Symptoms
- The domain is proxied behind Cloudflare
- 360 Monitoring says the site is down with a 403 Forbidden error
- The website is accessible both with cURL and the web browser
- Domain only has an IPv4 address assigned in Plesk
Cause
The domain resolves to an IPv6 address, but it doesn't have one assigned to it in Plesk:
# dig @8.8.8.8 +short -t AAAA example.com
2001:db8:f61:a1ff:0:0:0:80
Resolution
Configure the monitor to use IPv4 only
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In 360 Monitoring, go to Websites > example.com > Settings > Show advanced options
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Choose ipv4 under IP Version
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Hit the Update button and wait for the site to be rechecked
Alternatively, remove the AAAA records from Cloudflare, or add an IPv6 address in Plesk