When the domain configuration hosted by Cloudflare enables human verification to prevent attacks.
Method to enable human verification:
Enter the domain to be enabled, Security -- Settings -- Security Level, select I'm Under Attack!
At this time, all subdomains under this domain will be required to undergo human verification.
In addition, if you do not want all subdomains to enter Under Attack mode, you can create custom rules in the configuration rules.
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Add a rule name, select a custom filtering expression, for example: if www.example.com and trial.example.com are both under DDoS attack, you can configure two hostnames equal to these two domains, and select I'm Under Attack for security level.
You can also achieve this by configuring Cloudflare's [Page Rules]:
Once configured, it will take effect immediately.
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