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At its core, Apache is about communication over networks. Apache uses the TCP/IP protocol as its foundation, providing an implementation of HTTP. Developers who want to use Apache should have at least a foundation understanding of TCP/IP and may need more advanced skills if they need to integrate Apache servers with other network infrastructure like firewalls and proxy servers.
To understand the substance of this book, you need a modest knowledge of what TCP/IP is and what it does. You'll find more than enough information in Craig Hunt and Robert Bruce Thompson's books on TCP/IP,[7] but what follows is, we think, what is necessary to know for our book's purposes.
[7]Windows NT TCP/IP Network Administration, by Craig Hunt and Robert Bruce Thompson (O'Reilly & Associates, 1998), and TCP/IP Network Administration, Third Edition, by Craig Hunt (O'Reilly & Associates, 2002).
TCP/IP (Transmission Control Protocol/Internet Protocol) is a set of protocols enabling computers to talk to each other over networks. The two protocols that give the suite its name are among the most important, but there are many others, and we shall meet some of them later. These protocols are embodied in programs on your computer written by someone or other; it doesn't much matter who. TCP/IP seems unusual among computer standards in that the programs that implement it actually work, and their authors have not tried too much to improve on the original conceptions.
TCP/IP is generally only used where there is a network.[8] Each computer on a network that wants to use TCP/IP has an IP address, for example, 192.168.123.1.
[8]In the minimal case we could have two programs running on the same computer talking to each other via TCP/IP — the network is "virtual".
There are four parts in the address, separated by periods. Each part corresponds to a byte, so the whole address is four bytes long. You will, in consequence, seldom see any of the parts outside the range 0 -255.
Although not required by the protocol, by convention there is a dividing line somewhere inside this number: to the left is the network number and to the right, the host number. Two machines on the same physical network — usually a local area network (LAN) — normally have the same network number and communicate directly using TCP/IP.
How do we know where the dividing line is between network number and host number? The default dividing line used to be determined by the first of the four numbers, but a shortage of addresses required a change to the use of subnet masks. These allow us to further subdivide the network by using more of the bits for the network number and less for the host number. Their correct use is rather technical, so we leave it to the routing experts. (You should not need to know the details of how this works in order to run a host, because the numbers you deal with are assigned to you by your network administrator or are just facts of the Internet.)
Now we can think about how two machines with IP addresses X and Y talk to each other. If X and Y are on the same network and are correctly configured so that they have the same network number and different host numbers, they should be able to fire up TCP/IP and send packets to each other down their local, physical network without any further ado.
If the network numbers are not the same, the packets are sent to a router, a special machine able to find out where the other machine is and deliver the packets to it. This communication may be over the Internet or might occur on your wide area network (WAN). There are several ways computers use IP to communicate. These are two of them:
Let's look at a server from the outside. We have a box in which there is a computer, software, and a connection to the outside world — Ethernet or a serial line to a modem, for example. This connection is known as an interface and is known to the world by its IP address. If the box had two interfaces, they would each have an IP address, and these addresses would normally be different. A single interface, on the other hand, may have more than one IP address (see Chapter 3).
Requests arrive on an interface for a number of different services offered by the server using different protocols:
Network News Transfer Protocol (NNTP): news
Simple Mail Transfer Protocol (SMTP): mail
Domain Name Service (DNS)
HTTP: World Wide Web
The server can decide how to handle these different requests because the four-byte IP address that leads the request to its interface is followed by a two-byte port number. Different services attach to different ports:
NNTP: port number 119
SMTP: port number 25
DNS: port number 53
HTTP: port number 80
As the local administrator or webmaster, you can decide to attach any service to any port. Of course, if you decide to step outside convention, you need to make sure that your clients share your thinking. Our concern here is just with HTTP and Apache. Apache, by default, listens to port number 80 because it deals in HTTP business.
Port numbers below 1024 can only be used by the superuser (root, under Unix); this prevents other users from running programs masquerading as standard services, but brings its own problems, as we shall see.
Under Win32 there is currently no security directly related to port numbers and no superuser (at least, not as far as port numbers are concerned).
This basic setup is fine if our machine is providing only one web server to the world. In real life, you may want to host several, many, dozens, or even hundreds of servers, which appear to the world as completely different from each other. This situation was not anticipated by the authors of HTTP 1.0, so handling a number of hosts on one machine has to be done by a kludge, assigning multiple addresses to the same interface and distinguishing the virtual host by its IP address. This technique is known as IP-intensive virtual hosting. Using HTTP 1.1, virtual hosts may be created by assigning multiple names to the same IP address. The browser sends a Host header to say which name it is using.
