Virtual hosting
Encyclopedia
Virtual hosting is a method for hosting multiple domain name
s 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.
.
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
(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 inhttp://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.
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.
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.
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.
Domain name
A domain name is an identification string that defines a realm of administrative autonomy, authority, or control in the Internet. Domain names are formed by the rules and procedures of the Domain Name System ....
s on a server
Server (computing)
In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...
using a single IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
. 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 service
A shared web hosting service or virtual hosting service or derive host refers to a web hosting service where many websites reside on one web server connected to the Internet. Each site "sits" on its own partition, or section/place on the server, to keep it separate from other sites...
. Shared web hosting prices are lower than a dedicated web server
Web server
Web server can refer to either the hardware or the software that helps to deliver content that can be accessed through the Internet....
because many customers can be hosted on a single server.
Name-based
Name-based virtual hosts use multiple host names for the same webserver IP addressIP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
.
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
Uniform Resource Locator
In computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet 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
Domain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
to the same IP address. For www.example.com, the server would send the HTML file
HTML
HyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
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 SystemDomain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
(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
A workaround in this case is to add the IP address and hostname to the client system's hosts file
Hosts file
The hosts file is a computer file used in an operating system to map hostnames to IP addresses. The hosts file is a plain-text file and is conventionally named hosts.-Purpose:...
. 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
Handshaking
In information technology, telecommunications, and related fields, handshaking is an automated process of negotiation that dynamically sets parameters of a communications channel established between two entities before normal communication over the channel begins...
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
TCP and UDP port
In computer networking, a port is an application-specific or process-specific software construct serving as a communications endpoint in a computer's host operating system. A port is associated with an IP address of the host, as well as the type of protocol used for communication...
, 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
Transport Layer Security
Transport Layer Security and its predecessor, Secure Sockets Layer , are cryptographic protocols that provide communication security over the Internet...
(TLS 1.1 or later, which enables name-based virtual hosting
Server Name Indication
Server Name Indication is a feature that extends the SSL and TLS protocols. To properly secure the communication between a client and a server, the client requests a digital certificate from the server; once the server sends the certificate, the client examines it, uses it to encrypt the...
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 serviceShared web hosting service
A shared web hosting service or virtual hosting service or derive host refers to a web hosting service where many websites reside on one web server connected to the Internet. Each site "sits" on its own partition, or section/place on the server, to keep it separate from other sites...
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
Extranet
An extranet is a computer network that allows controlled access from the outside, for specific business or educational purposes. An extranet can be viewed as an extension of a company's intranet that is extended to users outside the company, usually partners, vendors, and suppliers...
, internal intranet
Intranet
An intranet is a computer network that uses Internet Protocol technology to securely share any part of an organization's information or network operating system within that organization. The term is used in contrast to internet, a network between organizations, and instead refers to a network...
, 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.