Nginx/Cloudflare 可以使用伺服器區塊而不是 DNS 進行代理

Question

我在設定 Cloudflare 的自訂錯誤頁面功能時遇到了一些有點煩人的問題，所以我想弄清楚如何僅當我的伺服器不返回錯誤代碼時才能 proxy_pass 到 cloudflare，因為我真的只想使用這些來顯示我創建的自訂頁面，而不是Cloudflare 的預設頁面。

這是 nginx 網站可用的設定：

map $http_upgrade $connection_upgrade {
    default upgrade;
    '' close;
}

upstream app {
    server 127.0.0.1:8000;
    # server CLOUDLFARE_ADDRESS_FOR_PROXY
}

server {
    listen 443 ssl;
    server_name example.com www.example.com;

        location / {
                proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
                proxy_pass http://app;
                proxy_http_version 1.1;
                proxy_set_header Upgrade $http_upgrade;
                proxy_set_header Connection "upgrade";

                proxy_redirect off;
                proxy_set_header X-Real-IP $remote_addr;
                proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
                proxy_set_header X-Forwarded-Host $server_name;
                proxy_headers_hash_max_size 512;
                proxy_headers_hash_bucket_size 128;
                # can this work somehow?
                # proxy_next_upstream [non-error] CLOUDLFARE_ADDRESS_FOR_PROXY
        }

        location ~*  \.(jpg|jpeg|png|gif|ico|css|js|pdf)$ {
                expires 1d;
        }

        error_page 500 502 503 504 /custom_50x.html;
        location = /custom_50x.html {
                        root /usr/share/nginx/html;
                        internal;
        }
        ssl_certificate /etc/letsencrypt/live/exactestate.com/fullchain.pem; # managed by Certbot
        ssl_certificate_key /etc/letsencrypt/live/exactestate.com/privkey.pem; # managed by Certbot
        include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
        ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot
}

我想要的是僅當我的伺服器不拋出錯誤代碼時才使用 Cloudflare 作為代理，我該如何實現這一點？

我認為使用 cloudlfare 介面的 DNS 設定比伺服器區塊更強大，所以我想要一種使用 Nginx 的伺服器區塊而不是完整的 DNS 代理來完成此操作的方法。