- Virtual hosting
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Not to be confused with Virtual private server.
Virtual hosting is a method for hosting multiple domain names on a server using a single IP address. This allows one server to share its resources, such as memory and processor cycles, in order to use its resources more efficiently.
One widely used application is shared web hosting. Shared web hosting prices are lower than a dedicated web server because many customers can be hosted on a single server.
Contents
Types
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 hostname from the address that the user typed into their browser's address bar along with the requested resource itself to the webserver. The server can use the Host header field to determine which web site (or virtual host), 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.example.com and www.example.net, both of which resolve to the same IP address. For www.example.com, the server would send the HTML file from the directory /var/www/user/Joe/site/, while requests for www.example.net would make the server serve pages from /var/www/user/Mary/site/.
Example: A blog server can be hosted using Name base hosting. blog1.example.com and blog2.example.com
Cons
If the Domain Name System (DNS) is not properly functioning, it becomes much harder to access a virtually-hosted website. The user could try to fall back to use the IP address to contact the system, as in http://10.23.45.67/. The web browser doesn't know which hostname to use when this happens; moreover, since the web server relies on the web browser client telling it what server name (vhost) to use, the server will respond with a default website—often not the site the user expects.
A workaround in this case is to add the IP address and hostname to the client system's hosts file. Accessing the server with the domain name should work again. Users should be careful when doing this, however, as any changes to the true mapping between hostname and IP address will be overridden by the local setting. 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.
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 June 2003, documented in RFC3546, and updated in RFC4366). Another option is to do IP aliasing, where a single network interface listens on more than one IP address.
IP-based
When IP-based virtual hosting is used, 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 web server can obtain the address the TCP connection was intended for using a standard API and use 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.
Cons
The server needs a different IP address for every web site which means increased costs for web site hosting and contribution to IPv4 address exhaustion.
Port-based
The default port number for HTTP is 80. However, most webservers can be configured to operate on almost any port number, provided the port number is not in use by any other program on the server.
For example, a server may host the website www.example.com. However, if the owner wishes to operate a second site, and does not have access to the domain name configuration for their domain name, and/or owns no other IP addresses which could be used to serve the site from, they could instead use another port number, for example, www.example.com:81 for port 81, www.example.com:8000 for port 8000, or www.example.com:8080 for port 8080.
Cons
Most internet users are not familiar with using non-standard port numbers, and more complicated port numbers may be more difficult to remember. Most web crawlers assume port 80 (default) when visiting a site and so may miss the non-standard port number. Using non-standard port numbers may also be seen as unprofessional and unattractive to users. In addition, some firewalls block all but the most common ports, causing a site hosted on a non-standard port to appear unavailable to some users.
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 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 technological or administrative reason to operate several separate websites, such as a customer extranet website, employee extranet, internal intranet, and 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.
See also
External links
Categories:- Web hosting
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