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You should keep your rkhunter database up-to-date.
This can be done automatically by putting this line to /etc/periodic.conf:

daily_rkhunter_update_enable="YES"
daily_rkhunter_update_flags="--update --nocolors"

Also, you can run rkhunter as a part of the daily security check by
putting this line to /etc/periodic.conf:

daily_rkhunter_check_enable="YES"
daily_rkhunter_check_flags="--checkall --nocolors --skip-keypress"

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