The robots protocol
robots.txt file is a text file. You can create and edit it using any common text editor, such as Notepad that comes with the Windows system. robots.txt is a protocol, not a command. Robots.txt is the first file to be viewed when accessing a website in a search engine. The robots.txt file tells the spider program what files can be viewed on the server. 
A more detailed reference: https://baike.baidu.com/item/robots%E5%8D%8F%E8%AE%AE/2483797?fr=aladdin
Here is a question about the robots protocol:
1.robots
observation topic It may be the first knowledge about the robots protocol, add /robots.txt to the URL, and come up with such an interface.
Search: http://159.138.137.79:60487/f1ag_1s_h3re.php to get the flag.
2.backup
directly opens the webpage, prompt: Do you know the backup file name of index.php? Here we must grasp the relevant knowledge about the backup file. Common backup file names have common backup file suffix names: .git .svn .swp .svn. ~ .Bak .bash_history. Try common file names one by one, and find out that they are .bak files, and get the flag after searching.
3.
Cookies Cookies are often used to identify users. A cookie is a small file that the server leaves on the user's computer. Whenever the same computer requests a page through a browser, this computer will send a cookie. With PHP, you can create and retrieve cookie values. 
Open the discovery prompt with Firefox, press F12 to open the developer tools, see the cookie value in the storage, continue to open /cookie.php, prompt to open to view the http response package, view the data package on the network, and get the flag.
Because of downloading some tools in the middle, the computer has a problem again. I believe that each topic will make my understanding of ctf further.
