Virtual hosting is the maintenance of multiple Web sites on a single machine. For example, you may run both www.mycompany1.com and www.mycompany2.com Web sites from the same server. You can define IP-based or name-based virtual hosts using the VirtualHost directive. IP-based virtual hosts have different IP addresses for each Web site. Name-based virtual hosts run multiple names on a single IP address.
You use the <VirtualHost> container to enclose a set of directives that apply only to a named virtual host. You may place any directive that can be used in a virtual host context in the virtual host container. When OHS receives a client request for a document that's on a virtual host, OHS will use the configuration directives for that virtual host's virtual host container. Using the <VirtualHost> container directive, you can specify the following alternative directives to the main HTTP server:
Here's how you use the <VirtualHost> directive to specify additional hosts:
<VirtualHost www.myhost1.com> DocumentRoot /usr/virtual/htdocs/info ServerName www.myhost1.com ErrorLog /usr/virtual/h1/logs/error_log </VirtualHost>
The foregoing example uses a name-based virtual hosting system. Here's an example showing a virtual host that's IP based:
<VirtualHost 126.96.36.199 188.8.131.52>
IP-Based Virtual Hosts
You must provide a different IP address for each IP-based virtual host, either by setting up your machine within multiple physical network connections, or by using virtual interfaces called "ip aliases." You can run multiple httpd daemons to support the different IP-based virtual hosts, or you can continue to use the default setup of a single httpd daemon. In our examples in this section, the single httpd daemon mode is assumed.
You can configure multiple hosts on a single server, using a separate VirtualHost directive for each virtual host. You can't use the following HTTP Server directives in a VirtualHost directive, because they can be used only in the server configuration context:
Here's an example showing how you use the <VirtualHost> container to set up different configuration values for the ServerAdmin, ServerName, DocumentRoot, ErrorLog, and TransferLog or CustomLog configuration directives for the two virtual hosts defined here:
Name-Based Virtual Hosts
In an IP-based virtual hosting system, each virtual host will need a separate IP address, because the virtual host uses the IP address to determine which host to serve. This forces you to have multiple IP addresses, one for each virtual host you wish to set up. When you use name-based virtual hosting, the HTTP Server expects the client to supply the hostname as part of the HTTP header information. This way, several hosts can share a single IP address.
It's very simple to configure name-based virtual hosting. All you need to do is to configure the DNS server to map host names to their correct IP addresses and then configure OHS so it can recognize the multiple host names. Name-based virtual hosting means you'll need fewer hard-to-acquire IP addresses. Unless you have strong reasons for doing so, you should use name-based virtual hosting. Following are some of those reasons:
You must use the NameVirtualHost directive to designate the IP address on the server that will accept requests for the name-based virtual hosts. If you want any or all IP addresses on the server to be used, you must provide * as the argument to the NameVirtualHost directive. Make sure that the IP address you specify is associated with a network interface on the server. You must provide a port argument (*:80, for example) if you intend to use multiple ports.
Once you do specify the NameVirtualHost directive, you must specify a <VirtualHost> block for each of the virtual hosts you wish to serve. You provide arguments to the <VirtualHost> directive that are similar to those you use for the NameVirtualHost directive. You need at least the following two directives inside each <VirtualHost> directive:
In the following example, originally you have the domain www.mydomain.com. You now wish to add the virtual host www.myotherdomain.com, pointing to the same IP address. Here's how your httpd.conf file will look:
The ServerAlias directive inside the first <VirtualHost> block, shown as follows, indicates that the name inside is an alternative name that clients can use to access the same Web site.
ServerAlias domain.com *.com.
Once you specify the <VirtualHost> directive in the manner shown in the preceding example, all requests for hosts in the domain.com domain will be served by the virtual host www.domain.com You must make sure that you configure your DNS server to map the names to an actual IP address on your server.
When a client connects to the Web address, the HTTP server will first check whether the client is the IP address specified by the NameVirtualHost directive. If so, the server will check for a matching IP address in one of the <VirtualHost> sections and look for a matching ServerName or ServerAlias for the requested host name. If it can't find any matching host names, it uses the first listed virtual host that matches the IP address. In order to provide a special configuration for client requests that don't match any of your virtual hosts, all you have to do is put that configuration in the first <VirtualHost> container in the httpd.conf file.
The first listed virtual host is the default virtual host. When an IP address matches the NameVirtualHost directive, the HTTP Server will not use the DocumentRoot for the main server.
Please check back next week for the conclusion of this article.
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