KRT: Allow to learn routes with RTPROT_KERNEL

The Kernel protocol, even with the option 'learn' enabled, ignores
direct routes created by the OS kernel (on Linux these are routes
with rtm_protocol == RTPROT_KERNEL).

Implement optional behavior where both OS kernel and third-party routes
are learned, it can be enabled by 'learn all' option.

Minor changes by committer.

doc/bird.sgml

@@ -3764,9 +3764,8 @@ on the <cf/learn/ switch, such routes are either ignored or accepted to our
 table).
 
 <p>Note that routes created by OS kernel itself, namely direct routes
-representing IP subnets of associated interfaces, are not imported even with
+representing IP subnets of associated interfaces, are imported only with
-<cf/learn/ enabled. You can use <ref id="direct" name="Direct protocol"> to
+<cf/learn all/ enabled.
-generate these direct routes.
 
 <p>If your OS supports only a single routing table, you can configure only one
 instance of the Kernel protocol. If it supports multiple tables (in order to
@@ -3797,10 +3796,12 @@ channels.
 	Time in seconds between two consecutive scans of the kernel routing
 	table.
 
-	<tag><label id="krt-learn">learn <m/switch/</tag>
+	<tag><label id="krt-learn">learn <m/switch/|all</tag>
 	Enable learning of routes added to the kernel routing tables by other
 	routing daemons or by the system administrator. This is possible only on
-	systems which support identification of route authorship.
+	systems which support identification of route authorship. By default,
+	routes created by kernel (marked as "proto kernel") are not imported.
+	Use <cf/learn all/ option to import even these routes.
 
 	<tag><label id="krt-kernel-table">kernel table <m/number/</tag>
 	Select which kernel table should this particular instance of the Kernel

sysdep/linux/netlink.c

@@ -1598,7 +1598,7 @@ nl_parse_route(struct nl_parse_state *s, struct nlmsghdr *h)
 
     case RTPROT_KERNEL:
       krt_src = KRT_SRC_KERNEL;
-      return;
+      break;
 
     case RTPROT_BIRD:
       if (!s->scan)

sysdep/unix/krt.Y

@@ -32,6 +32,7 @@ CF_DECLS
 CF_KEYWORDS(KERNEL, PERSIST, SCAN, TIME, LEARN, DEVICE, ROUTES, GRACEFUL, RESTART, KRT_SOURCE, KRT_METRIC, MERGE, PATHS)
 CF_KEYWORDS(INTERFACE, PREFERRED)
 
+%type <i> kern_learn
 %type <i> kern_mp_limit
 %type <cc> kern_channel
 
@@ -49,6 +50,11 @@ kern_proto_start: proto_start KERNEL {
 kern_proto: kern_proto_start proto_name '{' ;
 kern_proto: kern_proto kern_item ';' ;
 
+kern_learn:
+   bool { $$ = $1 ? KRT_LEARN_ALIEN : KRT_LEARN_NONE; }
+ | ALL  { $$ = KRT_LEARN_ALL; }
+ ;
+
 kern_mp_limit:
    /* empty */ { $$ = KRT_DEFAULT_ECMP_LIMIT; }
  | LIMIT expr  { $$ = $2; if (($2 <= 0) || ($2 > 255)) cf_error("Merge paths limit must be in range 1-255"); }
@@ -68,7 +74,7 @@ kern_item:
       /* Scan time of 0 means scan on startup only */
       THIS_KRT->scan_time = $3 S_;
    }
- | LEARN bool {
+ | LEARN kern_learn {
       THIS_KRT->learn = $2;
 #ifndef KRT_ALLOW_LEARN
       if ($2)

sysdep/unix/krt.c

@@ -639,12 +639,14 @@ krt_got_route(struct krt_proto *p, rte *e, s8 src)
 #ifdef KRT_ALLOW_LEARN
   switch (src)
     {
-    case KRT_SRC_KERNEL:
-      goto ignore;
-
     case KRT_SRC_REDIRECT:
       goto delete;
 
+    case KRT_SRC_KERNEL:
+      if (KRT_CF->learn != KRT_LEARN_ALL)
+        goto ignore;
+      /* fallthrough */
+
     case  KRT_SRC_ALIEN:
       if (KRT_CF->learn)
 	krt_learn_scan(p, e);
@@ -780,6 +782,11 @@ krt_got_route_async(struct krt_proto *p, rte *e, int new, s8 src)
       break;
 
 #ifdef KRT_ALLOW_LEARN
+    case KRT_SRC_KERNEL:
+      if (KRT_CF->learn != KRT_LEARN_ALL)
+        break;
+      /* fallthrough */
+
     case KRT_SRC_ALIEN:
       if (KRT_CF->learn)
 	{

sysdep/unix/krt.h

@@ -27,6 +27,10 @@ struct kif_proto;
 #define KRT_REF_SEEN	0x1	/* Seen in table */
 #define KRT_REF_BEST	0x2	/* Best in table */
 
+#define KRT_LEARN_NONE	0	/* Do not learn */
+#define KRT_LEARN_ALIEN	1	/* Learn KRT_SRC_ALIEN routes */
+#define KRT_LEARN_ALL	2	/* Learn both KRT_SRC_ALIEN and KRT_SRC_KERNEL routes */
+
 /* Whenever we recognize our own routes, we allow learing of foreign routes */
 
 #ifdef CONFIG_SELF_CONSCIOUS