src: add hash expression

This is special expression that transforms an input expression into a
32-bit unsigned integer. This expression takes a modulus parameter to
scale the result and the random seed so the hash result becomes harder
to predict.

You can use it to set the packet mark, eg.

 # nft add rule x y meta mark set jhash ip saddr . ip daddr mod 2 seed 0xdeadbeef

You can combine this with maps too, eg.

 # nft add rule x y dnat to jhash ip saddr mod 2 seed 0xdeadbeef map { \
	0 : 192.168.20.100, \
	1 : 192.168.30.100 \
   }

Currently, this expression implements the jenkins hash implementation
available in the Linux kernel:

 http://lxr.free-electrons.com/source/include/linux/jhash.h

But it should be possible to extend it to support any other hash
function type.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/include/expression.h b/include/expression.h
index b6005ec3..6a509b33 100644
--- a/include/expression.h
+++ b/include/expression.h
@@ -34,6 +34,7 @@
  * @EXPR_BINOP:		binary operations (bitwise, shifts)
  * @EXPR_RELATIONAL:	equality and relational expressions
  * @EXPR_NUMGEN:	number generation expression
+ * @EXPR_HASH:		hash expression
  */
 enum expr_types {
 	EXPR_INVALID,
@@ -57,6 +58,7 @@ enum expr_types {
 	EXPR_BINOP,
 	EXPR_RELATIONAL,
 	EXPR_NUMGEN,
+	EXPR_HASH,
 };
 
 enum ops {
@@ -174,6 +176,7 @@ enum expr_flags {
 #include <exthdr.h>
 #include <numgen.h>
 #include <meta.h>
+#include <hash.h>
 #include <ct.h>
 
 /**
@@ -285,6 +288,12 @@ struct expr {
 			enum nft_ng_types	type;
 			uint32_t		mod;
 		} numgen;
+		struct {
+			/* EXPR_HASH */
+			struct expr		*expr;
+			uint32_t		mod;
+			uint32_t		seed;
+		} hash;
 	};
 };