Virtual hosting is a method for hosting multiple

domain name A domain name is a string that identifies a realm of administrative autonomy, authority or control within the Internet. Domain names are often used to identify services provided through the Internet, such as websites, email services and more. As ...

s (with separate handling of each name) on a single

server Server may refer to: Computing *Server (computing), a computer program or a device that provides functionality for other programs or devices, called clients Role * Waiting staff, those who work at a restaurant or a bar attending customers and su ...

(or pool of servers). This allows one server to share its resources, such as memory and processor cycles, without requiring all services provided to use the same host name. The term virtual hosting is usually used in reference to

web server A web server is computer software and underlying hardware that accepts requests via HTTP (the network protocol created to distribute web content) or its secure variant HTTPS. A user agent, commonly a web browser or web crawler, initiate ...

s but the principles do carry over to other

Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...

services. One widely used application is shared web hosting. The price for shared web hosting is lower than for a dedicated

because many customers can be hosted on a single server. It is also very common for a single entity to want to use multiple names on the same machine so that the names can reflect services offered rather than where those services happen to be hosted. There are two main types of virtual hosting, name-based and IP-based. Name-based virtual hosting uses the host name presented by the client. This saves IP addresses and the associated administrative overhead but the protocol being served must supply the host name at an appropriate point. In particular, there are significant difficulties using name-based virtual hosting with SSL/TLS. IP-based virtual hosting uses a separate

IP address An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...

for each host name, and it can be performed with any protocol but requires a dedicated IP address per domain name served. Port-based virtual hosting is also possible in principle but is rarely used in practice because it is unfriendly to users. Name-based and IP-based virtual hosting can be combined: a server may have multiple IP addresses and serve multiple names on some or all of those IP addresses. This technique can be useful when using SSL/TLS with wildcard certificates. For example, if a server operator had two certificates, one for *.example.com and one for *.example.net, the operator could serve foo.example.com and bar.example.com off the same IP address but would need a separate IP address for baz.example.net.

Name-based

Name-based virtual hosts use multiple host names for the same

. A technical prerequisite needed for name-based virtual hosts is a web browser with

HTTP The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...

/1.1 support (commonplace today) to include the target hostname in the request. This allows a server hosting multiple sites behind one IP address to deliver the correct site's content. More specifically it means setting the

HTTP header The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, w ...

, which is mandatory in HTTP/1.1. For instance, a server could be receiving requests for two domains, and , both of which

resolve Resolve may refer to: * ''Resolve'' (Lagwagon album) * ''Resolve'' (Last Tuesday album) * "Resolve" (song), by the Foo Fighters *'' The Resolve'', a 1915 American silent short drama film * "Resolve" (''One Tree Hill'' episode) *''Resolve'', a Brit ...

to the same IP address. For , the server would send the HTML file from the directory , while requests for would make the server serve pages from . Equally two subdomains of the same domain may be hosted together. For instance, a blog server may host both blog1.example.com and blog2.example.com. The biggest issue with name-based virtual hosting is that it is difficult to host multiple secure websites running SSL/TLS. Because the SSL/TLS

handshake A handshake is a globally widespread, brief greeting or parting tradition in which two people grasp one of each other's like hands, in most cases accompanied by a brief up-and-down movement of the grasped hands. Customs surrounding handshakes a ...

takes place before the expected hostname is sent to the server, the server doesn't know which certificate to present in the handshake. It is possible for a single certificate to cover multiple names either through the "subjectaltname" field or through wildcards but the practical application of this approach is limited by administrative considerations and by the matching rules for wildcards. There is an extension to TLS called

Server Name Indication Server Name Indication (SNI) is an extension to the Transport Layer Security (TLS) computer networking protocol by which a client indicates which hostname it is attempting to connect to at the start of the handshaking process. This allows a serv ...

, that presents the name at the start of the handshake to circumvent that issue, except for some older clients (in particular

Internet Explorer Internet Explorer (formerly Microsoft Internet Explorer and Windows Internet Explorer, commonly abbreviated IE or MSIE) is a series of graphical web browsers developed by Microsoft which was used in the Windows line of operating systems ( ...

Windows XP Windows XP is a major release of Microsoft's Windows NT operating system. It was released to manufacturing on August 24, 2001, and later to retail on October 25, 2001. It is a direct upgrade to its predecessors, Windows 2000 for high-end and ...

or older Android versions) which do not implement

SNI SNI may refer to: Science and technology * Substitution nucleophilic internal, a chemistry reaction mechanism * Swedish Standard Industrial Classification, a Swedish economic classification system * Server Name Indication, an extension to the ...

. Furthermore, if the

Domain Name System The Domain Name System (DNS) is a hierarchical and distributed naming system for computers, services, and other resources in the Internet or other Internet Protocol (IP) networks. It associates various information with domain names assigned t ...

(DNS) is not properly functioning, it is difficult to access a virtually-hosted website even if the IP address is known. If the user tries to fall back to using the IP address to contact the system, as in , the web browser will send the IP address as the host name. 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 host name to the client system's

hosts file The computer file hosts is an operating system file that maps hostnames to IP addresses. It is a plain text file. Originally a file named HOSTS.TXT was manually maintained and made available via file sharing by Stanford Research Institute for the ...

. 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 host name 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.

IP-based

When IP-based virtual hosting is used, each site (either a DNS host name or a group of DNS host names 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 either open separate listening sockets for each IP address, or it can listen on all interfaces with a single socket and obtain the IP address the TCP connection was received on after accepting the connections. Either way, it can use the IP address 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. The downside of this approach is the server needs a different IP address for every web site. This increases administrative overhead (both assigning addresses to servers and justifying the use of those addresses to internet registries) and contributes to

IPv4 address exhaustion IPv4 address exhaustion is the depletion of the pool of unallocated IPv4 addresses. Because the original Internet architecture had fewer than 4.3 billion addresses available, depletion has been anticipated since the late 1980s, when the Interne ...

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. There is the

HTTP Secure Hypertext Transfer Protocol Secure (HTTPS) is an extension of the Hypertext Transfer Protocol (HTTP). It is used for secure communication over a computer network, and is widely used on the Internet. In HTTPS, the communication protocol is enc ...

special port 443 that needs special configuration (see

). Port based web sites are explicitly bound to a unique port number and an IP address. In this case the IP address is used for hosting multiple web sites. The unique port number used for a common IP address distinguish an individual web site from other web sites bound to the same IP address.

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 A web hosting service is a type of Internet hosting service that hosts websites for clients, i.e. it offers the facilities required for them to create and maintain a site and makes it accessible on the World Wide Web. Companies providing web ...

customer websites worldwide are hosted on shared servers, using virtual hosting technology. Many businesses companies 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 An extranet is a controlled private network that allows access to partners, vendors and suppliers or an authorized set of customers – normally to a subset of the information accessible from an organization's intranet. An extranet is similar to ...

, 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 In business, overhead or overhead expense refers to an ongoing expense of operating a business. Overheads are the expenditure which cannot be conveniently traced to or identified with any particular revenue unit, unlike operating expenses such as r ...

and the number of separate servers required to support the business.

References

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External links