How long are rights preserved if the domain controller goes down

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http://serverfault.com – I have a network with one domain controller and 2 NAS-servers (running on a built-in Linux OS). Permissions to the files on the NAS servers are handled through AD rights. I want to bring the DC down for a short while (up to half an hour) and need to know if people who already are accessing the NAS-servers are still able to do that while the DC is down (as long as they don't logoff, reboot etc their machines). (HowTos)