HTTP Strict Transport Security

Description
HTTP Strict Transport Security (HSTS) is an opt-in security enhancement that is specified by a web application through the use of a special response header. Once a supported browser receives this header that browser will prevent any communications from being sent over HTTP to the specified domain and will instead send all communications over HTTPS.

Examples
Example of the HTTP strict transport security header

Strict-Transport-Security: max-age=60000

If all subdomains are HTTPS to then the following header is applicable:

Strict-Transport-Security: max-age=60000; includeSubDomains

Server Side
The server side needs to inject the HSTS header.

For HTTP sites on the same domain it is recommended that a HSTS header be returned along with a permanent redirect to the HTTPS site.

An Apache HTTPd example is as follows:

 ServerAlias * Header Always set Strict-Transport-Security "max-age=16070400; includeSubDomains" RewriteEngine On       RewriteRule ^(.*)$ https://%{HTTP_HOST}$1 [redirect=301] 

Links
HSTS Spec

Wikipedia.org entry

MDN Docs for HSTS

OWASP TLS Protection Cheat Sheet