We recently ran into an issue setting up a new DNS entry on Cloudflare, using the orange-cloud (reverse proxying) feature, but we were receiving Error 520 and were curious what was wrong and how to fix it. The error page itself doesn’t give a lot of information and since it’s a custom error they’ve created, it wasn’t easy to find out or even intuit much information about what it might mean.
To give some backstory, we are using a SaaS provider of a service for our employees that we want to protect behind our own domain. For example, instead of using ourcompany.saascompany.com, we wanted to use something like saasservicename.ourcompany.com. The provider supported this and so we set up the record within Cloudflare but as soon as we tried to visit the page, we received Cloudflare’s infamous 520 error: “Web server is returning an unknown error”.
After trying to troubleshoot the problem through Cloudflare, we turned off the orange-cloud and figured out that the SaaS provider hadn’t installed our TLS certificate correctly and so when Cloudflare was attempting to retrieve our instance from their server, they were receiving the NET::ERR_CERT_COMMON_NAME_INVALID. In response to that, they were throwing their own custom error 520 (it is not an official error code).
As soon as the vendor fixed the certificate issue, the 520 went away and we were able to re-enable orange-cloud, confirm that the site was up and working, and continue on with life confident that an attacker would not be able to determine who is providing the SaaS service for us.
