A security appliance is any form of server appliance that is designed to protect computer networks from unwanted traffic.^[1]

• Active devices block unwanted traffic. Examples of such devices are firewalls, anti virus scanning devices, and content filtering devices.
• Passive devices detect and report on unwanted traffic, such as intrusion detection appliances.
• Preventative devices scan networks and identify potential security problems (such as penetration testing and vulnerability assessment appliances).
• Unified Threat Management (UTM) appliances combine features together into one system, such as some firewalls, content filtering, web caching etc.

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