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, 16 insertions, 0 deletions

diff --git a/src/netlink_linearize.c b/src/netlink_linearize.c
index a5d4502f..b5967d49 100644
--- a/src/netlink_linearize.c
+++ b/src/netlink_linearize.c
@@ -104,6 +104,20 @@ static void netlink_gen_concat(struct netlink_linearize_ctx *ctx,
 	}
 }
 
+static void netlink_gen_fib(struct netlink_linearize_ctx *ctx,
+			    const struct expr *expr,
+			    enum nft_registers dreg)
+{
+	struct nftnl_expr *nle;
+
+	nle = alloc_nft_expr("fib");
+	netlink_put_register(nle, NFTNL_EXPR_FIB_DREG, dreg);
+	nftnl_expr_set_u32(nle, NFTNL_EXPR_FIB_RESULT, expr->fib.result);
+	nftnl_expr_set_u32(nle, NFTNL_EXPR_FIB_FLAGS, expr->fib.flags);
+
+	nftnl_rule_add_expr(ctx->nlr, nle);
+}
+
 static void netlink_gen_hash(struct netlink_linearize_ctx *ctx,
 			     const struct expr *expr,
 			     enum nft_registers dreg)
@@ -663,6 +677,8 @@ static void netlink_gen_expr(struct netlink_linearize_ctx *ctx,
 		return netlink_gen_numgen(ctx, expr, dreg);
 	case EXPR_HASH:
 		return netlink_gen_hash(ctx, expr, dreg);
+	case EXPR_FIB:
+		return netlink_gen_fib(ctx, expr, dreg);
 	default:
 		BUG("unknown expression type %s\n", expr->ops->name);
 	}