I recently started using pi-hole, and it worked fine for a few weeks. Suddenly, I'm having intermittent problems with Firefox or Microsoft Edge, connecting to certain sites, including youtube.
Typical error message:
Did Not Connect: Potential Security Issue
Firefox detected a potential security threat and did not continue to www.youtube.com because this website requires a secure connection.
What can you do about it?
www.youtube.com has a security policy called HTTP Strict Transport Security (HSTS), which means that Firefox can only connect to it securely. You can’t add an exception to visit this site.
OR with Edge:
Your connection isn't private
Attackers might be trying to steal your information from youtu.be (for example, passwords, messages, or credit cards).
The problem is resolved by bypassing pi-hole as the DNS lookup, but I don't see how a DNS lookup could lead to such an error.
How can I debug this?
Firefox report for failed certificate: What has Cisco to do with youtube?
Certificate Subject Name Country US State/Province California Locality San Francisco Organization OpenDNS, Inc. Common Name www.youtube.com Issuer Name Organization Cisco Common Name Cisco Umbrella Secondary SubCA sea-SG Validity Not Before Tue, 11 May 2021 11:58:06 GMT Not After Sun, 16 May 2021 11:58:06 GMT Subject Alt Names DNS Name www.youtube.com Public Key Info Algorithm RSA Key Size 2048 Exponent 65537 Modulus BD:C7:34:77:DE:22:4B:0E:B3:CF:DB:A3:2F:B5:41:CC:BD:00:66:F7:A2:77:03:CA… Miscellaneous Serial Number 60:9D:14:A6 Signature Algorithm SHA-256 with RSA Encryption Version 3 Download PEM (cert) PEM (chain) Fingerprints SHA-256 36:1C:C0:F9:AB:2F:6E:58:81:BB:9B:AD:10:A4:B9:67:E3:37:9C:C7:4E:D0:41:AD… SHA-1 FD:97:F6:0E:F3:5B:60:01:61:50:68:BA:04:79:DC:99:67:2F:75:2F