Causes

This error indicates Cloudflare is unable to send requests to your server because its origin IP cannot resolve the A/AAAA or CNAME DNS record requested.

The common causes are :

• The origin host names (CNAMEs) in your Cloudflare Load Balancer default, region, and fallback pools are unresolvable.
• A missing DNS A record that mentions the origin IP address.
• A CNAME record in Cloudflare DNS points to an unresolvable external domain.

Fixes

Step 1: Uses A Fallback Pool

Use a fallback pool configured with an origin IP as a backup in case all other pools are unavailable (1st cause)

Step 2: Verify Cloudflare DNS

Verify that your Cloudflare DNS settings include an A record that points to a valid IP address. You can choose to edit existing A record or create a new one. (2nd Cause)

Step 3: Ensure Target Domain Can Be Resolved

Ensure that the target domain can be resolved by using command line tools as well as third-party tools to check DNS resolution, such as dig and nslookup. (3rd Cause)

• dig command
  • dig subdomain.example.com
  • dig +trace subdomain.example.com
  • dig subdomain.example.com @1.1.1.1
• nslookup
  • nslookup subdomain.example.com
    nslookup subdomain.example.com 1.1.1.1

Step 4: Try An Online Services

You can try an online services, such as whatsmydns.net, to check for DNS resolution worldwide. Take note that a response other than a valid IP address (e.g., timeout, SERVFAIL, NXDOMAIN or NOERROR) most likely points to the source of your issue.