Why Permission automatically changes?

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http://forums.fedoraforum.org – Hai to all I am working in a college where there is a lab in which 30 machines are installed fc11. During last 2 weeks I don't know because of what the users themselves loses the chmod 700 permission . When it is set to chmod 700 /home/username it works. But after a few days it again loses permission . How it can be resolved? (HowTos)