Virtual hosting
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Virtual hosting is a method that servers such as webservers use to host more than one domain name on the same computer, sometimes on the same IP address.
There are two basic methods of accomplishing virtual hosting: name-based, and IP address or ip-based.
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[edit] Name-based
Name-based virtual hosts use multiple host names for the same webserver IP address.
With web browsers that support HTTP/1.1 (as nearly all now do), upon connecting to a webserver, the browsers send the address that the user typed into their browser's address bar (the URL). The server can use this information to determine which web site, as well as page, to show the user. The browser specifies the address by setting the Host HTTP header with the host specified by the user. The Host header is required in all HTTP/1.1 requests.
For instance, a server could be receiving requests for two domains, www.site1.com and www.site2.com, both of which resolve to the same IP address. For www.site1.com, the server would send the HTML file file from the directory /www/JoeUser/site/, while requests for www.site2.com would make the server serve pages from /www/MaryUser/site/.
[edit] Cons
If the Domain Name System (DNS) is not properly functioning, it becomes much harder to access a virtually-hosted website. Ordinarily, in this case, the user could try and fall back to using the IP address to contact the system, as in http://12.37.56.78/. However, the web browser doesn't know what hostname to use when this happens, so the server will respond with a default website—often not the site the user expects. This workaround is not really useful for an average web user, but may be of some use to a site administrator while fixing DNS records.
A workaround in this case is to add the IP address and hostname to the client system's hosts file. At this point, accessing the server with the domain name should work again. However, users should be careful when doing this, as any changes to the true mapping between hostname and IP address will be overridden by the local setting.
Another issue with virtual hosting is the inability to host multiple secure websites running Secure Sockets Layer or SSL. Because the SSL handshake takes place before the expected hostname is sent to the server, the server doesn't know which certificate to present when the connection is made. One workaround is to run multiple web server programs, each listening to a different incoming port, which still allows the system to just use a single IP address. If running multiple web server programs is considered clumsy, a more efficient solution is to select TLS (TLS 1.1 or later, which enables name-based virtual hosting as of December 2006). Another option is to do IP aliasing, where a single computer listens on more than one IP address.
[edit] IP-based
In IP-based virtual hosting, each site (either a DNS hostname or a group of DNS hostnames that act the same) points to a unique IP address. The webserver is configured with multiple physical network interfaces, virtual network interfaces on the same physical interface or multiple IP addresses on one interface.
The webserver can obtain the address the TCP connection was intended for using a standard api and uses this to determine which website to serve. The client is not involved in this process and therefore (unlike with name based virtual hosting) there are no compatibility issues.
[edit] Uses
Virtual web hosting is often used on large scale in companies whose business model is to provide low cost website hosting for customers. The vast majority of such web hosting service customer websites worldwide are hosted on shared servers, using virtual hosting technology.
Many businesses utilize virtual servers for internal purposes, where there is a technology or administrative reason to keep several separate websites such as customer extranet website, employee extranet, internal intranet, intranets for different departments. If there are not security concerns in the website architectures, they can be merged into a single server using virtual hosting technology, which reduces management and administrative overhead and the number of separate servers required to support the business.
