This article explains the function of DNS protection under Proxy Options.
DNS protection on FortiProxy provides protection against DNS poisoning.
The feature is available under Proxy Options:
During transparent proxy deployment, every client performs DNS lookup to get servers IPs by themselves. However, DNS server settings cannot be controlled on every client. Some may use a compromised DNS server, which can forward connections to illegitimate IPs instead of the intended IP, creating a security compromise risk.
Enabling DNS Protection gives FortiProxy the ability to verify each server’s IP is legitimate by comparing domains with its own DNS lookup results. If the results do not match, FortiProxy will make a request to the server’s IP from its own DNS lookup.
Explicit proxy deployment does not carry this risk because every DNS lookup is performed by a proxy server which is configured to use only a trusted DNS server.