Important functions of a firewall include not only inspecting the data packets and responding appropriately to the results of this check, but also the logging of all actions triggered by the firewall. By evaluating these logs, the admin can draw conclusions about the attacks and further improve the configuration of the firewall.
But sometimes, logging alone can come too late. Often, quick intervention by the admins can prevent further damage. For this reason, firewalls usually have an alert function which reports firewall messages to the administrator, possibly by e-mail.
