Translations by Matthew East

Web Servers

A Web server is a software responsible for accepting HTTP requests from clients, which are known as Web browsers, and serving them HTTP responses along with optional data contents, which usually are Web pages such as HTML documents and linked objects (images, etc.).

HTTPD - Apache2 Web Server

The most common protocol used to transfer Web pages is the Hyper Text Transfer Protocol (HTTP). Protocols such as Hyper Text Transfer Protocol over Secure Sockets Layer (HTTPS), and File Transfer Protocol (FTP), a protocol for uploading and downloading files, are also supported.

Apache Web Servers are often used in combination with the <application>MySQL</application> database engine, the HyperText Preprocessor (<application>PHP</application>) scripting language, and other popular scripting languages such as <application>Python</application> and <application>Perl</application>. This configuration is termed LAMP (Linux, Apache, MySQL and Perl/Python/PHP) and forms a powerful and robust platform for the development and deployment of Web-based applications.

Installation

The <application>Apache2</application> web server is available in Ubuntu Linux. To install Apache2:

At a terminal prompt enter the following command:

Configuration

Apache2 is configured by placing <emphasis>directives</emphasis> in plain text configuration files. These <emphasis>directives</emphasis> are separated between the following files and directories:

<emphasis>apache2.conf:</emphasis> the main Apache2 configuration file. Contains settings that are <emphasis>global</emphasis> to Apache2.

<emphasis>envvars:</emphasis> file where Apache2 <emphasis>environment</emphasis> variables are set.

<emphasis>mods-available:</emphasis> this directory contains configuration files to both load <emphasis>modules</emphasis> and configure them. Not all modules will have specific configuration files, however.

<emphasis>mods-enabled:</emphasis> holds <emphasis>symlinks</emphasis> to the files in <filename>/etc/apache2/mods-available</filename>. When a module configuration file is symlinked it will be enabled the next time <application>apache2</application> is restarted.

<emphasis>ports.conf:</emphasis> houses the directives that determine which TCP ports Apache2 is listening on.

<emphasis>sites-available:</emphasis> this directory has configuration files for Apache2 <emphasis>Virtual Hosts</emphasis>. Virtual Hosts allow Apache2 to be configured for multiple sites that have separate configurations.

<emphasis>sites-enabled:</emphasis> like mods-enabled, <filename role="directory">sites-enabled</filename> contains symlinks to the <filename>/etc/apache2/sites-available</filename> directory. Similarly when a configuration file in sites-available is symlinked, the site configured by it will be active once Apache2 is restarted.

In addition, other configuration files may be added using the <emphasis>Include</emphasis> directive, and wildcards can be used to include many configuration files. Any directive may be placed in any of these configuration files. Changes to the main configuration files are only recognized by Apache2 when it is started or restarted.

Basic Settings

The directives set for a virtual host only apply to that particular virtual host. If a directive is set server-wide and not defined within the virtual host settings, the default setting is used. For example, you can define a Webmaster email address and not define individual email addresses for each virtual host.

If you wish to configure a new virtual host or site, copy that file into the same directory with a name you choose. For example:

Edit the new file to configure the new site using some of the directives described below.

The <emphasis>ServerAdmin</emphasis> directive specifies the email address to be advertised for the server's administrator. The default value is webmaster@localhost. This should be changed to an email address that is delivered to you (if you are the server's administrator). If your website has a problem, Apache2 will display an error message containing this email address to report the problem to. Find this directive in your site's configuration file in /etc/apache2/sites-available.

The <emphasis>Listen</emphasis> directive specifies the port, and optionally the IP address, Apache2 should listen on. If the IP address is not specified, Apache2 will listen on all IP addresses assigned to the machine it runs on. The default value for the Listen directive is 80. Change this to 127.0.0.1:80 to cause Apache2 to listen only on your loopback interface so that it will not be available to the Internet, to (for example) 81 to change the port that it listens on, or leave it as is for normal operation. This directive can be found and changed in its own file, <filename>/etc/apache2/ports.conf</filename>

You may also want your site to respond to www.ubunturocks.com, since many users will assume the www prefix is appropriate. Use the <emphasis>ServerAlias</emphasis> directive for this. You may also use wildcards in the ServerAlias directive.

