Server Name Indication

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.

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.

However it may be difficult - 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. 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.

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