Firewalling Basics; Susefirewall2 - Novell LINUX ENTERPRISE DESKTOP 10 SP2 - DEPLOYMENT GUIDE 08-05-2008 Deployment Manual

39.3 Firewalling Basics

Firewall is probably the term most widely used to describe a mechanism that provides
and manages a link between networks while also controlling the data flow between
them. Strictly speaking, the mechanism described in this section is called a packet filter.
A packet filter regulates the data flow according to certain criteria, such as protocols,
ports, and IP addresses. This allows you to block packets that, according to their ad-
dresses, are not supposed to reach your network. To allow public access to your Web
server, for example, explicitly open the corresponding port. However, a packet filter
does not scan the contents of packets with legitimate addresses, such as those directed
to your Web server. For example, if incoming packets were intended to compromise a
CGI program on your Web server, the packet filter would still let them through.
A more effective but more complex mechanism is the combination of several types of
systems, such as a packet filter interacting with an application gateway or proxy. In
this case, the packet filter rejects any packets destined for disabled ports. Only packets
directed to the application gateway are accepted. This gateway or proxy pretends to be
the actual client of the server. In a sense, such a proxy could be considered a masquerad-
ing host on the protocol level used by the application. One example for such a proxy
is Squid, an HTTP proxy server. To use Squid, the browser must be configured to
communicate via the proxy. Any HTTP pages requested are served from the proxy
cache and pages not found in the cache are fetched from the Internet by the proxy. As
another example, the SUSE proxy suite (proxy-suite) provides a proxy for the FTP
protocol.
The following section focuses on the packet filter that comes with SUSE Linux Enter-
prise. For further information about packet filtering and firewalling, read the Firewall
HOWTO included in the howto package. If this package is installed, read the HOWTO
with
less /usr/share/doc/howto/en/txt/Firewall-HOWTO.gz

39.4 SuSEfirewall2

SuSEfirewall2 is a script that reads the variables set in /etc/sysconfig/
SuSEfirewall2 to generate a set of iptables rules. It defines three security zones,
although only the first and the second one are considered in the following sample con-
figuration:
736 Deployment Guide