T5169: Add PoC for generating CGNAT rules rfc6888

Add PoC for generating CGNAT rules
https://datatracker.ietf.org/doc/html/rfc6888
Not all requirements are implemented, but some of them.
Implemented:

REQ-2
```
A CGN MUST have a default "IP address pooling" behavior of "Paired"
CGN must use the same external IP
      address mapping for all sessions associated with the same internal
      IP address, be they TCP, UDP, ICMP, something else, or a mix of
      different protocols.
```

REQ-3
```
The CGN function SHOULD NOT have any limitations on the size
      or the contiguity of the external address pool
```

REQ-4
```
A CGN MUST support limiting the number of external ports (or,
      equivalently, "identifiers" for ICMP) that are assigned per
      subscriber
```

CLI:
```
set nat cgnat pool external ext1 external-port-range '1024-65535'
set nat cgnat pool external ext1 per-user-limit port '1000'
set nat cgnat pool external ext1 range 192.0.2.222/32
set nat cgnat pool internal int1 range '100.64.0.0/28'
set nat cgnat rule 10 source pool 'int1'
set nat cgnat rule 10 translation pool 'ext1'
```

1 files changed, 47 insertions, 0 deletions

diff --git a/data/templates/firewall/nftables-cgnat.j2 b/data/templates/firewall/nftables-cgnat.j2
new file mode 100644
index 000000000..79a8e3d5a
--- /dev/null
+++ b/data/templates/firewall/nftables-cgnat.j2
@@ -0,0 +1,47 @@
+#!/usr/sbin/nft -f
+
+add table ip cgnat
+flush table ip cgnat
+
+add map ip cgnat tcp_nat_map { type ipv4_addr: interval ipv4_addr . inet_service ; flags interval ;}
+add map ip cgnat udp_nat_map { type ipv4_addr: interval ipv4_addr . inet_service ; flags interval ;}
+add map ip cgnat icmp_nat_map { type ipv4_addr: interval ipv4_addr . inet_service ; flags interval ;}
+add map ip cgnat other_nat_map { type ipv4_addr: interval ipv4_addr ; flags interval ;}
+flush map ip cgnat tcp_nat_map
+flush map ip cgnat udp_nat_map
+flush map ip cgnat icmp_nat_map
+flush map ip cgnat other_nat_map
+
+table ip cgnat {
+    map tcp_nat_map {
+        type ipv4_addr : interval ipv4_addr . inet_service
+        flags interval
+        elements = { {{ proto_map_elements }} }
+    }
+
+    map udp_nat_map {
+        type ipv4_addr : interval ipv4_addr . inet_service
+        flags interval
+        elements = { {{ proto_map_elements }} }
+    }
+
+    map icmp_nat_map {
+        type ipv4_addr : interval ipv4_addr . inet_service
+        flags interval
+        elements = { {{ proto_map_elements }} }
+    }
+
+    map other_nat_map {
+        type ipv4_addr : interval ipv4_addr
+        flags interval
+        elements = { {{ other_map_elements }} }
+    }
+
+    chain POSTROUTING {
+        type nat hook postrouting priority srcnat; policy accept;
+        ip protocol tcp counter snat ip to ip saddr map @tcp_nat_map
+        ip protocol udp counter snat ip to ip saddr map @udp_nat_map
+        ip protocol icmp counter snat ip to ip saddr map @icmp_nat_map
+        counter snat ip to ip saddr map @other_nat_map
+    }
+}