Chapter 6: Security Mechanisms
This chapter describes NetDefendOS security features.
• Access Rules, page 421
• ALGs, page 425
• Web Content Filtering, page 503
• Email Filtering and Anti-Spam, page 526
• Anti-Virus Scanning, page 541
• Intrusion Detection and Prevention, page 552
• Denial-of-Service Attacks, page 566
• Blacklisting Hosts and Networks, page 571
6.1. Access Rules
6.1.1. Overview
One of the principal functions of NetDefendOS is to allow only authorized connections access to
protected data resources. Access control is primarily addressed by the NetDefendOS IP rule set in
which a range of protected LAN addresses are treated as trusted hosts, and traffic flow from
untrusted sources is restricted from entering trusted areas.
Before a new connection is checked against the IP rule set, NetDefendOS checks the connection
source against a set of Access Rules. Access Rules can be used to specify what traffic source is
expected on a given interface and also to automatically drop traffic originating from specific
sources. AccessRules provide an efficient and targeted initial filter of new connection attempts.
The Default Access Rule
Even if the administrator does not explicitly specify any custom Access Rules, an access rule is
always in place which is known as the Default Access Rule.
This default rule is not really a true rule but operates by checking the validity of incoming traffic
by performing a reverse lookup in the NetDefendOS routing tables. This lookup validates that the
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