A firewall is a system that provides network security by filtering incoming and outgoing network traffic based on a set of user-defined rules. In general, the purpose of a firewall is to reduce or eliminate the occurrence of unwanted network communications while allowing all legitimate communication to flow freely. In most server infrastructures, firewalls provide an essential layer of security that, combined with other measures, prevent attackers from accessing your servers in malicious ways.
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If you are running a mission critical web server, or maintaining a storage server loaded with sensitive data, you probably want to closely monitor file access activities within the server. For example, you want to track any unauthorized change in system configuration files such as /etc/passwd.