Nginx/Cloudflare 我可以使用服务器块而不是 DNS 进行代理吗

Question

我在设置 Cloudflare 的自定义错误页面功能时遇到了一些烦人的问题，所以我想弄清楚如何仅当我的服务器未返回错误代码时才将 proxy_pass 到 cloudflare，因为我真的只想使用这些来显示我创建的自定义页面，而不是 Cloudflare 的默认页面。

以下是 nginx 站点可用配置：

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
}

我想要的是仅当我的服务器没有抛出错误代码时才使用 Cloudflare 作为代理，我该如何实现呢？

我认为使用 cloudlfare 接口的 DNS 设置比服务器块更强大，因此我希望使用 Nginx 的服务器块而不是完整的 DNS 代理来做到这一点。