core: use router preference for IPv6 routes

For routes and the default-route from NDisc, set the router preference
RTA_PREF.

Also, previously, we would only configure one IPv6 default-route. That by itself
was not really a problem, as long as NetworkManager would always make sure that
it configured the route to the ~best~ router.

Actually, NM should have done that already. It keeps the list of gateways
sorted, and prefers them according to their preference. But maybe
it didn't, so we have bug rh#1445417 (??).

Change that by configuring a default-route for all gateways, with
appropriate router prefrence. In case, kernel doesn't support RTA_PREF
yet, only configure all routes that share the same maxiumum preference.

https://bugzilla.redhat.com/show_bug.cgi?id=1445417

src/devices/nm-device.c

@@ -7397,7 +7397,9 @@ ndisc_config_changed (NMNDisc *ndisc, const NMNDiscData *rdata, guint changed_in
 		                                  rdata->routes,
 		                                  rdata->routes_n,
 		                                  nm_device_get_route_table (self, AF_INET6, TRUE),
-		                                  nm_device_get_route_metric (self, AF_INET6));
+		                                  nm_device_get_route_metric (self, AF_INET6),
+		                                  nm_platform_check_kernel_support (nm_device_get_platform (self),
+		                                                                    NM_PLATFORM_KERNEL_SUPPORT_RTA_PREF));
 	}
 
 	if (changed & NM_NDISC_CONFIG_DNS_SERVERS) {

src/nm-iface-helper.c

@@ -208,7 +208,9 @@ ndisc_config_changed (NMNDisc *ndisc, const NMNDiscData *rdata, guint changed_in
 		                                  rdata->routes,
 		                                  rdata->routes_n,
 		                                  RT_TABLE_MAIN,
-		                                  global_opt.priority_v6);
+		                                  global_opt.priority_v6,
+		                                  nm_platform_check_kernel_support (NM_PLATFORM_GET,
+		                                                                    NM_PLATFORM_KERNEL_SUPPORT_RTA_PREF));
 	}
 
 	if (changed & NM_NDISC_CONFIG_DHCP_LEVEL) {

src/nm-ip6-config.c

@@ -1769,7 +1769,8 @@ nm_ip6_config_reset_routes_ndisc (NMIP6Config *self,
                                   const NMNDiscRoute *routes,
                                   guint routes_n,
                                   guint32 route_table,
-                                  guint32 route_metric)
+                                  guint32 route_metric,
+                                  gboolean kernel_support_rta_pref)
 {
 	NMIP6ConfigPrivate *priv;
 	guint i;
@@ -1800,6 +1801,8 @@ nm_ip6_config_reset_routes_ndisc (NMIP6Config *self,
 		r->rt_source  = NM_IP_CONFIG_SOURCE_NDISC;
 		r->table_coerced = nm_platform_route_table_coerce (route_table);
 		r->metric     = route_metric;
+		r->rt_pref    = ndisc_route->preference;
+		nm_assert ((NMIcmpv6RouterPref) r->rt_pref == ndisc_route->preference);
 
 		if (_nm_ip_config_add_obj (priv->multi_idx,
 		                           &priv->idx_ip6_routes_,
@@ -1814,28 +1817,40 @@ nm_ip6_config_reset_routes_ndisc (NMIP6Config *self,
 		new_best_default_route = _nm_ip_config_best_default_route_find_better (new_best_default_route, obj_new);
 	}
 
-	/* Use the first gateway as ordered in neighbor discovery cache. */
 	if (gateways_n) {
 		const NMPObject *obj_new;
-		const NMPlatformIP6Route r = {
+		NMPlatformIP6Route r = {
 			.rt_source     = NM_IP_CONFIG_SOURCE_NDISC,
 			.ifindex       = priv->ifindex,
-			.gateway       = gateways[0].address,
 			.table_coerced = nm_platform_route_table_coerce (route_table),
 			.metric        = route_metric,
 		};
+		const NMIcmpv6RouterPref first_pref = gateways[0].preference;
 
-		if (_nm_ip_config_add_obj (priv->multi_idx,
+		for (i = 0; i < gateways_n; i++) {
-		                           &priv->idx_ip6_routes_,
+			r.gateway = gateways[i].address;
-		                           priv->ifindex,
+			r.rt_pref = gateways[i].preference;
-		                           NULL,
+			nm_assert ((NMIcmpv6RouterPref) r.rt_pref == gateways[i].preference);
-		                           (const NMPlatformObject *) &r,
+			if (_nm_ip_config_add_obj (priv->multi_idx,
-		                           FALSE,
+			                           &priv->idx_ip6_routes_,
-		                           TRUE,
+			                           priv->ifindex,
-		                           NULL,
+			                           NULL,
-		                           &obj_new))
+			                           (const NMPlatformObject *) &r,
-			changed = TRUE;
+			                           FALSE,
-		new_best_default_route = _nm_ip_config_best_default_route_find_better (new_best_default_route, obj_new);
+			                           TRUE,
+			                           NULL,
+			                           &obj_new))
+				changed = TRUE;
+			new_best_default_route = _nm_ip_config_best_default_route_find_better (new_best_default_route, obj_new);
+
+			if (   first_pref != gateways[i].preference
+			    && !kernel_support_rta_pref) {
+				/* We are unable to configure a router preference. Hence, we skip all gateways
+				 * with a different preference from the first gateway. Note, that the gateways
+				 * are sorted in order of highest to lowest preference. */
+				break;
+			}
+		}
 	}
 
 	if (nm_dedup_multi_index_dirty_remove_idx (priv->multi_idx, &priv->idx_ip6_routes, FALSE) > 0)

src/nm-ip6-config.h

@@ -219,6 +219,7 @@ void nm_ip6_config_reset_routes_ndisc (NMIP6Config *self,
                                        const struct _NMNDiscRoute *routes,
                                        guint routes_n,
                                        guint32 route_table,
-                                       guint32 route_metric);
+                                       guint32 route_metric,
+                                       gboolean kernel_support_rta_pref);
 
 #endif /* __NETWORKMANAGER_IP6_CONFIG_H__ */