Virtual hosting

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Virtual hosting is the simultaneous operation ( hosting ) of several domains or IP addresses on a server or host . There are two basic variants of virtual hosting for the virtualization of internet-based services. They differ in the procedure that the server uses when making a request to determine which of the services it provides has been addressed.

IP-based virtual hosting

A host can have several network interfaces . Several IP addresses (interface aliases) can be configured on each network interface of a host. The servers or virtual servers running on the host can bind to one or more of these IP addresses. This means that different servers can be addressed on this host under different IP addresses. From the client's point of view, the servers are independent of one another. This method is more robust than the name-based virtual hosting discussed below .

Name-based virtual hosting

Many servers offer the option of offering their services for different domains at the same time, regardless of whether the server is bound to one or more IP addresses. The distinction for which domain the service was requested is made at the application level. So z. B. a mail server can recognize the desired domain of the recipient on the SMTP handshake. With HTTP , the distinction is made using the HTTP header field Host .

This process is known as name-based virtual hosting or also as a virtual domain . The latter designation is misleading because it is not the domain that is virtualized, but the services running under its name.

Pure name-based virtual hosting is only used with virtual servers , since several non- virtual servers cannot receive connections on a common IP address and therefore a differentiation by name is of no advantage.

This method is less robust than IP-based virtual hosting. With the HTTP protocol, for example, the sending of a "host" header is required on the client side, but this is only mandatory from HTTP / 1.1 and which is usually beyond the control of the server operator. In the case of HTTPS connections, the support of Server Name Indication on the server and client side is also necessary for a suitable assignment of the certificates to the domains .

The advantage of name-based virtual hosting is the lower server-side maintenance effort and the lower consumption of IP addresses, which is an important argument, especially for large servers that accommodate services for several hundred or even thousands of domains.