src: add fib expression

This adds the 'fib' expression which can be used to
obtain the output interface from the route table based on either
source or destination address of a packet.

This can be used to e.g. add reverse path filtering:

 # drop if not coming from the same interface packet
 # arrived on
 # nft add rule x prerouting fib saddr . iif oif eq 0 drop

 # accept only if from eth0
 # nft add rule x prerouting fib saddr . iif oif eq "eth0" accept

 # accept if from any valid interface
 # nft add rule x prerouting fib saddr oif accept

Querying of address type is also supported.  This can be used
to e.g. only accept packets to addresses configured in the same
interface:
 # fib daddr . iif type local

Its also possible to use mark and verdict map, e.g.:
 # nft add rule x prerouting meta mark set 0xdead fib daddr . mark type vmap {
   blackhole : drop,
   prohibit : drop,
   unicast : accept
 }

Signed-off-by: Florian Westphal <fw@strlen.de>

1 files changed, 7 insertions, 0 deletions

diff --git a/include/expression.h b/include/expression.h
index 3ae4e804..3a52a45c 100644
--- a/include/expression.h
+++ b/include/expression.h
@@ -61,6 +61,7 @@ enum expr_types {
 	EXPR_NUMGEN,
 	EXPR_HASH,
 	EXPR_RT,
+	EXPR_FIB,
 };
 
 enum ops {
@@ -180,6 +181,7 @@ enum expr_flags {
 
 #include <payload.h>
 #include <exthdr.h>
+#include <fib.h>
 #include <numgen.h>
 #include <meta.h>
 #include <rt.h>
@@ -306,6 +308,11 @@ struct expr {
 			uint32_t		mod;
 			uint32_t		seed;
 		} hash;
+		struct {
+			/* EXPR_FIB */
+			uint32_t		flags;
+			uint32_t		result;
+		} fib;
 	};
 };