KRT: Add krt_scope attribute

Add a new route attribute, krt_scope, to expose the Linux kernel route
scope. Constants from /etc/iproute2/rt_scopes (prefixed by "ips_") are
expected to be used with the attribute. Both import and export are
supported.

Also, the patch fixes device route export to the kernel, by setting link
scope automatically.

1 files changed, 9 insertions, 0 deletions

diff --git a/doc/bird.sgml b/doc/bird.sgml
index 9bd7df86..af5b7980 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -2443,6 +2443,15 @@ these attributes:
 
 	<tag>int <cf/krt_realm/</tag> (Linux)
 	The realm of the route. Can be used for traffic classification.
+
+	<tag>int <cf/krt_scope/</tag> (Linux IPv4)
+	The scope of the route. Valid values are 0-254, although Linux kernel
+	may reject some values depending on route type and nexthop. It is
+	supposed to represent `indirectness' of the route, where nexthops of
+	routes are resolved through routes with a higher scope, but in current
+	kernels anything below <it/link/ (253) is treated as <it/global/ (0).
+	When not present, global scope is implied for all routes except device
+	routes, where link scope is used by default.
 </descrip>
 
 <p>In Linux, there is also a plenty of obscure route attributes mostly focused