Understanding NAT & DNS Interaction on Palo Alto Networks Firewalls (PCNSE Focus)

Network Address Translation (NAT) is fundamental for connecting private networks to the public internet and publishing internal services. However, the interplay between NAT and the Domain Name System (DNS) can create specific challenges, particularly the "split-DNS" problem where internal and external users need different IP address resolutions for the same Fully Qualified Domain Name (FQDN). Palo Alto Networks firewalls offer features like DNS Proxy and NAT Rule DNS Rewrite to address these challenges.

Mastering how these features work, when to use them, and how they interact with NAT and Security policies is crucial for effective firewall administration and for success on the PCNSE exam. This article provides a detailed explanation focused on these interactions.

     The PCNSE exam often includes scenarios testing your understanding of split-DNS issues and how to solve them using DNS Proxy or NAT Rule DNS Rewrite. Knowing the configuration details and the resulting traffic flow, including Security Policy implications, is essential.
    

The Problem: Internal Access via Public FQDN

Consider this common setup:

An internal web server hosts `www.mycorp.com` at private IP 10.1.1.100 (DMZ Zone).
A Destination NAT (DNAT) rule on the firewall translates incoming traffic to public IP 203.0.113.50 (Untrust Zone) to the internal server 10.1.1.100 .
The public DNS record for `www.mycorp.com` correctly points to the public IP 203.0.113.50 .

The Challenge: What happens when an internal user (e.g., 192.168.1.20 in the Trust Zone) tries to browse to `www.mycorp.com`?

DNS Resolution: The internal client queries its configured DNS server.
- If using an external DNS server (e.g., 8.8.8.8), it receives the public IP 203.0.113.50 .
- If using an internal DNS server that only holds the public record, it also receives the public IP 203.0.113.50 .
Connection Attempt: The client initiates a connection towards the public IP 203.0.113.50 .
Firewall Handling: The traffic goes from the Trust zone towards the Untrust zone (based on the route to the public IP). The firewall must now handle this "hairpin" or "U-turn" traffic.
U-Turn NAT Requirement: To make this work, a potentially complex U-Turn NAT configuration is often needed, involving:
- A DNAT rule to translate the public destination back to the private server IP.
- An SNAT rule (often required if client/server are in the same zone) to make the traffic appear to originate from the firewall itself, ensuring return traffic flows correctly.

This U-Turn approach works but adds complexity to NAT policy and can be less efficient as traffic must loop back through the firewall.

     Recognize this "split-DNS" problem where internal clients resolving a public FQDN for an internal resource get the public IP, necessitating complex U-Turn NAT.
    

Solution: DNS Proxy for Split-DNS

The Palo Alto Networks firewall's DNS Proxy feature provides an elegant solution to the split-DNS problem, often eliminating the need for U-Turn NAT for internal access scenarios.

When configured:

The firewall acts as an intermediary DNS server for specific internal clients/zones.
It can intercept DNS queries sent to its own interface IP.
Based on configured rules and static entries, it can provide different responses to internal clients than what external clients would receive.
Queries for internal resources can be answered directly by the firewall with the resource's private IP address.
Queries for external resources can be forwarded by the proxy to actual external DNS servers.

     Key Requirement: For DNS Proxy to work, internal clients must be configured (e.g., via DHCP) to use the firewall's internal interface IP address as their DNS server.
    

How DNS Proxy Solves the Split-DNS Problem

Using the previous example (`www.mycorp.com` = Public 203.0.113.50 / Private 10.1.1.100 ):

Client Configuration: Internal client 192.168.1.20 (Trust Zone) is configured to use the firewall's Trust interface IP (e.g., 192.168.1.1 ) as its DNS server.
Firewall DNS Proxy Config: A DNS Proxy Object is configured with a Static Entry mapping `www.mycorp.com` to the private IP `10.1.1.100`. This object is applied via a DNS Proxy Rule for traffic from the Trust zone destined for the firewall's IP (`192.168.1.1`).
Client DNS Query: Client sends DNS query for `www.mycorp.com` to 192.168.1.1 .
DNS Proxy Rule Match: The firewall's DNS Proxy Rule matches the incoming DNS query.
Static Entry Lookup: The DNS Proxy Object finds the static entry for `www.mycorp.com`.
DNS Proxy Reply: The firewall directly replies to the client with the private IP 10.1.1.100 .
Client Connection: The client now initiates its connection directly towards the private IP 10.1.1.100 .
Result: Traffic flows directly from the Trust zone towards the DMZ zone (where the server resides). No U-Turn NAT is required for this internal access, simplifying NAT and security policies.

