Classic CTF topic about FTP protocol

Gave one to pcap package , The prompt uses ftp agreement ：

1. After filtration , track tcp Flow get :
   
   It can be seen that ftp Passive mode transfers two files , Again because ftp The protocol transmits a file using a connection , So two ports are opened before and after , Here is how to calculate the open port number ：
   The following is an example
   The open port is 159*256+247 = 40951

2. Now that you know the port for transferring files , Then get these two files out , See what it is .
   First look at hello.txt
   The open port is 33303, Then filter the port first 33303 Data packets of ：
   
   track TCP flow ：
   
   Look at the other one （ We can also see from the title flag It must be here ）：
   
   save as-> 2.png
   see ：