Apache Web Server !

Apache Rocks On
What Apache has accomplished is simply amazing! Who knew that an open source
Web server could consistently beat two major commercial competitors, Microsoft
and Netscape as a Web server platform! Everyone has his or her own reason for
why Apache is so popular. Here are mine:
✦ Apache is a highly configurable Web Server with a modular design. It is
very easy to extend the capabilities of Apache Web server. Anyone with decent
C or Perl programming expertise can write a module to perform a special
function. This means that there are tons of Apache modules available for use.
✦ Apache is a free, open source technology. Being free is important but not as
important as being open source.
✦ Apache works great with Perl, PHP, and other scripting languages. Most
Web applications are still scripts. Perl excels in the script world and Apache
makes using Perl a piece of cake with both CGI support and mod_perl support.
✦ Apache runs on Linux and other Unix systems. Linux used to be an underdog
operating system, which has now found itself in enterprise computing arena.
Linux and Apache go hand-in-hand in the enterprise world today. I believe
Linux’s acceptance in the business world has made Apache’s entry into such
territory easy. However, there are people who would argue that it was
Apache’s fame that made Linux find its way into the business world easier.
Either way, Apache and Linux is a powerful combination. Other Unix systems
such as FreeBSD and Solaris, and the new Mac OS X also play a great role in
expanding Apache’s user base horizon

Configuring Kernel for IP accounting

 

The Linux IP accounting feature is very closely related to the Linux firewall software. The places you want to collect accounting data are the same places that you would be interested in performing firewall filtering: into and out of a network host, and in the software that does the routing of datagrams. If you haven't read the section on firewalls, now is probably a good time to do so, as we will be using some of the concepts described in Chapter 9, TCP/IP Firewall .

To activate the Linux IP accounting feature, you should first see if your Linux kernel is configured for it. Check to see if the /proc/net/ip_acct file exists. If it does, your kernel already supports IP accounting. If it doesn't, you must build a new kernel, ensuring that you answer "Y" to the options in 2.0 and 2.2 series kernels: Networking options ---> [*] Network firewalls [*] TCP/IP networking ... [*] IP: accounting or in 2.4 series kernels: Networking options ---> [*] Network packet filtering (replaces ipchains)

Configuring IP accounting.

Because IP accounting is closely related to IP firewall, the same tool was designated to configure it, so ipfwadm , ipchains or iptables are used to configure IP accounting. The command syntax is very similar to that of the firewall rules, so we won't focus on it, but we will discuss what you can discover about the nature of your network traffic using this feature.

The general syntax for IP accounting with ipfwadm is: # ipfwadm -A [ direction ] [ command ] [ parameters ]

The direction argument is new. This is simply coded as in , out , or both . These directions are from the perspective of the linux machine itself, so in means data coming into the machine from a network connection and out means data that is being transmitted by this host on a network connection. The both direction is the sum of both the incoming and outgoing directions.

The general command syntax for ipchains and iptables is: # ipchains -A chain rule-specification
# iptables -A chain rule-specification

The ipchains and iptables commands allow you to specify direction in a manner more consistent with the firewall rules. IP Firewall Chains doesn't allow you to configure a rule that aggregates both directions, but it does allow you to configure rules in the forward chain that the older implementation did not. We'll see the difference that makes in some examples a little later.

The commands are much the same as firewall rules, except that the policy rules do not apply here. We can add, insert, delete, and list accounting rules. In the case of ipchains and iptables , all valid rules are accounting rules, and any command that doesn't specify the -j option performs accounting only.

The rule specification parameters for IP accounting are the same as those used for IP firewall. These are what we use to define precisely what network traffic we wish to count and total.

Digital Signal and Gates

 

While the binary numeration system is an interesting mathematical abstraction, we haven't
yet seen its practical application to electronics. This chapter is devoted to just that: practically
applying the concept of binary bits to circuits. What makes binary numeration so important to
the application of digital electronics is the ease in which bits may be represented in physical
terms. Because a binary bit can only have one of two different values, either 0 or 1, any
physical medium capable of switching between two saturated states may be used to represent a
bit. Consequently, any physical system capable of representing binary bits is able to represent
numerical quantities, and potentially has the ability to manipulate those numbers. This is the
basic concept underlying digital computing.
Electronic circuits are physical systems that lend themselves well to the representation of
binary numbers. Transistors, when operated at their bias limits, may be in one of two different
states: either cutoff (no controlled current) or saturation (maximum controlled current). If
a transistor circuit is designed to maximize the probability of falling into either one of these
states (and not operating in the linear, or active, mode), it can serve as a physical representation
of a binary bit. A voltage signal measured at the output of such a circuit may also serve as
a representation of a single bit, a low voltage representing a binary ”0” and a (relatively) high
voltage representing a binary ”1.”