Explanation
To prevent anonymous FTP users from listing uploaded file names, the upload directory must not have the read or execute permission set for the anonymous user or group. The read permission allows the user to view the contents of a file or directory, while the execute permission allows the user to enter or search a directory.
Therefore, removing both permissions will prevent the user from accessing or listing the files in the upload directory. However, the write permission can be left on, as it allows the user to create or modify files in the directory, which is the purpose of an upload directory. References:
LPIC-2 Exam 202-450 Objectives, Topic 211: File Sharing, Objective 211.2: Manage FTP Service, Weight: 3 LPIC-2 Linux Professional Institute Certification Study Guide: Exam 201 and Exam 202, Chapter 9:
File Sharing, FTP, p. 353-354
Linux File Permissions Explained, Read, Write, and Execute Permissions, p. 2-4