What type of information is best harvested using a keylogger?

A keylogger is designed to capture the keystrokes made by a user on their keyboard. This makes it particularly effective for harvesting sensitive information that relies on user input, such as user IDs and passwords used to access various accounts and systems. When a user types in their credentials, a keylogger can record those keystrokes in real time, allowing the person or entity using the keylogger to collect important login information.

In contrast, the other choices relate to network and system information rather than direct user input. File names of databases, for instance, would typically require access to file systems and directories rather than just capturing keystrokes. IP addresses and MAC addresses are identifiers related to network configurations and hardware, which keyloggers do not inherently capture without additional functionality beyond their primary design purpose. Hence, user IDs are the most relevant type of information for which a keylogger is specifically suited to retrieve.