     Understand that DNS Proxy intercepts queries sent *to the firewall* and uses static entries to provide internal IPs for specific FQDNs, thereby directing internal clients to connect directly to private IPs.
    

DNS Proxy Configuration Overview

Implementing DNS Proxy involves two main components:

DNS Proxy Object ( Objects > DNS Proxy ):
- Name: A descriptive name (e.g., `Internal_DNS_Proxy`).
- Interfaces/IP Addresses: Specify the firewall interface(s) and IP address(es) that will listen for DNS queries from clients. Typically, this is the Layer 3 interface IP address facing the internal clients.
- DNS Proxy Rules (within the Object): Define domains to handle.
  - Static Entries: Map specific FQDNs to their internal private IP addresses (e.g., `www.mycorp.com` -> `10.1.1.100`). This is the core of split-DNS.
  - Domain Forwarding: Define rules to forward queries for other domains (e.g., `*.google.com`, `*.*` for everything else) to specific external DNS servers (Primary/Secondary).
- Cache Settings: Configure DNS caching options (optional).
DNS Proxy Policy Rule ( Policies > DNS Proxy ):
- Name: A descriptive name (e.g., `Allow_Trust_to_DNS_Proxy`).
- Source Zones/Addresses: Specify the internal zones/subnets whose clients will use this DNS Proxy (e.g., Source Zone: `Trust`).
- Destination Interface/Address: Must match the interface/IP configured in the DNS Proxy Object where the firewall is listening for queries.
- Action: Select the DNS Proxy Object configured in step 1.

     Key Config Locations: Objects > DNS Proxy (for mappings & forwarding) and Policies > DNS Proxy (to activate the object for specific zones/interfaces).
    

     Know the two main configuration areas (Objects vs. Policies) and the purpose of Static Entries (internal mapping) and Domain Forwarding (external resolution) within the DNS Proxy Object.
    

DNS Proxy Interaction with Security Policy

When DNS Proxy is used effectively for split-DNS, it simplifies Security Policy configuration for internal access to internal resources.

DNS Traffic Policy: You need a Security Policy rule to allow the internal clients to send DNS queries (UDP/53, potentially TCP/53) to the firewall interface IP acting as the DNS Proxy server.
- Source Zone: e.g., `Trust`
- Source Address: e.g., `Trust_Subnet`
- Destination Zone: The zone containing the firewall interface acting as DNS server (e.g., `Trust` if the interface is in that zone).
- Destination Address: The firewall interface IP address configured in the DNS Proxy Object.
- Service: `service-dns` (or udp/53, tcp/53)
- Action: `Allow`
Data Traffic Policy: Since the client now connects directly to the server's private IP , the Security Policy rule allows this direct inter-zone or intra-zone traffic.
- Source Zone: e.g., `Trust`
- Source Address: e.g., `Trust_Subnet`
- Destination Zone: e.g., `DMZ` (Server's actual zone)
- Destination Address: e.g., `10.1.1.100` (Server's private IP)
- Application/Service: e.g., `ssl`, `web-browsing`
- Action: `Allow`

This avoids the complexity of the U-Turn NAT Security Policy, which needs to match the public destination IP but the internal destination zone.

     Remember that when DNS Proxy is used for split-DNS, the resulting data connection is directly between the client and the server's private IP. The Security Policy must allow this direct internal flow (inter-zone or intra-zone based on the server's actual location).
    

Alternative: NAT Rule DNS Rewrite Option

Separate from the DNS Proxy feature, individual NAT rules have a "DNS Rewrite" checkbox. This feature serves a different purpose than the primary split-DNS goal addressed by DNS Proxy.

NAT Rule DNS Rewrite inspects the payload of DNS responses that pass through the firewall *as part of the flow matching that specific NAT rule*. It modifies IP addresses found *within* the DNS response data (e.g., in A records, PTR records, TXT records) to be consistent with the address translation performed by that NAT rule itself.

