Mechanism of transparent DNS proxy on the USG9000

The process of DNS transparent proxy is described as follows:
1.An administrator determines which DNS requests require DNS transparent proxy based on a DNS transparent proxy policy. As the policy is matched based only on the source and destination addresses of the DNS requests, DNS transparent proxy works no matter what DNS server address is on the client (an extreme situation is that no DNS server address is set), implementing DNS server redirection and error correction functions.
If the FW has multiple DNS transparent proxy policies, DNS requests are matched in the policy configuration order. As long as one policy is matched, the action specified in this policy is taken, and the policy matching activity finishes. Therefore, you are advised to first configure policies with narrow matching scopes.
2.When a DNS request matches a DNS transparent proxy policy, if the DNS request requires DNS transparent proxy, the FW first checks whether the domain name is an exception. If so, the FW does not perform DNS transparent proxy. If not, the FW marks DNS transparent proxy on the DNS request for the subsequent process.
For an exception, if another DNS server is required to parse this domain name, the FW changes the destination address of the DNS request to the desired DNS server address.
3.The FW searches for a route for the DNS request (the route can be a policy-based route, static route, or dynamic route) to determine the outgoing interface.
If intelligent uplink selection (Global Route Selection Policy or PBR-based Intelligent Uplink Selection) is configured on the FW and the DNS request matches the corresponding equal-cost route or policy-based route, the FW forwards the DNS request based on the intelligent uplink selection result. Note that the intelligent uplink selection result is dynamic and determined by the uplink selection mode and real-time link status. The result may vary even if a user accesses the same domain name twice.
4.A maximum of two DNS servers can be bound to each outgoing interface on the FW, with one primary DNS server and the other secondary DNS server. Both DNS servers belong to the ISP network directly connected to the outgoing interface. After the FW determines the outgoing interface of the DNS request, the DNS transparent proxy function preferentially replaces the destination address of the DNS request with the primary DNS server address. The secondary DNS server address is used only when the primary DNS server is Down.
The FW performs DNS transparent proxy only when a DNS server is bound to the outgoing interface and the DNS request has a DNS transparent proxy mark.

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