files: add load balance example

Include this example file in the tarball on how to do load balancing with
nftables, inspired from https://wiki.nftables.org

Signed-off-by: Arturo Borrero Gonzalez <arturo@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

diff --git a/files/examples/load_balancing.nft b/files/examples/load_balancing.nft
new file mode 100755 (executable)
index 0000000..2f03d27
--- /dev/null
+++ b/files/examples/load_balancing.nft
@@ -0,0 +1,54 @@
+#!/usr/sbin/nft -f
+
+# This example file shows how to implement load balancing using the nftables
+# framework.
+# This script is meant to be loaded with `nft -f <file>`
+# You require linux kernel >= 4.12 and nft >= 0.7
+# For up-to-date information please visit https://wiki.nftables.org
+
+flush ruleset
+
+table ip nat {
+       chain prerouting {
+               type nat hook prerouting priority -300;
+               # round-robing load balancing between the 2 IPv4 addresses:
+               dnat to numgen inc mod 2 map {
+                               0 : 192.168.10.100, \
+                               1 : 192.168.20.200 }
+               # emulate flow distribution with different backend weights using intervals:
+               dnat to numgen inc mod 10 map {
+                               0-5 : 192.168.10.100, \
+                               6-9 : 192.168.20.200 }
+               # tcp port based distribution is also possible:
+               ip protocol tcp dnat to 192.168.1.100 : numgen inc mod 2 map {
+                               0 : 4040 ,\
+                               1 : 4050 }
+               # consistent hash-based distribution:
+               dnat to jhash ip saddr . tcp dport mod 2 map {
+                               0 : 192.168.20.100, \
+                               1 : 192.168.30.100 }
+       }
+}
+
+table ip raw {
+       chain prerouting {
+               type filter hook prerouting priority -300;
+               # using stateless NAT, round-robing distribution (you could use hashing too):
+               tcp dport 80 notrack ip daddr set numgen inc mod 2 map { 0 : 192.168.1.100, 1 : 192.168.1.101 }
+       }
+}
+
+table netdev mytable {
+       chain ingress {
+               # mind the NIC devices, they must exist in the system
+               type filter hook ingress device eth0 priority 0;
+               # using Direct Server Return (DSR), connectionless approach:
+               udp dport 53 ether saddr set aa:bb:cc:dd:ff:ee ether daddr set numgen inc mod 2 map {
+                                       0 : aa:aa:aa:aa:aa:aa,
+                                       1 : bb:bb:bb:bb:bb:bb } fwd to eth1
+               # using Direct Server Return (DSR), connection-oriented flows:
+                tcp dport 80 ether saddr set aa:bb:cc:dd:ff:ee ether daddr set jhash ip saddr . tcp sport mod 2 map {
+                                       0 : aa:aa:aa:aa:aa:aa,
+                                       1 : bb:bb:bb:bb:bb:bb } fwd to eth1
+       }
+}