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In the preceding chapters, you learned about various aspects of Java s access control mechanism and its access specifiers For example, you already know that access to a private member of a class is granted only to other members of that class Packages add another dimension to access control As you will see, Java provides many levels of protection to allow fine-grained control over the visibility of variables and methods within classes, subclasses, and packages Classes and packages are both means of encapsulating and containing the name space and scope of variables and methods Packages act as containers for classes and other subordinate packages Classes act as containers for data and code The class is Java s smallest unit of abstraction Because of the interplay between classes and packages, Java addresses four categories of visibility for class members:

line-numbers This option will display the line numbers next to each rule in a chain This is useful when you need to insert a rule in the middle of a chain and need a quick list of the rules and their corresponding rule numbers An example of this is

Subclasses in the same package Non-subclasses in the same package Subclasses in different packages Classes that are neither in the same package nor subclasses

The three access specifiers, private, public, and protected, provide a variety of ways to produce the many levels of access required by these categories Table 9-1 sums up the interactions While Java s access control mechanism may seem complicated, we can simplify it as follows Anything declared public can be accessed from anywhere Anything declared private cannot be seen outside of its class When a member does not have an explicit access

One of the most powerful aspects of Netfilter is the fact that it offers a pluggable design For developers, this means that it is possible to make extensions to Netfilter using an application programming interface (API) rather than having to dive deep into the kernel code and hack away For users of Netfilter, this means a wide variety of extensions are available beyond the basic feature set These extensions are accomplished with the Match feature in the iptables commandline tool By specifying a desired module name after the -m parameter, iptables will take care of loading the necessary kernel modules and then offer an extended commandline parameter set These parameters are used to offer richer packet-matching features In this section, we discuss the use of a few of these extensions that have, as of this writing, been sufficiently well tested so that they are commonly included with Linux distributions

9:

TABLE 9-1

TIP To get help for a match extension, simply specify the extension name after the -m parameter and then give the -h parameter For example, to get help for the ICMP module, use [root@serverA ~]# iptables -m icmp -h icmp This module provides an extra match parameter for the ICMP protocol:

specification, it is visible to subclasses as well as to other classes in the same package This is the default access If you want to allow an element to be seen outside your current package, but only to classes that subclass your class directly, then declare that element protected Table 9-1 applies only to members of classes A non-nested class has only two possible access levels: default and public When a class is declared as public, it is accessible by any other code If a class has default access, then it can only be accessed by other code within its same package When a class is public, it must be the only public class declared in the file, and the file must have the same name as the class