Andrew Hudson - Fedora™ Unleashed, 2008 edition

One of the more popular services to provide with a web server is to host a virtual domain. Also known as a virtual host , a virtual domain is a complete website with its own domain name, as if it were a standalone machine, but it's hosted on the same machine as other websites. Apache implements this capability in a simple way with directives in the httpd.confconfiguration file.

Apache now can dynamically host virtual servers by using the mod_vhost_aliasmodule you read about in the preceding section of the chapter. The module is primarily intended for ISPs and similar large sites that host a large number of virtual sites. This module is for more advanced users and, as such, it is outside the scope of this introductory chapter. Instead, this section concentrates on the traditional ways of hosting virtual servers.

After you've configured your Linux machine with multiple IP addresses, setting up Apache to serve them as different websites is simple. You need only put a VirtualHostdirective in your httpd.conffile for each of the addresses you want to make an independent website:

ServerName gnulix.org

DocumentRoot /home/virtual/gnulix/public_html

TransferLog /home/virtual/gnulix/logs/access_log

ErrorLog /home/virtual/gnulix/logs/error_log

Use the IP address, rather than the hostname, in the VirtualHosttag.

You can specify any configuration directives within the tags. For example, you might want to set AllowOverridesdirectives differently for virtual hosts than you do for your main server. Any directives that aren't specified default to the settings for the main server.

Name-based virtual hosts enable you to run more than one host on the same IP address. You must add the names to your DNS as CNAMEs of the machine in question. When an HTTP client (web browser) requests a document from your server, it sends with the request a variable indicating the server name from which it's requesting the document. Based on this variable, the server determines from which of the virtual hosts it should serve content.

Some older browsers are unable to see name-based virtual hosts because this is a feature of HTTP 1.1 and the older browsers are strictly HTTP 1.0-compliant. However, many other older browsers are partially HTTP 1.1-compliant, and this is one of the parts of HTTP 1.1 that most browsers have supported for a while.

Name-based virtual hosts require just one step more than IP address-based virtual hosts. You must first indicate which IP address has the multiple DNS names on it. This is done with the NameVirtualHostdirective:

NameVirtualHost 212.85.67.67

You must then have a section for each name on that address, setting the configuration for that name. As with IP-based virtual hosts, you need to set only those configurations that must be different for the host. You must set the ServerNamedirective because it's the only thing that distinguishes one host from another:

ServerName bugserver.gnulix.org

ServerAlias bugserver

DocumentRoot /home/bugserver/htdocs

ScriptAlias /home/bugserver/cgi-bin

TransferLog /home/bugserver/logs/access_log

ServerName pts.gnulix.org

ServerAlias pts

DocumentRoot /home/pts/htdocs

ScriptAlias /home/pts/cgi-bin

TransferLog /home/pts/logs/access_log

ErrorLog /home/pts/logs/error_log

If you're hosting websites on an intranet or internal network, users are likely to use the shortened name of the machine rather than the FQDN. For example, users might type http://bugserver/index.htmlin their browser location fields rather than http://bugserver.gnulix.org/index.html. In that case, Apache would not recognize that those two addresses should go to the same virtual host. You could get around this by setting up VirtualHostdirectives for both bugserverand bugserver.gnulix.org, but the easy way around it is to use the ServerAliasdirective, which lists all valid aliases for the machine:

ServerAlias bugserver

For more information about VirtualHost, refer to the help system on http://localhost/_manual.

Apache provides for logging just about any web access information in which you might be interested. Logging can help with the following:

► System resource management, by tracking usage

► Intrusion detection, by documenting bad HTTP requests

► Diagnostics, by recording errors in processing requests

Two standard log files are generated when you run your Apache server: access_logand error_log. They are found under the /var/log/httpddirectory. (Others include the SSL logs ssl_access_log, ssl_error_log, and ssl_request_log.) All logs except for the error_log(by default, this is just the access_log) are generated in a format specified by the CustomLogand LogFormatdirectives. These directives appear in your httpd.conffile.

A new log format can be defined with the LogFormatdirective:

LogFormat "%h %l %u %t \"%r\" %>s %b" common

The common log format is a good starting place for creating your own custom log formats. Note that most of the available log analysis tools assume that you are using the commonlog format or the combinedlog format — both of which are defined in the default configuration files.

The following variables are available for LogFormatstatements:

You can put a conditional in front of each variable to determine whether the variable is displayed. If the variable isn't displayed, — is displayed instead. These conditionals are in the form of a list of numerical return values. For example, %!401udisplays the value of REMOTE_USER unless the return code is 401.