Security policy protects network assets from threats and disruptions and helps to optimally allocate network resources for enhancing productivity and efficiency in business processes. On a Palo Alto Networks firewall, individual Security policy rules determine whether to block or allow a session based on traffic attributes, such as the source and destination security zone, the source and destination IP address, the application, the user, and the service.
All traffic passing through the firewall is matched against a session and each session is matched against a Security policy rule. When a session match occurs, the firewall applies the matching Security policy rule to bidirectional traffic in that session (client to server and server to client). For traffic that doesn’t match any defined rules, the default rules apply. The default rules—displayed at the bottom of the security rulebase—are predefined to allow all intrazone traffic (within a zone) and deny all interzone traffic (between zones). Although these rules are part of the predefined configuration and are read-only by default, you can override them and change a limited number of settings, including the tags, action (allow or block), log settings, and security profiles.
Security policy rules are evaluated left to right and from top to bottom. A packet is matched against the first rule that meets the defined criteria and, after a match is triggered, subsequent rules are not evaluated. Therefore, the more specific rules must precede more generic ones in order to enforce the best match criteria. Traffic that matches a rule generates a log entry at the end of the session in the traffic log if you enable logging for that rule. The logging options are configurable for each rule and can, for example, be configured to log at the start of a session instead of, or in addition to, logging at the end of a session.
After an administrator configures a rule, you can View Policy Rule Usage to determine when and how many times traffic matches the Security policy rule to determine its effectiveness. As your rulebase evolves, change and audit information get lost over time unless you archived this information at the time the rule is created or modified. You can Enforce Policy Rule Description, Tag, and Audit Comment to ensure that all administrators enter audit comments so that you can view the audit comment archive and review comments and configuration log history and can compare rule configuration versions for a selected rule. Together, you now have more visibility into and control over the rulebase.