To change the FTP server banner message in the vsftpd server, you can edit the banner_file directive in the server's configuration file, which is typically located at /etc/vsftpd.conf.
The banner_file directive specifies the path to a file that contains the banner message that will be displayed to clients when they connect to the server. You can create a banner file with the message you want to display, and then specify the path to the file in the banner_file directive.
For example, to display the message "Welcome to the FTP server" when clients connect to the server, you can create a file named banner.txt with the following contents:
Welcome to the FTP server
Then, add the following line to the /etc/vsftpd.conf configuration file:
banner_file=/path/to/banner.txt
Save the configuration file and restart the vsftpd server for the changes to take effect.
You can find more information about the banner_file directive and other configuration options for the vsftpd server in the vsftpd man page by running man vsftpd in a terminal.