doc: add nat examples

nftables nat is much more capable than what the existing
documentation describes.

In particular, nftables can fully emulate iptables
NETMAP target and can perform n:m address mapping.

Add a new example section extracted from commit log
messages when those features got added.

Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>

1 files changed, 51 insertions, 2 deletions

diff --git a/doc/statements.txt b/doc/statements.txt
index 3fc70f86..19672805 100644
--- a/doc/statements.txt
+++ b/doc/statements.txt
@@ -359,8 +359,8 @@ NAT STATEMENTS
 ~~~~~~~~~~~~~~
 [verse]
 ____
-*snat* [[*ip* | *ip6*] *to*] 'ADDR_SPEC' [*:*'PORT_SPEC'] ['FLAGS']
-*dnat* [[*ip* | *ip6*] *to*] 'ADDR_SPEC' [*:*'PORT_SPEC'] ['FLAGS']
+*snat* [[*ip* | *ip6*] [ *prefix* ] *to*] 'ADDR_SPEC' [*:*'PORT_SPEC'] ['FLAGS']
+*dnat* [[*ip* | *ip6*] [ *prefix* ] *to*] 'ADDR_SPEC' [*:*'PORT_SPEC'] ['FLAGS']
 *masquerade* [*to :*'PORT_SPEC'] ['FLAGS']
 *redirect* [*to :*'PORT_SPEC'] ['FLAGS']
 
@@ -398,6 +398,9 @@ Before kernel 4.18 nat statements require both prerouting and postrouting base c
 to be present since otherwise packets on the return path won't be seen by
 netfilter and therefore no reverse translation will take place.
 
+The optional *prefix* keyword allows to map to map *n* source addresses to *n*
+destination addresses.  See 'Advanced NAT examples' below.
+
 .NAT statement values
 [options="header"]
 |==================
@@ -457,6 +460,52 @@ add rule inet nat postrouting meta oif ppp0 masquerade
 
 ------------------------
 
+.Advanced NAT examples
+----------------------
+
+# map prefixes in one network to that of another, e.g. 10.141.11.4 is mangled to 192.168.2.4,
+# 10.141.11.5 is mangled to 192.168.2.5 and so on.
+add rule nat postrouting snat ip prefix to ip saddr map { 10.141.11.0/24 : 192.168.2.0/24 }
+
+# map a source address, source port combination to a pool of destination addresses and ports:
+add rule nat postrouting dnat to ip saddr . tcp dport map { 192.168.1.2 . 80 : 10.141.10.2-10.141.10.5 . 8888-8999 }
+
+# The above example generates the following NAT expression:
+#
+# [ nat dnat ip addr_min reg 1 addr_max reg 10 proto_min reg 9 proto_max reg 11 ]
+#
+# which expects to obtain the following tuple:
+# IP address (min), source port (min), IP address (max), source port (max)
+# to be obtained from the map. The given addresses and ports are inclusive.
+
+# This also works with named maps and in combination with both concatenations and ranges:
+table ip nat {
+	map ipportmap {
+		typeof ip saddr : interval ip daddr . tcp dport
+		flags interval
+		elements = { 192.168.1.2 : 10.141.10.1-10.141.10.3 . 8888-8999, 192.168.2.0/24 : 10.141.11.5-10.141.11.20 . 8888-8999 }
+	}
+
+	chain prerouting {
+		type nat hook prerouting priority dstnat; policy accept;
+		ip protocol tcp dnat ip to ip saddr map @ipportmap
+	}
+}
+
+@ipportmap maps network prefixes to a range of hosts and ports.
+The new destination is taken from the range provided by the map element.
+Same for the destination port.
+
+Note the use of the "interval" keyword in the typeof description.
+This is required so nftables knows that it has to ask for twice the
+amount of storage for each key-value pair in the map.
+
+": ipv4_addr . inet_service" would allow associating one address and one port
+with each key.  But for this case, for each key, two addresses and two ports
+(The minimum and maximum values for both) have to be stored.
+
+------------------------
+
 TPROXY STATEMENT
 ~~~~~~~~~~~~~~~~
 Tproxy redirects the packet to a local socket without changing the packet header