|
Server Name Indication (SNI) is an extension to the 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 server to present multiple certificates on the same IP address and TCP port number and hence allows multiple secure (HTTPS) websites (or any other Service over TLS) to be served off the same IP address without requiring all those sites to use the same certificate. It is the conceptual equivalent to HTTP/1.1 name-based virtual hosting, but for HTTPS. The desired hostname is not encrypted,〔 so an eavesdropper can see which site is being requested. To make use of SNI practical, the vast majority of users must use web browsers that implement it. Users whose browsers do not implement SNI are presented with a default certificate and hence are likely to receive certificate warnings, unless the server is equipped with a wildcard certificate that matches the name of the website. ==Background of the problem== When making a TLS connection the client requests a digital certificate from the web server; once the server sends the certificate, the client examines it and compares the name it was trying to connect to with the name(s) included in the certificate. If a match occurs the connection proceeds as normal. If a match is not found the user may be warned of the discrepancy and the connection may abort as the mismatch may indicate an attempted man-in-the-middle attack. However, some applications allow the user to bypass the warning to proceed with the connection, with the user taking on the responsibility of trusting the certificate and, by extension, the connection. It is possible for one certificate to cover multiple hostnames. The X.509 v3 specification introduced the ''subjectAltName'' field which allows one certificate to specify more than one domain and the usage of wildcards in both the common name and ''subjectAltName'' fields. However it may be impractical—or even impossible, due to lack of a full list of all names in advance—to obtain a single certificate that covers all names a server will be responsible for. A server that is responsible for multiple hostnames is likely to need to present a different certificate for each name (or small group of names). Since 2005, CAcert has run experiments on different methods of using TLS on virtual servers.〔(【引用サイトリンク】 CAcert VHostTaskForce )〕 Most of the experiments are unsatisfactory and impractical. For example, it is possible to use ''subjectAltName'' to contain multiple domains controlled by one person in a single certificate. Such "unified communications certificates" must be reissued every time the list of domains changes. Name-based virtual hosting allows multiple DNS hostnames to be hosted by a single server (usually a web server) on the same IP address. To achieve this the server uses a hostname presented by the client as part of the protocol (for HTTP the name is presented in the host header). However, when using HTTPS the TLS handshake happens before the server sees any HTTP headers. Therefore, it is not possible for the server to use the information in the HTTP host header to decide which certificate to present and as such only names covered by the same certificate can be served from the same IP address. In practice, this means that an HTTPS server can only serve one domain (or small group of domains) per IP address for secured browsing. Assigning a separate IP address for each site increases the cost of hosting, since requests for IP addresses must be justified to the regional internet registry and IPv4 addresses are now in short supply. The result is that many websites are effectively constrained from using secure communications over IPv4. IPv6 address space is not in short supply so websites served using IPv6 are unaffected by this issue. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Server Name Indication」の詳細全文を読む スポンサード リンク
|