nginx not serving my error_page - nginx

Question

I have a Sinatra application hosted with Unicorn, and nginx in front of it. When the Sinatra application errors out (returns 500), I'd like to serve a static page, rather than the default "Internal Server Error". I have the following nginx configuration:
server {
listen 80 default;
server_name *.example.com;
root /home/deploy/www-frontend/current/public;
location / {
proxy_pass_header Server;
proxy_set_header Host $http_host;
proxy_redirect off;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Scheme $scheme;
proxy_connect_timeout 5;
proxy_read_timeout 240;
proxy_pass http://127.0.0.1:4701/;
}
error_page 500 502 503 504 /50x.html;
}
The error_page directive is there, and I have sudo'd as www-data (Ubuntu) and verified I can cat the file, thus it's not a permission problem. With the above config file, and service nginx reload, the page I receive on error is still the same "Internal Server Error".
What's my error?

Answer 1

error_page handles errors that are generated by nginx. By default, nginx will return whatever the proxy server returns regardless of http status code.
What you're looking for is proxy_intercept_errors
This directive decides if nginx will intercept responses with HTTP
status codes of 400 and higher.
By default all responses will be sent as-is from the proxied server.
If you set this to on then nginx will intercept status codes that are
explicitly handled by an error_page directive. Responses with status
codes that do not match an error_page directive will be sent as-is
from the proxied server.

Answer 2

You can set proxy_intercept_errors especially for that location
location /some/location {
proxy_pass_header Server;
proxy_set_header Host $http_host;
proxy_redirect off;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Scheme $scheme;
proxy_connect_timeout 5;
proxy_read_timeout 240;
proxy_pass http://127.0.0.1:4701/;
proxy_intercept_errors on; # see http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_intercept_errors
error_page 400 500 404 ... other statuses ... =200 /your/path/for/custom/errors;
}
and you can set instead 200 other status what you need

Answer 3

People who are using FastCGI as their upstream need this parameter turned on
fastcgi_intercept_errors on;
For my PHP application, I am using it in my upstream configuration block
location ~ .php$ { ## Execute PHP scripts
fastcgi_pass php-upstream;
fastcgi_intercept_errors on;
error_page 500 /500.html;
}

Answer 4

As mentioned by Stephen in this response, using proxy_intercept_errors on; can work.
Though in my case, as seen in this answer, using uwsgi_intercept_errors on; did the trick...