Справочник Пользователя для ZyXEL Communications 5 Series

Chapter 15 Anti-Virus Screens

ZyWALL 5/35/70 Series User’s Guide

The following table describes some of the common computer viruses. 

The following describes a simple life cycle of a computer virus. 

1 A computer gets a copy of a virus from a source such as the Internet, e-mail, file sharing 

or any removable storage media. The virus is harmless until the execution of an infected 
program. 

2 The virus spreads to other files and programs on the computer. 
3 The infected files are unintentionally sent to another computer thus starting the spread of 

the virus. 

4 Once the virus is spread through the network, the number of infected networked 

computers can grow exponentially. 

The section describes two types of anti-virus scanner: host-based and network-based. 

A host-based anti-virus (HAV) scanner is often software installed on computers and/or servers 
in the network. It inspects files for virus patterns as they are moved in and out of the hard 
drive. However, host-based anti-virus scanners cannot eliminate all viruses for a number of 
reasons:

• HAV scanners are slow in stopping virus threats through real-time traffic (such as from 

the Internet). 

• HAV scanners may reduce computing performance as they also share the resources (such 

as CPU time) on the computer for file inspection. 

• You have to update the virus signatures and/or perform virus scans on all computers in the 

network regularly.

Table 87   Common Computer Virus Types

File Infector

This is a small program that embeds itself in a legitimate program. A file infector is 
able to copy and attach itself to other programs that are executed on an infected 
computer. 

Boot Sector Virus

This type of virus infects the area of a hard drive that  a computer reads and 
executes during startup. The virus causes computer crashes and to some extend 
renders the infected computer inoperable. 

Macro Virus

Macro viruses or Macros are small programs that are created to perform repetitive 
actions. Macros run automatically when a file to which they are attached is 
opened. Macros spread more rapidly than other types of viruses as data files are 
often shared on a network. 

E-mail Virus

E-mail viruses are malicious programs that spread through e-mail.  

Polyrmorphic 
Virus

A polymorphic virus (also known as a mutation virus) tries to evade detection by 
changing a portion of its code structure after each execution or self replication. 
This makes it harder for an anti-virus scanner to detect or intercept it. 
A polymorphic virus can also belong to any of the virus types discussed above.