Key Differences from DNS Proxy:

Trigger: DNS Rewrite acts on DNS responses matching the specific NAT rule it's enabled on. DNS Proxy acts on DNS queries sent *to* the firewall interface configured as a proxy.
Purpose: DNS Rewrite fixes inconsistencies *within DNS payloads* caused by NAT. DNS Proxy provides different DNS answers based on the querier's location (split-DNS).
Configuration: DNS Rewrite is an option within a NAT rule. DNS Proxy uses separate Objects and Policies.

While DNS Rewrite *can* sometimes be manipulated to achieve a split-DNS-like effect, DNS Proxy is the dedicated and recommended feature for the standard split-DNS use case (internal clients getting internal IPs).

     Distinguish clearly between DNS Proxy (intercepts queries to the firewall, provides different answers) and NAT Rule DNS Rewrite (modifies IPs within DNS response payloads passing through a specific NAT rule). DNS Proxy is preferred for typical split-DNS.
    

NAT Rule DNS Rewrite (Reverse)

Direction: Translates IPs in the DNS response payload from the Translated (External/Public) IP back to the Original (Internal/Private) IP defined in the NAT rule.

Primary Use Case: Used with Static Source NAT .

Scenario: An internal server ( 10.1.1.10 ) has a Static SNAT rule mapping its source IP to public IP 203.0.113.10 for outbound connections. If this server queries something externally (e.g., performs a PTR lookup on 203.0.113.10 , or interacts with a service that reflects its public IP back in a DNS record), the DNS response packet coming back towards the server (destined for 203.0.113.10 before reverse NAT) might contain 203.0.113.10 within its data payload.

Action: With DNS Rewrite (Reverse) enabled on the Static SNAT rule, as the firewall processes the return DNS response (destined for 10.1.1.10 after reverse DNAT of the destination), it inspects the payload. If it finds 203.0.113.10 , it rewrites it to 10.1.1.10 before delivering the packet to the internal server.

Configuration: Check "DNS Rewrite" and select "Reverse" in the Translated Packet tab of the Static Source NAT rule.

     Understand DNS Rewrite (Reverse) is mainly for Static SNAT scenarios to ensure internal servers see their correct private IP in DNS response payloads related to their translated public IP.
    

NAT Rule DNS Rewrite (Forward)

Direction: Translates IPs in the DNS response payload from the Original (Internal/Private) IP to the Translated (External/Public) IP defined in the NAT rule.

Primary Use Case: Used with Static Destination NAT , although less common than using DNS Proxy for the client-side resolution.

Scenario: An internal authoritative DNS server ( 10.2.2.20 ) is published using DNAT ( 203.0.113.20 -> 10.2.2.20 ). An external client queries this DNS server. The internal DNS server responds with an 'A' record containing the *private* IP of another internal server (e.g., 10.1.1.100 ). This private IP is useless to the external client.

Action: Assume another DNAT rule exists mapping public 203.0.113.100 to private 10.1.1.100 . With DNS Rewrite (Forward) enabled on the *outbound* flow of the DNS response (often tied to the DNAT rule for the DNS server or a broader rule), the firewall inspects the payload. It finds 10.1.1.100 and rewrites it to its corresponding public IP 203.0.113.100 before sending the response to the external client.

Configuration: Check "DNS Rewrite" and select "Forward" in the Translated Packet tab of the relevant NAT rule (could be the DNAT for the DNS server or a related rule handling the response flow).

     Understand DNS Rewrite (Forward) is mainly for ensuring external clients receive public IPs in DNS payloads when querying internal resources through DNAT, but DNS Proxy is often the cleaner solution for internal client resolution.
    

Choosing Between DNS Proxy and NAT DNS Rewrite

For Split-DNS (Internal clients needing internal IPs for internal FQDNs): Use DNS Proxy . This is the dedicated feature. Configure clients to use the firewall as DNS server, create static entries in the DNS Proxy Object, and apply via DNS Proxy Policy.
For fixing DNS payload IPs related to SNAT: Use NAT Rule DNS Rewrite (Reverse) on the relevant Static Source NAT rule.
For fixing DNS payload IPs related to DNAT (less common): Use NAT Rule DNS Rewrite (Forward) on the relevant DNAT rule (or associated outbound rule), but consider if DNS Proxy for internal clients is a better overall approach.

     Know when to apply DNS Proxy (primary split-DNS solution) versus NAT Rule DNS Rewrite (fixing payload inconsistencies tied to a specific NAT rule).
    

