Virtual Wire Source NAT Example

Virtual wire deployment of a Palo Alto Networks firewall includes the benefit of providing security transparently to the end devices. It is possible to configure NAT for interfaces configured in a virtual wire. All of the NAT types are allowed: source NAT (Dynamic IP, Dynamic IP and Port, static) and destination NAT.
Because interfaces in a virtual wire do not have an IP address assigned, it is not possible to translate an IP address to an interface IP address. You must configure an IP address pool.
When performing NAT on virtual wire interfaces, it is recommended that you translate the source address to a different subnet than the one on which the neighboring devices are communicating. The firewall will not proxy ARP for NAT addresses. Proper routing must be configured on the upstream and downstream routers in order for the packets to be translated in virtual wire mode. Neighboring devices will only be able to resolve ARP requests for IP addresses that reside on the interface of the device on the other end of the virtual wire. See Proxy ARP for NAT Address Pools for more explanation about proxy ARP.
In the source NAT example below, security policies (not shown) are configured from the virtual wire zone named vw-trust to the zone named vw-untrust.
In the following topology, two routers are configured to provide connectivity between subnets 192.0.2.0/24 and 172.16.1.0/24. The link between the routers is configured in subnet 198.51.100.0/30. Static routing is configured on both routers to establish connectivity between the networks. Before the firewall is deployed in the environment, the topology and the routing table for each router look like this:
vwire_nat_no_fw.png
Route on R1:
Destination
Next Hop
172.16.1.0/24
198.51.100.2
Route on R2:
Destination
Next Hop
192.0.2.0/24
198.51.100.1
Now the firewall is deployed in virtual wire mode between the two Layer 3 devices. A NAT IP address pool with range 198.51.100.9 to 198.51.100.14 is configured on the firewall. All communications from clients in subnet 192.0.2.0/24 accessing servers in network 172.16.1.0/24 will arrive at R2 with a translated source address in the range 198.51.100.9 to 198.51.100.14. The response from servers will be directed to these addresses.
vwire_nat_source_nat.png
In order for source NAT to work, you must configure proper routing on R2, so that packets destined for other addresses are not dropped. The routing table below shows the modified routing table on R2; the route ensures traffic to the destinations 198.51.100.9-198.51.100.14 (that is, hosts on subnet 198.51.100.8/29) will be sent back through the firewall to R1.
Route on R2:
Destination
Next Hop
198.51.100.8/29
198.51.100.1

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