src: add numgen expression

This new expression allows us to generate incremental and random numbers
bound to a specified modulus value.

The following rule sets the conntrack mark of 0 to the first packet seen,
then 1 to second packet, then 0 again to the third packet and so on:

 # nft add rule x y ct mark set numgen inc mod 2

A more useful example is a simple load balancing scenario, where you can
also use maps to set the destination NAT address based on this new numgen
expression:

 # nft add rule nat prerouting \
	dnat to numgen inc mod 2 map { 0 : 192.168.10.100, 1 : 192.168.20.200 }

So this is distributing new connections in a round-robin fashion between
192.168.10.100 and 192.168.20.200. Don't forget the special NAT chain
semantics: Only the first packet evaluates the rule, follow up packets
rely on conntrack to apply the NAT information.

You can also emulate flow distribution with different backend weights
using intervals:

 # nft add rule nat prerouting \
	dnat to numgen inc mod 10 map { 0-5 : 192.168.10.100, 6-9 : 192.168.20.200 }

So 192.168.10.100 gets 60% of the workload, while 192.168.20.200 gets 40%.

We can also be mixed with dynamic sets, thus weight can be updated in
runtime.

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

1 files changed, 4 insertions, 0 deletions

diff --git a/src/scanner.l b/src/scanner.l
index 53b79aa5..cff375f3 100644
--- a/src/scanner.l
+++ b/src/scanner.l
@@ -467,6 +467,10 @@ addrstring	({macaddr}|{ip4addr}|{ip6addr})
 "proto-dst"		{ return PROTO_DST; }
 "label"			{ return LABEL; }
 
+"numgen"		{ return NUMGEN; }
+"inc"			{ return INC; }
+"mod"			{ return MOD; }
+
 "dup"			{ return DUP; }
 "fwd"			{ return FWD; }