anonymous_enable=NO  
local_enable=YES  
write_enable=YES  
chroot_local_user=YES  
pasv_enable=YES  
pasv_min_port=40000  
pasv_max_port=50000  
ssl_enable=YES  
rsa_cert_file=/etc/ssl/certs/vsftpd.pem  
rsa_private_key_file=/etc/ssl/private/vsftpd.pem  
force_local_data_ssl=YES  
force_local_logins_ssl=YES  

add_filter('wp_handle_upload', function($upload) {
    $file = $upload【'file'】;
    $remote_path = '/files/' . basename($file);
    $conn = ftp_ssl_connect('ftp.example.com', 21, 10);
    if (!$conn || !ftp_login($conn, 'wpftp', 'YOUR_PASSWORD')) {
        return $upload; // 回退到本地
    }
    ftp_pasv($conn, true);
    ftp_put($conn, $remote_path, $file, FTP_BINARY);
    ftp_close($conn);
    $upload【'url'】 = 'https://cdn.example.com' . $remote_path;
    @unlink($file); // 清理本地零时文件
    return $upload;
});

server {
    listen 443 ssl http2;
    server_name cdn.example.com;
    ssl_certificate /etc/ssl/certs/cdn.pem;
    ssl_certificate_key /etc/ssl/private/cdn.key;
    location /files/ {
        proxy_pass https://ftp.example.com:21/files/; # 注意ftp over https需额外模块
        proxy_cache_path /var/cache/nginx levels=1:2 keys_zone=ftp:10m inactive=60m;
        proxy_cache ftp;
        proxy_cache_valid 200 302 1d;
        add_header X-Cache-Status $upstream_cache_status;
    }
}