server {
    listen 80;
    listen 8080;
    server_name a.com b.com;

    # 公共代理配置
    location / {
        set $pass_host backend;
        set $pass_port;
        proxy_pass http://$pass_host:$pass_port;
        proxy_set_header   Host              $host;
        proxy_set_header   X-Real-IP         $remote_addr;
        proxy_set_header   X-Forwarded-Proto $scheme;
        proxy_set_header   X-Forwarded-For   $proxy_add_x_forwarded_for;
        proxy_connect_timeout   3s;
    }

    error_page 500 502 503 504  /50x.html;
}