Sams Teach Yourself PHP, MySQL and Apache All in One (2012)

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Additional Files

The httpd.pid file contains the process ID of the running Apache server. You can use this number to send signals to Apache manually, as described in the next section. The scoreboard file, the present on Linux/UNIX Apache, is used by the process-based MPMs to communicate with their children. In general, you do not need to worry about these files.

Apache-Related Commands

The Apache distribution includes several executables. This section covers only the server binary and related scripts. Chapter 25, “Restricting Access to Your Applications,” and Chapter 29, “Apache Performance Tuning and Virtual Hosting,” cover additional utilities included with the Apache distribution.

Apache Server Binary

The name of the Apache executable is httpd in Linux/UNIX and Mac OS X, and httpd.exe in Windows. It accepts several command-line options, some of which are described in Table 3.1. You can get a complete listing of options by typing /usr/local/apache2/bin/httpd -h on Linux/UNIX, by typing /usr/sbin/httpd -h on Mac OS X, or by typing httpd.exe -h from a command prompt on Windows.

Table 3.1 Some httpd Options

After Apache is running, you can use the kill command on Linux/UNIX and Mac OS X to send signals to the parent Apache process. Signals provide a mechanism to send commands to a process. To send a signal, execute the following command:

# kill -SIGNAL pid

In this syntax, pid is the process ID, and SIGNAL is one of the following:

• HUP—Stop the server

• USR1 or WINCH—Graceful restart; which signal to use depends on the underlying operating system

• SIGHUP—Restart

If you make some changes to the configuration files and you want them to take effect, you must signal Apache that the configuration has changed. You can do this by stopping and starting the server or by sending a restart signal. This tells Apache to reread its configuration.

A normal restart can result in a momentary pause in service. A graceful restart takes a different approach: Each thread or process serving a client continues processing the current request, but when it finishes, it is killed and replaced by a new thread or process with the new configuration. This allows seamless operation of the web server with no downtime.

On Windows, you can signal Apache using the httpd.exe executable. Some commands are listed here:

• httpd.exe -k restart—Tells Apache to restart

• httpd.exe -k graceful—Tells Apache to do a graceful restart

• httpd.exe -k stop—Tells Apache to stop

You can access shortcuts to these commands in the Start menu entries that the Apache installer created. If you installed Apache as a service, you can start or stop Apache by using the Windows service interface: In Control Panel, select Administrative Tasks and then click the Services icon.

Apache Control Script

Although it is possible to control Apache on Linux/UNIX using the httpd binary, it is recommended that you use the apachectl tool. The apachectl support program wraps common functionality in an easy-to-use script. To use apachectl, type the following:

# /usr/local/apache2/bin/apachectl command

In this syntax, command can be stop, start, restart, or graceful. You can also edit the contents of the apachectl script to add extra command-line options. Some OS distributions provide you with additional scripts to control Apache; check the documentation included with your distribution.

Starting Apache for the First Time

Before you start Apache, verify that the minimal set of information is present in the Apache configuration file, httpd.conf. The following sections describe the basic information needed to configure Apache and to start the server.

Check Your Configuration File

You can edit the Apache httpd.conf file with your favorite text editor. In Linux/UNIX and Mac OS X, this probably means vi or emacs. In Windows, you can use Notepad or WordPad. You must remember to save the configuration file in plaintext, which is the only format Apache understands.

You might need to change just two parameters to enable you to start Apache for the first time: the name of the server and the address and port to which it is listening. The name of the server is the one Apache will use when it needs to refer to itself (for example, when redirecting requests).

Apache can usually figure out its server name from the IP address of the machine, but not always. If the server does not have a valid DNS (domain name service) entry, you might need to specify one of the IP addresses of the machine. If the server is not connected to a network (you might want to test Apache on a standalone machine), you can use the value 127.0.0.1, which is the loopback address. The default port value is 80. You might need to change this value if a server is already running in the machine at port 80 or if you do not have administrator permissions—on Linux/UNIX and Mac OS X systems, only the root user can bind to privileged ports (those with port numbers lower than 1024).

You can change both the listening address and the port values with the Listen directive. The Listen directive takes either a port number or an IP address and a port, separated by a colon. If you specify only the port, Apache listens on that port at all available IP addresses in the machine. If you provide an additional IP address, Apache listens at only that address and port combination. For example, Listen 80 tells Apache to listen for requests at all IP addresses on port 80. Listen 10.0.0.1:443 tells Apache to listen at only 10.0.0.1 on port 443.

The ServerName directive enables you to define the name the server will report in any self-referencing URLs. The directive accepts a DNS name and an optional port, separated by a colon. Make sure that ServerName has a valid value. Otherwise, the server will not function properly; for example, it will issue incorrect redirects.

