Firewall
BAT54-Rail/F..
Release
7.54
06/08
8.2
What is a Firewall?
253
Above all, the aspect of the “central” positioning is very Important: Only when
the entire data traffic between “inside” and “outside” goes through the Fire-
wall, it can fulfil its task reliably under any circumstances. Each alternative
way can reduce or even turn off the security of the Firewall. This central po-
sition of the Firewall simplifies by the way also the maintenance: One Firewall
as common passage between two networks is certainly easier to maintain
than a “Personal Firewall” on each of the workstations belonging to the LAN.
Note: In principle, Firewalls operate at the interconnection between two or
more networks. For the following explanation, we only look as example at
the passage between a local network of a company and the Internet.
These explanations can be transferred however in a general manner also
to other network constellations, e.g. for the protection of a subnetwork of
the personnel department of a company against the remaining network
users.
U Logging and alerting
An important function of the Firewall is beside the checking of data packets
and the right reaction to the results of this checking also the logging of all ac-
tions triggered by the Firewall. By analyzing these protocols, the administra-
tor can draw conclusions from the occurred attacks and on the basis of this
information he can, if necessary, go on to improve the configuration of the
Firewall.
But sometimes, logging alone comes too late. Often, an immediate interven-
tion of the administrator can prevent a major danger. That is why Firewalls
have mostly an alerting function, by which the Firewall notifies the adminis-
trator e.g. by e-mail.
8.2.2 Different types of Firewalls
During the last years, the operating principles of Firewalls have more and
more evolved. Under the generic term “Firewall”, a whole range of different
technical concepts is offered to protect the LAN. Here we introduce the most
important ones.
U Packet filters
One speaks about a packet filter-based Firewall, if the router only checks the
details in the header of the data packets and decides on the basis of this in-
formation, whether the packet may pass or not. The following details belong
to the analyzed information:
D IP address of source and destination
D Transfer protocol (TCP, UDP or ICMP)
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