Where are firewall servers physically located?
Hardware firewall - the external security system
The most important role in the functioning of the various hardware firewall types is played by the Packet filtering. The firewall decides on the basis of a manually configured set of rules which data packets should be forwarded and which should not. It works on OSI layers 3 and 4, i.e. the switching and transport layers, where it checks the packets for properties that it takes from the respective protocol header. Here, for example, exact IP addresses or ports are possible that are either allowed or blocked in the rules.
With the help of the bridge mentioned or with a switch, which represents a kind of bridge extension, the packet filtering can also be carried out on the data link layer, the second level of the OSI model. There the data packets are not filtered on the basis of the IP addresses, but rather based on the MAC addressesused for hardware addressing.
Furthermore, after expansion, firewalls can also filter with stateful inspection methods (Stateful Packet Inspection, SPI). For this purpose, the packet filtering, which is usually limited to layers 3 and 4, also includes the application level (layer 7) and the user data recorded there. In contrast to a proxy firewall, which also has access to this level, the SPI technology does not allow this data to be changed.
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