In one way the Web is like the telephone system: each site has a number that uniquely identifies it — for instance, 192.168.123.5. In another way it is not: since these numbers are hard to remember, they are automatically linked to domain names — www.amazon.com, for instance, or www.butterthlies.com, which we shall meet later in examples in this book.
When you surf to http://www.amazon.com, your browser actually goes first to a specialist server called a Domain Name Server (DNS), which knows (how it knows doesn't concern us here) that this name translates into 208.202.218.15.It then asks the Web to connect it to that IP number. When you get an error message saying something like "DNS not found," it means that this process has broken down. Maybe you typed the URL incorrectly, or the server is down, or the person who set it up made a mistake — perhaps because he didn't read this book.
A DNS error impacts Apache in various ways, but one that often catches the beginner is this: if Apache is presented with a URL that corresponds to a directory, but does not have a / at the end of it, then Apache will send a redirect to the same URL with the trailing / added. In order to do this, Apache needs to know its own hostname, which it will attempt to determine from DNS (unless it has been configured with the ServerName directive, covered in Chapter 2. Often when beginners are experimenting with Apache, their DNS is incorrectly set up, and great confusion can result. Watch out for it! Usually what will happen is that you will type in a URL to a browser with a name you are sure is correct, yet the browser will give you a DNS error, saying something like "Cannot find server." Usually, it is the name in the redirect that causes the problem. If adding a / to the end of your URL causes it, then you can be pretty sure that's what has happened.
It is fortunate that the crucial Unix utility ifconfig, which binds IP addresses to physical interfaces, often allows the binding of multiple IP numbers to a single interface so that people can switch from one IP number to another and maintain service during the transition. This is known as "IP aliasing" and can be used to maintain multiple "virtual" web servers on a single machine.
In practical terms, on many versions of Unix, we run ifconfig to give multiple IP addresses to the same interface. The interface in this context is actually the bit of software — the driver — that handles the physical connection (Ethernet card, serial port, etc.) to the outside. While writing this book, we accessed the practice sites through an Ethernet connection between a Windows 95 machine (the client) and a FreeBSD box (the server) running Apache.
Our environment was very untypical, since the whole thing sat on a desktop with no access to the Web. The FreeBSD box was set up using ifconfig in a script lan_setup, which contained the following lines:
ifconfig ep0 192.168.123.2 ifconfig ep0 192.168.123.3 alias netmask 0xFFFFFFFF ifconfig ep0 192.168.124.1 alias
The first line binds the IP address 192.168.123.2 to the physical interface ep0. The second binds an alias of 192.168.123.3 to the same interface. We used a subnet mask (netmask 0xFFFFFFFF) to suppress a tedious error message generated by the FreeBSD TCP/IP stack. This address was used to demonstrate virtual hosts. We also bound yet another IP address, 192.168.124.1, to the same interface, simulating a remote server to demonstrate Apache's proxy server. The important feature to note here is that the address 192.168.124.1 is on a different IP network from the address 192.168.123.2, even though it shares the same physical network. No subnet mask was needed in this case, as the error message it suppressed arose from the fact that 192.168.123.2 and 192.168.123.3 are on the same network.
Unfortunately, each Unix implementation tends to do this slightly differently, so these commands may not work on your system. Check your manuals!
In real life, we do not have much to do with IP addresses. Web sites (and Internet hosts generally) are known by their names, such as www.butterthlies.com or sales.butterthlies.com , which we shall meet later. On the authors' desktop system, these names both translate into 192.168.123.2. The distinction between them is made by Apache' Virtual Hosting mechanism — see Chapter 4.
As far as we can discern, it is not
possible to assign multiple IP addresses to a single interface under
a standard Windows 95 system. On Windows NT it can be done via
Control Panel 
Networks
Protocols TCP/IP/Properties...
IP Address Advanced. Later versions of Windows,
notably Windows 2000 and XP, support multiple IP addresses through
the TCP/IP properties dialog of the Local Area Network in the Network
and Dial-up Settings area of the Start menu.
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| 1.2. How Apache Works |  | 1.4. How HTTP Clients Work |
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