NAT Policy Rules
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Next-Generation Firewall

NAT Policy Rules

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NAT Policy Rules

Understand NAT policy rules and NAT rule capacities.
Where Can I Use This?What Do I Need?
  • NGFW (Managed by PAN-OS or Panorama)
You configure a NAT rule to match a packet’s source zone and destination zone, at a minimum. In addition to zones, you can configure matching criteria based on the packet’s destination interface, source and destination address, and service. You can configure multiple NAT rules. The firewall evaluates the rules in order from the top down. Once a packet matches the criteria of a single NAT rule, the packet is not subjected to additional NAT rules. Therefore, your list of NAT rules should be in order from most specific to least specific so that packets are subjected to the most specific rule you created for them.
It is important to understand that in firewall policy rules (including NAT), the set of IPv4 addresses is treated as a subset of the set of IPv6 addresses. However, the set of IPv6 addresses is not a subset of the set of IPv4 addresses. An IPv4 address can match a set or range of IPv6 addresses; but an IPv6 address cannot match a set or range of IPv4 addresses.
In all policy types, the keyword any for a source or destination address means any IPv4 or IPv6 address. The keyword any is equivalent to ::/0. If you want to express "any IPv4 address", specify 0.0.0.0/0.
During policy matching, the firewall converts an IPv4 address into an IPv6 prefix where the first 96 bits are 0. An address of ::/8 means, match the rule if the first 8 bits are 0. All IPv4 addresses will match ::/8, ::/9, ::/10, ::/11, ... ::/16, ... ::/32, ... through ::/96.
If you want to express "any IPv6 address, but no IPv4 addresses", you must configure two rules. The first rule denies 0.0.0.0/0 to deny any IPv4 address (as the source or destination address), and the second rule has ::/0 to mean any IPv6 address (as the source or destination address), to satisfy your requirement.
Static NAT rules do not have precedence over other forms of NAT. Therefore, for static NAT to work, the static NAT rules must be above all other NAT rules in the list on the firewall.
NAT rules provide address translation, and are different from security policy rules, which allow or deny packets. It is important to understand the firewall’s flow logic when it applies NAT rules and security policy rules so that you can determine what rules you need, based on the zones you have defined. You must configure security policy rules to allow the NAT traffic.
Upon ingress, the firewall inspects the packet and does a route lookup to determine the egress interface and zone. Then the firewall determines if the packet matches one of the NAT rules that have been defined, based on source and/or destination zone. It then evaluates and applies any security policies that match the packet based on the original (pre-NAT) source and destination addresses, but the post-NAT zones. Finally, upon egress, for a matching NAT rule, the firewall translates the source and/or destination address and port numbers.
Keep in mind that the translation of the IP address and port do not occur until the packet leaves the firewall. The NAT rules and security policies apply to the original IP address (the pre-NAT address). A NAT rule is configured based on the zone associated with a pre-NAT IP address.
Security policies differ from NAT rules because security policies examine post-NAT zones to determine whether the packet is allowed or not. Because the very nature of NAT is to modify source or destination IP addresses, which can result in modifying the packet’s outgoing interface and zone, security policies are enforced on the post-NAT zone.
A SIP call sometimes experiences one-way audio when going through the firewall because the call manager sends a SIP message on behalf of the phone to set up the connection. When the message from the call manager reaches the firewall, the SIP ALG must put the IP address of the phone through NAT. If the call manager and the phones are not in the same security zone, the NAT lookup of the IP address of the phone is done using the call manager zone. The NAT policy should take this into consideration.
No-NAT rules are configured to allow exclusion of IP addresses defined within the range of NAT rules defined later in the NAT policy. To define a no-NAT policy, specify all of the match criteria and select No Source Translation in the source translation column.
You can verify the NAT rules processed by selecting DeviceTroubleshooting and testing the traffic matches for the NAT rule. For example:

NAT Address Pools Identified as Address Objects

When configuring a Dynamic IP or Dynamic IP and Port NAT address pool in a NAT policy rule, it is typical to configure the pool of translated addresses with address objects. Each address object can be a host IP address, IP address range, or IP subnet.
Because both NAT rules and security policy rules use address objects, it is a best practice to distinguish between them by naming an address object used for NAT with a prefix, such as “NAT-name.”

Proxy ARP for NAT Address Pools

NAT address pools are not bound to any interfaces. The following figure illustrates the behavior of the firewall when it is performing proxy ARP for an address in a NAT address pool.
The firewall performs source NAT for a client, translating the source address 10.1.1.1 to the address in the NAT pool, 192.168.2.2. The translated packet is sent on to a router.
For the return traffic, the router does not know how to reach 192.168.2.2 (because that IP address is just an address in the NAT address pool), so it sends an ARP request packet to the firewall.
In our first scenario, when the NAT pool address (192.168.2.2) is in the same subnet as the egress/ingress interface IP address (192.168.2.3/24), the firewall can send a proxy ARP reply to the router, indicating the Layer 2 MAC address for 192.168.2.2 is 54:22:07:33:98:21, as shown in the figure above.

No Proxy ARP When the NAT Pool Address Isn't a Subnet of the Egress/Ingress Interface

In our second scenario, the NAT pool address (192.168.2.2) isn't a subnet of an interface on the firewall, so the firewall won't send a proxy ARP reply to the router. This means that the router must be configured with the necessary route to know where to send packets destined for 192.168.2.2, in order to ensure the return traffic is routed back to the firewall, as shown in the figure below.

NAT Rule Capacities

The number of NAT rules allowed is based on the firewall model. Individual rule limits are set for static, Dynamic IP (DIP), and Dynamic IP and Port (DIPP) NAT. The sum of the number of rules used for these NAT types cannot exceed the total NAT rule capacity. For DIPP, the rule limit is based on the oversubscription setting (8, 4, 2, or 1) of the firewall and the assumption of one translated IP address per rule. To see model-specific NAT rule limits and translated IP address limits, use the Compare Firewalls tool.
Consider the following when working with NAT rules:
  • If you run out of pool resources, you cannot create more NAT rules, even if the model’s maximum rule count has not been reached.
  • If you consolidate NAT rules, the logging and reporting will also be consolidated. The statistics are provided per the rule, not per all of the addresses within the rule. If you need granular logging and reporting, do not combine the rules.