1.3. How Does Apache Use TCP/IP?
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 -- a piece of 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. One interface, on the other hand, may have more than one IP address (see Chapter 3, "Toward a Real Web Site").
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 (if you really want) 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 WWW and Apache. Apache, by default, listens to port number 80 because it deals in WWW 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 real security beyond what you can provide yourself (using file
permissions) and no superuser (at least, not as far as port numbers
are concerned).
This 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 to be 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, which is to assign multiple addresses to the same interface and distinguish 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.
1.3.1. Multiple Sites: Unix
By happy accident, the crucial Unix utility ifconfig , which binds IP addresses to physical interfaces, often allows the binding of multiple IP numbers so that people can switch from one IP number to another and maintain service during the transition.
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.[8]
[8]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:
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 in order 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.ifconfig ep0 192.168.123.2 ifconfig ep0 192.168.123.3 alias netmask 0xFFFFFFFF ifconfig ep0 192.168.124.1 aliasUnfortunately, 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' system, these names both translate into 192.168.123.2.
1.3.2. Multiple Sites: Win32
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. This means,
of course, that IP-intensive virtual hosting is not possible on
Windows 95.
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| 1.2. What to Know About TCP/IP |  | 1.4. What the Client Does |
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