Diagram: Sequence - DNS Proxy Flow (Split-DNS)

Sequence diagram illustrating how DNS Proxy intercepts a query to the firewall, uses a static entry to reply with the internal IP, and allows direct internal connection, avoiding U-Turn NAT.

Diagram: Sequence - NAT Rule DNS Rewrite (Reverse)

Sequence diagram showing DNS Rewrite (Reverse) modifying a DNS response payload containing the server's NAT'd public IP back to its private IP before delivery.

Diagram: Sequence - NAT Rule DNS Rewrite (Forward)

Sequence diagram illustrating DNS Rewrite (Forward) modifying a DNS response payload from an internal DNS server, changing an internal service IP to its public equivalent.

Diagram: Flowchart - DNS Proxy vs NAT DNS Rewrite

Flowchart guiding the choice between DNS Proxy and NAT Rule DNS Rewrite based on the problem being solved.

PCNSE Exam Focus Summary (NAT & DNS)

Key takeaways for the PCNSE exam regarding NAT and DNS interaction:

Split-DNS Problem: Understand why internal clients resolving public FQDNs for internal resources can lead to issues requiring U-Turn NAT.
DNS Proxy for Split-DNS: Know this is the primary solution. Understand its mechanism (intercepting queries to the firewall, using static entries). Know the configuration requirements (clients use firewall IP, DNS Proxy Object, DNS Proxy Policy).
DNS Proxy Security Policy: Understand that the data traffic flows directly internal-to-internal, and the security policy must allow this based on private IPs and actual zones. Remember the policy needed to allow DNS queries *to* the proxy itself.
NAT Rule DNS Rewrite: Understand its distinct purpose (modifying DNS response payloads to match NAT rule translations). Differentiate 'Reverse' (for SNAT) and 'Forward' (for DNAT) use cases. Know it's configured within the NAT rule itself.
When to Use Which: Prioritize DNS Proxy for standard split-DNS. Use NAT Rule DNS Rewrite for fixing payload inconsistencies directly related to a NAT rule's translation.
U-Turn NAT Awareness: Recognize scenarios where DNS Proxy/Rewrite is *not* used, meaning U-Turn NAT might still be required, and recall its configuration principles (DNAT, conditional SNAT, "Pre-NAT IP, Post-NAT Zone" Security Policy).

     Expect scenario questions asking you to identify the split-DNS problem and choose the appropriate solution (usually DNS Proxy). Be prepared to configure DNS Proxy Objects/Policies conceptually and determine the correct Security Policy rules when DNS Proxy is in use. Understand the less common use cases for NAT Rule DNS Rewrite.
    

PCNSE Style Quiz: NAT & DNS Interaction

Test your understanding with these 20 questions focused on NAT, DNS Proxy, and DNS Rewrite:

1. What problem does using DNS Proxy primarily solve when internal users try to access internal resources using their public FQDNs?

It encrypts DNS traffic between the client and the firewall. It allows external users to resolve internal IP addresses. It provides internal clients with the internal IP address of the resource, avoiding the need for U-Turn NAT. It caches DNS responses to speed up external website access.

2. To implement split-DNS using DNS Proxy on a Palo Alto firewall, where do you configure the specific mapping between an FQDN (e.g., `internal.myco.com`) and its internal private IP address (e.g., `10.2.2.20`)?

In the Security Policy rule allowing DNS traffic. In a Destination NAT rule with DNS Rewrite enabled. Within the DNS Proxy Object's "Static Entries" table. As a Custom URL Category object.

3. An internal client uses the firewall (configured as a DNS Proxy) for DNS resolution. The client queries `server.internal.lan`, and the DNS Proxy provides the internal IP `10.50.1.100`. Which destination IP address will the client use when initiating the actual connection to `server.internal.lan`?

The firewall's internal interface IP. The firewall's external public IP. The IP address of the external DNS server. The internal IP address `10.50.1.100` provided by the DNS Proxy.

4. Following the scenario in Question 3, where the internal client connects directly to the internal server's IP `10.50.1.100` (located in the DMZ zone) after DNS Proxy resolution, what should the Destination Address be in the Security Policy rule allowing this connection?

The firewall's external public IP address. The FQDN address object for `server.internal.lan`. The internal IP address `10.50.1.100`. `any`

5. Compared to configuring U-Turn NAT, what is a primary advantage of using DNS Proxy for internal clients accessing internal resources via public FQDNs?

