Apache HTTPD Virtual Hosts allow a single server to host many web sites with different addresses.
Virtual hosts are easy to set up, just check the documentation at http://httpd.apache.org/docs/ . That said, there are two main ways to configure virtual hosting, which you have to keep in mind when starting out. One method involves matching the request host name, IP address, port, or any combination of them to a separate block of HTTPD configuration statements. The other method specifies a directory pattern to use for the document root and cgi-bin based on parts of the host name.
For the former method of configuring virtual hosts, all one needs to do is add a wildcard ServerAlias directive to the VirtualHost block for your domain name.
<VirtualHost 1.2.3.4>
ServerName domain.tld
ServerAlias *.domain.tld
DocumentRoot /var/www/
</VirtualHost>
For the latter, all one needs to do is match against only the domain name, or include subdomains in the pattern, making sure to create the appropriate directory structure.
VirtualDocumentRoot /var/www/%-2/
VirtualDocumentRoot /var/www/%-2/%-3/
Web hosts tend to use the larger VirtualHost method. Smaller shops, or generic mass hosts (departmental or employee hosting within an organization,for example) will find the latter very helpful, particularly when serving out of user’s home directories.
All this is great, but what about SSL? You could start up a separate instance of Apache HTTPD to serve over an SSL connection, but you probably don’t want to do that. There are some advantages, which but that is beyond the scope of this piece. The easiest way is to use a VirtualHost block to match against connections on port 443, the default HTTPS port. Contrary to popular belief, you do not need a separate IP address to do this.
<VirtualHost *:443>
SSLEngine On
SSLCertificateFile /etc/httpd/ssl.pem
DocumentRoot /var/www/
</VirtualHost>
This works just fine if you are only serving one site over HTTPS. The problem comes in when you have multiple domain names being served from the same server which need SSL. Since the SSL certificate needs to be used before the web browser sends a request to the server, the server has no way of picking a domain-specific SSL certificate to use. Name-based matching just won’t work for SSL. This is why proprietors of shared web hosting services demand that you purchase a dedicated IP address if you want to use SSL. IP addresses are known before SSL certificates are used, so by matching based on IP address, we can use domain-specific SSL certificates.
<VirtualHost 1.2.3.4:443>
SSLEngine On
SSLCertificateFile /etc/httpd/dom1-ssl.pem
DocumentRoot /var/www/dom1/
</VirtualHost>
<VirtualHost 1.2.3.5:443>
SSLEngine On
SSLCertificateFile /etc/httpd/dom2-ssl.pem
DocumentRoot /var/www/dom2/
</VirtualHost>
So, to recap, you do not need a separate IP address to use HTTPS. You do need separate IP addresses to use HTTPS on servers with multiple domains using SSL.
