Network Security & Firewalls
A firewall is a piece
of hardware and/or software which functions in a networked environment to
prevent some communications forbidden by the security policy. They are
analogous to the function of firewalls in building construction. A firewall has
the basic task of controlling traffic between different zones of trust. Typical
zones of trust include the Internet (a zone with no trust) and an internal
network (a zone with high trust). The ultimate goal is to provide controlled
connectivity between zones of differing trust levels through the enforcement of
a security policy and connectivity model based on the least privilege principle.
There are three basic types of firewalls depending on whether the
communication is being done between a single node and the network, or between
two or more networks whether the communication is intercepted at the network
layer, or at the application layer; whether the communication state is being
tracked at the firewall or not
With regard to the scope of filtered communication there exist:
personal firewalls, a software application which normally filters traffic
entering or leaving a single computer through the Internet
network firewalls, normally running on a dedicated network device or computer
positioned on the boundary of two or more networks or DMZs (demilitarized
zones). Such a firewall filters all traffic entering or leaving the connected
networks.
The latter definition corresponds to the conventional, traditional meaning of
"firewall" in networking.
In reference to the layers where the traffic can be intercepted, three main
categories of firewalls exist:
network layer firewalls
application layer firewalls
application firewalls
These network-layer and application-layer types of firewall may overlap, even
though the personal firewall does not serve a network; indeed, single systems
have implemented both together.
There's also the notion of application firewalls which are sometimes used during
wide area network (WAN) networking on the world-wide web and govern the system
software. An extended description would place them lower than application layer
firewalls, indeed at the Operating System layer, and could alternately be called
operating system firewalls.
Lastly, depending on whether the firewalls track packet states, two additional
categories of firewalls exist: stateful firewalls and stateless firewalls.
A stateful firewall (any firewall that
performs stateful packet inspection or stateful inspection) is a firewall that
keeps track of the state of network connections (such as TCP streams) traveling
across it. The firewall is programmed to know what legitimate packets are for
different types of connections. Only packets which match a known connection
state will be allowed by the firewall; others will be rejected.
Proper configuration of firewalls demands skill from the administrator. It
requires considerable understanding of network protocols and of computer
security. Small mistakes can render a firewall worthless as a security tool.
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