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Ensure good physical security for computers, network cables, and connectors.
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If user quotas are available on your system, enable them.
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Configure appropriate process and user limits on your system.
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Don't test new software while running as root .
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Educate your users on polite methods of sharing system resources.
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Run long-running tasks in the background, setting the nice to a positive value.
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Partition disks to isolate critical partitions from those that might be filled by mail or file uploads.
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Configure disk partitions to have sufficient inodes and storage.
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Make sure that you have appropriate swap space configured.
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Monitor disk usage and encourage users to archive and delete old files.
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Consider investing in a network monitor appropriate for your network. Have a spare network connection available, in case you need it.
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Install a firewall to prevent and react to network problems.
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Keep an up-to-date paper list of low-level network addresses (e.g., Ethernet addresses), IP addresses, and machine names available.
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Enable SYN cookies if your kernel supports them.
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Use egress filters on border routers to prevent spoofed packets from being sent out from your network.