penalty */
+            .metric  = ndisc_route->duplicate && !ndisc_route->on_link ? 5 : 0,
+            .rt_pref = ndisc_route->preference,
         };
         nm_assert((NMIcmpv6RouterPref) r.rt_pref == ndisc_route->preference);
 
@@ -349,12 +350,47 @@ _ASSERT_data_gateways(const NMNDiscDataInternal *data)
 }
 
 /*****************************************************************************/
+static bool
+is_duplicate_route(const NMNDiscRoute *r0, const NMNDiscRoute *r1)
+{
+    return IN6_ARE_ADDR_EQUAL(&r0->network, &r1->network) && r0->plen == r1->plen;
+}
+
+static void
+_data_complete_prepare_routes(GArray *routes)
+{
+    guint i, j;
+
+    for (i = 0; i < routes->len; i++) {
+        NMNDiscRoute *r0 = &nm_g_array_index(routes, NMNDiscRoute, i);
+
+        r0->duplicate = FALSE;
+    }
+    for (i = 0; i < routes->len; i++) {
+        NMNDiscRoute *r0 = &nm_g_array_index(routes, NMNDiscRoute, i);
+
+        for (j = i + 1; j < routes->len; j++) {
+            NMNDiscRoute *r1 = &nm_g_array_index(routes, NMNDiscRoute, j);
+
+            if (!is_duplicate_route(r0, r1))
+                continue;
+
+            r0->duplicate = TRUE;
+            r1->duplicate = TRUE;
+
+            /* Maybe after index j, there is yet another duplicate. But we
+            * will find that later, when i becomes j. */
+            break;
+        }
+    }
+}
 
 static const NMNDiscData *
 _data_complete(NMNDiscDataInternal *data)
 {
     _ASSERT_data_gateways(data);
 
+    _data_complete_prepare_routes(data->routes);
 #define _SET(data, field)                                      \
     G_STMT_START                                               \
     {                                                          \
@@ -658,8 +694,17 @@ nm_ndisc_add_route(NMNDisc *ndisc, const NMNDiscRoute *new_item, gint64 now_msec
     for (i = 0; i < rdata->routes->len;) {
         NMNDiscRoute *item = &nm_g_array_index(rdata->routes, NMNDiscRoute, i);
 
-        if (IN6_ARE_ADDR_EQUAL(&item->network, &new_item->network)
-            && item->plen == new_item->plen) {
+        /*
+         * It is possible that two entries in rdata->routes have
+         * the same prefix as well as the same prefix length.
+         * One of them, however, refers to the on-link prefix,
+         * and the other one to a route from the route information field.
+         * Moreover, they might have different route preferences.
+         * Hence, if both routes differ in the on-link flag,
+         * comparison is aborted, and both routes are added.
+         */
+        if (IN6_ARE_ADDR_EQUAL(&item->network, &new_item->network) && item->plen == new_item->plen
+            && item->on_link == new_item->on_link) {
             if (new_item->expiry_msec <= now_msec) {
                 g_array_remove_index(rdata->routes, i);
                 return TRUE;

src/core/ndisc/nm-ndisc.h

@@ -115,6 +115,8 @@ typedef struct _NMNDiscRoute {
     gint64             expiry_msec;
     NMIcmpv6RouterPref preference;
     guint8             plen;
+    bool               on_link : 1;
+    bool               duplicate : 1;
 } NMNDiscRoute;
 
 typedef struct {