Nginx反代DOH服务实现CDN加速功能

部分场景下访问DOH服务速度较慢，最常见的做法为就近部署CDN节点进行加速，推荐使用Nginx反代DOH服务实现CDN加速功能。

nginx配置如下：

#http块内容
upstream google_dns_backend {
    server dns.google:443;
    keepalive 32;
}
#server块内容
location /secure {
        proxy_http_version 1.1;
        proxy_set_header Connection "";
        proxy_pass https://google_dns_backend/dns-query;
        proxy_set_header Host dns.google;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;

        proxy_connect_timeout 5s;
        proxy_read_timeout 10s;
        proxy_send_timeout 10s;
        proxy_buffering off; 
    }
location / { return 444; }