DNS Proxy works for both IPv4 and IPv6 simultaneously. DNS Proxy allows traffic to flow directly between internal zones, potentially simplifying NAT rules and improving efficiency. DNS Proxy encrypts the actual data connection to the server. DNS Proxy requires fewer Security Policy rules than U-Turn NAT.

6. What is the primary function of the NAT Rule DNS Rewrite option?

To modify IP addresses within the payload of DNS responses related to the NAT rule's translation. To provide internal clients with internal IP addresses for internal FQDNs. To forward DNS queries to specific upstream servers. To cache DNS query results on the firewall.

7. In which configuration section are DNS Proxy Objects (containing Static Entries and Forwarding rules) created?

Policies > DNS Proxy Network > DNS Proxy Device > Setup > Services Objects > DNS Proxy

8. What activates a specific DNS Proxy Object for traffic from certain clients?

A DNS Proxy Policy rule under Policies > DNS Proxy. A Security Policy rule with action "DNS Proxy". Enabling DNS Proxy on the client's interface. Associating the object with a Virtual Router.

9. The NAT Rule DNS Rewrite (Reverse) option is most commonly associated with which type of NAT?

Destination NAT (Static) Source NAT (Static) Source NAT (DIPP) NPTv6

10. For DNS Proxy to function for internal clients, what must be configured as their DNS server address?

An external DNS server IP (e.g., 8.8.8.8). The internal DNS server's IP address. The firewall's management interface IP address. The firewall's Layer 3 interface IP address on the client's segment.

11. A DNS Proxy Object is configured with a static entry `app.internal -> 10.10.10.10` and a forwarding rule for `*.*` to `8.8.8.8`. An internal client using the proxy queries `www.google.com`. What happens?

The proxy replies with 10.10.10.10. The proxy forwards the query to 8.8.8.8 and relays the response. The query is dropped. The proxy replies with its own IP address.

12. Which feature inspects and modifies IP addresses *within the DNS packet payload* based on an associated NAT rule?

DNS Security Subscription DNS Proxy NAT Rule DNS Rewrite Policy Based Forwarding

13. You want internal clients to use the firewall for DNS, getting internal IPs for internal servers and external IPs for everything else. Which feature is most appropriate?

DNS Proxy NAT Rule DNS Rewrite (Forward) NAT Rule DNS Rewrite (Reverse) U-Turn NAT

14. If DNS Proxy is correctly configured and provides an internal client with the private IP of an internal server, U-Turn NAT rules are typically:

Still required for security. Required, but only the DNAT portion. Required, but only the SNAT portion. Not required for that client accessing that server via FQDN.

15. A Security Policy rule allowing internal clients to query the firewall's DNS Proxy listener IP must have which service/port?

service-https service-dns (or udp/53, tcp/53) service-icmp service-ssh

16. The NAT Rule DNS Rewrite (Forward) option would translate an IP found in a DNS response payload from:

Public IP to Private IP Private IP to Private IP Private IP to Public IP Public IP to Public IP

17. An internal server (10.1.1.5) is subject to Static SNAT (-> 203.0.113.5). It receives a DNS response containing its public IP (203.0.113.5) in the payload. Which feature fixes this?

NAT Rule DNS Rewrite (Reverse) NAT Rule DNS Rewrite (Forward) DNS Proxy Destination NAT

18. Can DNS Proxy be used to block access to certain external domains?

No, it only forwards or provides static entries. Yes, by creating a static entry pointing to 0.0.0.0. Yes, using DNS Security signatures. Yes, while not its primary purpose, you could potentially sinkhole domains by creating static entries pointing to a non-routable/sinkhole IP or by not forwarding queries for them (though DNS Security is better for blocking).

19. If an internal client uses an external DNS server directly (bypassing the firewall's DNS Proxy), can DNS Proxy provide a split-DNS response?

Yes, the firewall intercepts all DNS traffic. No, DNS Proxy only handles queries sent directly to its listener IP address. Yes, if DNS Rewrite (Forward) is enabled on the outbound NAT rule. Only if the external DNS server forwards back to the firewall.

20. The NAT Rule DNS Rewrite feature modifies which part of the DNS packet?

The IP addresses within the DNS answer/additional/authority sections (payload). The source or destination IP address in the IP header. The source or destination UDP/TCP port in the transport header. The DNS transaction ID in the DNS header.