For example, the following configuration will cause your site to respond to any domain request ending in <emphasis>.ubunturocks.com</emphasis>.

ServerAlias *.ubunturocks.com

Enable the new <emphasis>VirtualHost</emphasis> using the <application>a2ensite</application> utility and restart Apache2:

sudo a2ensite mynewsite

Be sure to replace <emphasis>mynewsite</emphasis> with a more descriptive name for the VirtualHost. One method is to name the file after the <emphasis>ServerName</emphasis> directive of the VirtualHost.

Similarly, use the <application>a2dissite</application> utility to disable sites. This is can be useful when troubleshooting configuration problems with multiple VirtualHosts:

sudo a2dissite mynewsite

Default Settings

This section explains configuration of the Apache2 server default settings. For example, if you add a virtual host, the settings you configure for the virtual host take precedence for that virtual host. For a directive not defined within the virtual host settings, the default value is used.

The <emphasis>DirectoryIndex</emphasis> is the default page served by the server when a user requests an index of a directory by specifying a forward slash (/) at the end of the directory name.

For example, when a user requests the page http://www.example.com/this_directory/, he or she will get either the DirectoryIndex page if it exists, a server-generated directory list if it does not and the Indexes option is specified, or a Permission Denied page if neither is true. The server will try to find one of the files listed in the DirectoryIndex directive and will return the first one it finds. If it does not find any of these files and if <emphasis>Options Indexes</emphasis> is set for that directory, the server will generate and return a list, in HTML format, of the subdirectories and files in the directory. The default value, found in <filename>/etc/apache2/mods-available/dir.conf</filename> is "index.html index.cgi index.pl index.php index.xhtml index.htm". Thus, if Apache2 finds a file in a requested directory matching any of these names, the first will be displayed.

Some options are specified on a per-directory basis rather than per-server. <emphasis>Options</emphasis> is one of these directives. A Directory stanza is enclosed in XML-like tags, like so:

The <emphasis>Options</emphasis> directive within a Directory stanza accepts one or more of the following values (among others), separated by spaces:

Most files should not be executed as CGI scripts. This would be very dangerous. CGI scripts should kept in a directory separate from and outside your DocumentRoot, and only this directory should have the ExecCGI option set. This is the default, and the default location for CGI scripts is <filename>/usr/lib/cgi-bin</filename>.

<emphasis role="bold">ExecCGI</emphasis> - Allow execution of CGI scripts. CGI scripts are not executed if this option is not chosen. <placeholder-1/>

<emphasis role="bold">IncludesNOEXEC</emphasis> - Allow server-side includes, but disable the <emphasis>#exec</emphasis> and <emphasis>#include</emphasis> commands in CGI scripts.

For security reasons, this should usually not be set, and certainly should not be set on your DocumentRoot directory. Enable this option carefully on a per-directory basis only if you are certain you want users to see the entire contents of the directory.

<emphasis role="bold">Indexes</emphasis> - Display a formatted list of the directory's contents, if no <emphasis>DirectoryIndex</emphasis> (such as index.html) exists in the requested directory. <placeholder-1/>

<emphasis role="bold">SymLinksIfOwnerMatch</emphasis> - Only follow symbolic links if the target file or directory has the same owner as the link.

httpd Settings

This section explains some basic <application>httpd</application> daemon configuration settings.

<emphasis role="bold">LockFile</emphasis> - The LockFile directive sets the path to the lockfile used when the server is compiled with either USE_FCNTL_SERIALIZED_ACCEPT or USE_FLOCK_SERIALIZED_ACCEPT. It must be stored on the local disk. It should be left to the default value unless the logs directory is located on an NFS share. If this is the case, the default value should be changed to a location on the local disk and to a directory that is readable only by root.

<emphasis role="bold">PidFile</emphasis> - The PidFile directive sets the file in which the server records its process ID (pid). This file should only be readable by root. In most cases, it should be left to the default value.

Unless you know exactly what you are doing, do not set the User directive to root. Using root as the User will create large security holes for your Web server.

Apache2 Modules