Detailed explanation of how PHP prevents Thunder downloads

How does PHP prevent Thunder downloads? This article mainly introduces the detailed explanation of PHP's methods to prevent hot links and Thunder downloads, as a reference for everyone. I hope to be helpful.

Prevent hotlinking:

Principle: When the server asks us to download a file, we will get a link, and then we find the file through this link, and then Download it. So that means this link must be there, so how to prevent hot links? That means the link given to you is not a real file link. This is easy to think of, so how can we let you download the file without giving you a real link?

When we use PHP to make web pages, we always insert the PHP code in the middle of the HTML code, then use PHP to generate the remaining code, and then pass it to the client, which means that the client will accept it to the result of running our php script. That is to say: if the result of running my php script is the file you want to download. In this way, we have achieved file hotlink protection. That is to say, the link you can get is this php file, and then this ever-changing php file is turned into the file you want for you to download.

The code is as follows:

$FileAddress : 文件的相对路径。
$DownloadName : 文件下载到客户端的名字。
if(file_exists($FileAddress) && $file=fopen($FileAddress,'r')) { //首先要判断文件是否存在，如果文件跟本不存在的话，后边的代码也是白费。
 Header('content-type:application/octet-stream'); //声明文件类型，这里是为了让客户端下载它，而不是打开它，所以声明为未知二进制文件。否则客户端会根据其文件类型在线打开它。
 Header('content-Length:'.filesize($FileAddress)); //声明文件的大小，告诉客户端这个文件的大小，否则客户端下载的时候看不到进度。
 Header('content-disposition:attachment;filename='.$DownloadName); //声明文件名，这里就是告诉客户端它要下载的文件的名字，否则名字就会是你php文件的名字。
 echo fread($file,filesize($FileAddress)); //这里就是将加载的文件echo出来，因此这个php文件不能出现其他任何的文字，就是说这里若是出现了任何其他的输出的话都会输出到客户端下载的文件里。
 fclose($file); //最后关闭句柄。
}

The above code has successfully completed the task of preventing hotlinking, and only needs to define those two variables. Those two variables can be obtained through GET. For example, if we map the actual link of the file to its number in the database, we only need to GET a file ID to download the file, ensuring the security of our real file address. . Of course, you can also encrypt the real link to the file. In short, just don't mention the real link and put it where the client can see it.

Prevent Thunder downloads

In fact, through the above code, we can only hide the link, but cannot prevent the client from using Thunder and other tools to download it. So how to prevent it from being downloaded using tools such as Thunder?

As I said before, we can use various ways to get the path of this file to the php file, so we just need not to add this information to the link. For example: the file ID can be transmitted through POST, and the file ID can be transmitted through session.

This is what I call: fancy authorized downloads.

1. We can write the client session in the downloaded header page to store its authorization code and file ID, and then add the code to verify the session in the downloaded php, so that even if the client It is useless to enter the connection into Thunder download.

2. We can add a hidden form to the download header page and use POST to submit it to php that implements the download function. This can also achieve the purpose of preventing third-party download tools from downloading.

In short, there are many such methods. The above two methods provide reference. The main idea is to separate the information and links of the files to be downloaded, so that the file cannot be downloaded with only one link.

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