KRT: Fixes learning of preferred kernel routes.

When a new route was imported from kernel and chosen as preferred, then
the old best route was propagated as a withdraw to the kernel protocol.
Under some circumstances such withdraw propagated to the BSD kernel could
remove the new alien route and thus reverting the import.

sysdep/cf/README

@@ -5,6 +5,7 @@ CONFIG_AUTO_ROUTES	Device routes are added automagically by the kernel
 CONFIG_SELF_CONSCIOUS	We're able to recognize whether route was installed by us
 CONFIG_MULTIPLE_TABLES	The kernel supports multiple routing tables
 CONFIG_ALL_TABLES_AT_ONCE	Kernel scanner wants to process all tables at once
+CONFIG_SINGLE_ROUTE	There is only one route per network
 
 CONFIG_MC_PROPER_SRC	Multicast packets have source address according to socket saddr field
 CONFIG_SKIP_MC_BIND	Don't call bind on multicast socket (def for *BSD)

sysdep/cf/bsd-v6.h

@@ -11,6 +11,7 @@
 #define CONFIG_AUTO_ROUTES
 #define CONFIG_SELF_CONSCIOUS
 #define CONFIG_MULTIPLE_TABLES
+#define CONFIG_SINGLE_ROUTE
 
 #define CONFIG_SKIP_MC_BIND
 #define CONFIG_NO_IFACE_BIND

sysdep/cf/bsd.h

@@ -9,6 +9,7 @@
 #define CONFIG_AUTO_ROUTES
 #define CONFIG_SELF_CONSCIOUS
 #define CONFIG_MULTIPLE_TABLES
+#define CONFIG_SINGLE_ROUTE
 
 #define CONFIG_SKIP_MC_BIND
 #define CONFIG_NO_IFACE_BIND

sysdep/unix/krt.c

@@ -961,7 +961,21 @@ krt_import_control(struct proto *P, rte **new, ea_list **attrs, struct linpool *
   rte *e = *new;
 
   if (e->attrs->src->proto == P)
+  {
+#ifdef CONFIG_SINGLE_ROUTE
+    /*
+     * Implicit withdraw - when the imported kernel route becomes the best one,
+     * we know that the previous one exported to the kernel was already removed,
+     * but if we processed the update as usual, we would send withdraw to the
+     * kernel, which would remove the new imported route instead.
+     *
+     * We will remove KRT_INSTALLED flag, which stops such withdraw to be
+     * processed in krt_rt_notify() and krt_replace_rte().
+     */
+    e->net->n.flags &= ~KRF_INSTALLED;
+#endif
     return -1;
+  }
 
   if (!KRT_CF->devroutes &&
       (e->attrs->dest == RTD_DEVICE) &&