Hi all,
I am having getting intermittent 521 error code. Here is a bit explanation about the architecture. I have a wordpress multisite server to serve client sites. The webserver is NGINX with PHP-FPM Every site has its own domain, so for every client site, I have one Cloudflare account that connects to the wordpress.
Somewhat I have been getting intermittent 521 errors in all the sites to the server after some change in configuration (which I kind of forgot which one).
Before the errors, basically the Cloudflare configuration has
- A name from the site to the multisite IP address
- flexible ssl enabled
- https rewrite and https redirect enabled
- 1 page rule to redirect *.domain to www.domain.
- 1 page rule to bypass cache to wp-admin
- 1 page rule to cache all content under wp-content/*
But then suddenly the 521 errors happening to all microsites. What I tried to fix are
- install SSL in multisite, set the domain to CNAME, set the HTTPS to Full (from Flexibe).
- test enabling the development mode
Somewhat the problem still persist, so I tried curl one of the site and see the log in the nginx.
Above is example of the log. On the first section, on the left, I tried to curl based on timestamp while the right is the nginx log. In the 155xxxxxxx033 I got 521 Origin Down but there is no entry in the right. Only the previous 155xxxxxxxx00994 and next 155xxxxxx8901.
After that, I tried again with changing the HTTPS configuration from Flexible to Full. Still the same. See in the second section of the picture above (new user can only post 1 picture 
)
I am now stuck in where to investigate next and what configuration I am missing. Really appreciate help and ideas.