On Linux/UNIX and Mac OS X platforms, you can use the User and Group directives to specify which user and group IDs the server will run as. The nobody user is a good choice for most platforms. However, there are problems in the HP-UX platform with this user ID, so you must create and use a different user ID, such as www.

Starting Apache

To start Apache on Linux/UNIX, change to the directory containing the apachectl script and execute the following command:

# /usr/local/apache2/bin/apachectl start

Mac OS X users can type the following at the prompt:

# /usr/sbin/httpd

To manually start Apache on Windows, click the Start link in the Control Apache Server section, within the Apache HTTP Server 2.2 program group in the Start menu. If you installed Apache as a service, you must start the Apache service instead.

If everything goes well, you can access Apache using a browser. A default installation page displays, such as the one shown in Figure 3.4. If you cannot start the web server or an error page appears instead, consult the “Troubleshooting” section that follows. Make sure that you are accessing Apache in one of the ports specified in the Listen directive—usually port 80 or 8080.

Figure 3.4 Apache has been installed.

Troubleshooting

The following subsections describe several common problems that you might encounter the first time you start Apache.

Already an Existing Web Server

If a server is already running on the machine and is listening to the same IP address and port combination, Apache cannot start successfully. You will get an entry in the error log file indicating that Apache cannot bind to the port:

[crit] (48)Address already in use: make_sock: could not bind...[alert] no
listening sockets available, shutting down

To solve this problem, you need to stop the running server or change the Apache configuration to listen on a different port.

No Permission to Bind to Port

You will get an error if you do not have administrator permissions and you try to bind to a privileged port (between 0 and 1024):

[crit] (13)Permission denied: make_sock: could not bind to address 10.0.0.2:80
[alert] no listening sockets available, shutting down

To solve this problem, you must either log on as the administrator before starting Apache or change the port number; 8080 is a commonly used nonprivileged port.

Access Denied

You might not be able to start Apache if you do not have permission to read the configuration files or to write to the log files. You will get an error similar to the following:

(13)Permission denied: httpd: could not open error log file

This problem can arise if the user who built and installed Apache is different from the user trying to run it.

Wrong Group Settings

You can configure Apache to run under a certain username and group. Apache has default values for the running server username and group. Sometimes the default value is not valid and you will get an error containing setgid: unable to set group id.

To solve this problem on Linux/UNIX and Mac OS X, you must change the value of the Group directive in the configuration file to a valid value. Check the /etc/groups file for existing groups.

Summary

This chapter explained different ways of getting an Apache web server installed and running on your Linux/UNIX, Mac OS X, or Windows machine. It covered both binary and source installation and explained the basic build-time options. In addition, you learned the location of the server configuration files and the syntax of the commands used to modify your Apache configuration. You learned about the two main log files: access_log and error_log. You also saw how to start and stop the server using the Apache control scripts or the Apache server binary on Linux/UNIX, Mac OS X, and Windows platforms.

Q&A

Q. How can I start a clean build?

A. If you need to build a new Apache from source and do not want the result of earlier builds to affect the new one, it is always a good idea to run the make clean command. Doing so takes care of cleaning up any existing binaries, intermediate object files, and so on.

Q. Why are per-directory configuration files useful?

A. Although per-directory configuration files have an effect on server performance, they can be useful for delegated administration. Because per-directory configuration files are read every time a request is made, there is no need to restart the server when a change is made to the configuration.

You can allow users of your website to make configuration changes on their own without granting them administrator privileges. In this way, they can password-protect sections of their home pages, for example.

Q. What do you mean by a valid ServerName directive?

A. The DNS system associates IP addresses with domain names. The value of ServerName is returned when the server generates a URL. If you are using a certain domain name, you must make sure that it is included in your DNS system and will be available to clients visiting your site.

Workshop

The workshop is designed to help you review what you’ve learned and begin putting your knowledge into practice.

Quiz

1. How can you specify the location where you want to install Apache?

2. What is the main difference between the <Location> and <Directory> sections?

3. What is the difference between a restart and a graceful restart?

Answers

1. Linux/UNIX users can use the --prefix option of the configure script. If an existing installation is present at that location, the configuration files are preserved, but the binaries are replaced. On Windows, this location is set in the Installation Wizard.

2. Directory sections refer to filesystem objects; Location sections refer to elements in the address bar of the web page.

3. During a normal restart, the server is stopped and then started, causing some requests to be lost. A graceful restart allows Apache children to continue to serve their current requests until they can be replaced with children running the new configuration.

Activities

1. Practice the various types of server shutdown and restart procedures.

2. Make some configuration changes, such as different port assignments and ServerName changes.