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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>
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