I would need some solution to setup document root for particular ftp user or restrict access for particular FTP user to particular directory. I would like to let particular FTP user upload files just to particular directory but restrict access for all other directories. For example I have /www/test, /www/pictures. I would like to make for FTP user documentroot /www/pictures. Or something similar. Thank you for your advice.
Create a new user with the useradd command. That user must have /www/pictures as homedir. Make sure you give the user a UID below 10000 (otherwise his UID will interfere with the users created by ISPConfig).
