Virtual hosting

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Virtual hosting is a method for hosting multiple domain names on a computer using a single IP address. This allows one machine to share its resources, such as memory and processor cycles, 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 this allows many customers to 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 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.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. www.blog1.example.com and www.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 using 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 computer listens on more than one IP address.

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 web server 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 higher costs of web site hosting and leads to IP 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 instance, a server may host the website www.example.com. However, if they wish to operate a second site, do not have access to the domain name configuration for their domain name, and/or own no other IP addresses which they could use to serve the site from, they could instead use another port number, for example, www.example.com:81 for port 81, or www.example.com:8000 for port 8,000.

Cons

Most people are not familiar with using non-standard port numbers, and more complicated port numbers may be harder to remember. Most webcrawlers assume port 80 (default) when attempting to crawl a site and so may miss the non-standard port number. Humans may also be unaware of the non-standard port number and may be unaware as to where to look to find the website. Using non-standard port numbers may also be seen as unprofessional and unattractive to users. Some firewalls, either hardware or software, block all but the most common ports. this would cause a site hosted on a non standard port to appear unavailable to some users.

However, non-standard port numbers have found applications in HTTP based software backends such as Bit Torrent tracker announce scripts, which are part of the software's backend and not usually fully visible to the user.

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.

See also

External links

fa:میزبان مجازی pt:Hospedagem virtual zh:虚拟主机

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