HTTP/2 on Backend Causes Sporadic 520 Errors

Website, Application, Performance DNS & Network
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Hey everyone,

My friend who I debugged this with has opened a ticket so I’m sure Cloudflare will fix this soon, but in the meantime I wanted to post this here for anyone having this issue.

Do not enable HTTP/2 on your backend server. This causes sporadic 520 errors on the CF side. We found that sending an OPTIONS request would trigger the 520 about 100% of the time, but occasionally it would happen to other requests too. The backend logs show a stream of normal HTTP/1.1 requests and then randomly CF uses a HTTP/2 request, and the server responds correctly with HTTP/2 but Cloudflare shows a 520.

Looking into the CF docs there is a page that says that HTTP/2 is not used between origin and CF: https://support.cloudflare.com/hc/en-us/articles/200168076-Understanding-Cloudflare-HTTP-2-and-HTTP-3-Support

However, it seems this is an issue with CF since they are negotiating a HTTP/2 request in the first place.

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Our servers are Nginx, but looking at these past forum posts, it seems to affect Apache too so probably related to HTTP/2 and not just Nginx.

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