A 523 error means that Cloudflare could not reach your host web server. The origin web server is not reachable.
DNS looks up or searches for the correct IP address for your website.
DNS resolution converts your hostname (such as www.example.com) into a computer-friendly IP address (such as 192.168.1.1).
Your browser then uses the IP address to communicate with your origin server.
4 DNS servers load a webpage:
- DNS recursor
- Root nameserver
- TLD nameserver
- Authoritative nameserver
These four DNS servers work together in harmony to complete the task of delivering the IP address for a specified domain to the client. A DNS resolver, DNS root server, DNS TLD server, and DNS authoritative nameserver must all provide information to complete the lookup.
In my case, I suspect one (or more) of these four servers is failing.
Is the DNS recursor the problem?
Unlikely. “Cloudflare’s 22.214.171.124 is the world’s fastest and most reliable public DNS resolver”
Is it the ROOT server?
No. Root servers are DNS nameservers that operate in the root zone. Ultimate authority over the root zone belongs to the National Telecommunications and Information Administration (NTIA), which is a part of the US Department of Commerce. Root servers are very reliable.
Is the TLD nameserver the problem?
Maybe. TLD nameservers are DNS nameservers that operate in the root zone. ICANN looks after most top-level domain. It operates the Internet Assigned Numbers Authority (IANA). .eu is the country code top level domain name (ccTLD) of the European Union. It is managed and operated by EURid under the authority of the European Commission. My domain ends in .eu.
Error message from https://dnsviz.net/
- eu zone: The server(s) were not responsive to queries over UDP. (2001:978:2:1::93:2)
Is the Authoritative nameserver the problem?
On DNS Zones
The DNS is broken up into many different zones.
The DNS namespace includes a DNS zone. An org or admin manages each zone.
On DNS Zone Files
A DNS zone file holds zone info.
A reverse lookup zone maps an IP address to the host. This zone is used for troubleshooting, spam filtering, and bot detection.
If many requests are made simultaneously to the same origin server, the server may become overwhelmed with traffic and be unable to respond efficiently to additional incoming requests.
Per Microsoft - DNS troubleshooting