Server Name Indication
Server Name Indication (SNI) is an extension to the TLS protocol that indicates what hostname the client 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 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 virtual hosting for HTTPS.
To make use of SNI practical, it is necessary that the vast majority of users use web browsers that support it. Users whose browsers do not support SNI will be 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. As of November 2012, the only major user bases whose browsers do not support SNI appear to be users of Android 2.x (default browser), Internet Explorer on Windows XP and versions of Java before 1.7 on any operating system.
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 is found the connection proceeds as normal. If a match is not found the user may be warned of the discrepancy and the connection may be aborted 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. As such 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.
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 prevented from using secure communications.
How SNI fixes the problem
An extension to TLS called Server Name Indication (SNI) addresses this issue by sending the name of the virtual domain as part of the TLS negotiation. This enables the server to select the correct virtual domain early and present the browser with the certificate containing the correct name. Therefore with clients and servers that support SNI, a single IP address can be used to serve a group of domain names for which it is impractical to get a common certificate.
In 2004, a patch for adding TLS/SNI into OpenSSL was created by the EdelKey project. In 2006, this patch was then ported to the development branch of OpenSSL, and in 2007 it was back-ported to OpenSSL 0.9.8.
To support SNI the TLS library used by an application must support it and the application must pass the hostname to the TLS library. Further complicating matters the TLS library may be either shipped as part of the application or may be a component of the operating system. As a result of this some browsers support SNI on all operating systems while others only support it on a subset of operating systems. As of 2011 most web browsers and TLS libraries have implemented support for SNI but there are still a large number of users still using combinations of browser and operating system that do not support it.
- Internet Explorer 7 or later, on Windows Vista or higher. Does not work on Windows XP, even Internet Explorer 8 (because the support of this feature is not browser version dependent, it depends on SChannel system component which introduced the support of TLS SNI extension, starting from Windows Vista, not XP).
- Mozilla Firefox 2.0 or later
- Opera 8.0 (2005) or later (the TLS 1.1 protocol must be enabled)
- Opera Mobile at least version 10.1 beta on Android
- Google Chrome (Vista or higher. XP on Chrome 6 or newer. OS X 10.5.7 or higher on Chrome 5.0.342.1 or newer)
- Safari 3.0 or later (Mac OS X 10.5.6 or higher and Windows Vista or higher)
- Konqueror/KDE 4.7 or later 
- MobileSafari in Apple iOS 4.0 or later
- Android default browser on Honeycomb (v3.x) or newer
- Windows Phone 7
- MicroB on Maemo
- Odyssey on MorphOS
- Apache 2.2.12 or later using mod_ssl (or alternatively with experimental mod_gnutls)
- Microsoft Internet Information Server IIS 8
- Nginx with an accompanying OpenSSL built with SNI support
- Apache Traffic Server 3.2.0 or later.
- Cherokee if compiled with TLS support
- Versions of lighttpd 1.4.x and 1.5.x with patch, or 1.4.24+ without patch
- F5 Networks Local Traffic Manager running version 11.1 or later 
- Hiawatha (web server) 8.6 or later
- LiteSpeed 4.1 or later
- Pound 2.6 or later
- Apache Tomcat on Java 7 or later
- PageKite tunneling reverse proxy
- Saetta Web Server via OpenSSL
- Citrix NetScaler 9.2 or later (9.3 Enhanced)
- Radware Alteon ADC running AlteonOS 28.1 or later 
- HAProxy 1.5 or later 
- Mozilla NSS 3.11.1 client-side only
- 0.9.8f (released 11 Oct 2007) - not compiled in by default, can be compiled in with config option '--enable-tlsext'.
- 0.9.8j (released 07 Jan 2009) through 1.0.0 (released 29 March 2010) - compiled in by default
- CyaSSL - not compiled in by default, can be compiled in with config option '--enable-sni' or '--enable-tlsx'.
- PolarSSL since 1.2.0 - compiled in by default
- libcurl / cURL since 7.18.1 (released 30 Mar 2008) when compiled against an SSL/TLS toolkit with SNI support
- Python 3.2 (
- Qt 4.8
- Oracle Java 7 JSSE 
- wget 1.14
- Android 2.3 (Gingerbread) has partial support if application uses HttpsURLConnection class.
- Android 3.0 (Honeycomb) has full support
- IO::Socket::SSL (Perl/CPAN module, since version 1.83 )
- Pike 7.9.5 (SSL module) 
The following combinations do not support SNI:
- Internet Explorer (any version) on Windows XP or Internet Explorer 6 or earlier
- Safari on Windows XP
- BlackBerry Browser
- Windows Mobile up to 6.5
- Android default browser on Android 2.x (Fixed in Honeycomb for tablets and Ice Cream Sandwich for phones)
- wget before 1.14
- Nokia Browser for Symbian at least on Series60
- Opera Mobile for Symbian at least on Series60
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