The HTTP Strict-Transport-Security response header is a security measure that informs clients that the site and its resources shall only be accessed using HTTPS.
When the HTTP Strict-Transport-Security response header is included, clients must only interact with the site using the HTTPS protocol. If HTTP is used then the connections will be automatically upgraded to HTTPS. If the more secure protocol is not available then access to the resources is blocked. There are three directives including
max-age=<seconds> attribute is required and it is used to set the number of seconds which the client can access the site using HTTPS. The recommended value is two years, or 63,072,000 seconds.
includeSubDomains attribute is present, the HTTPS requirement applies to all of the site’s subdomains.
preload attribute is not part of the standard. Rather, it is an HSTS preload service that is maintained by Google. If a domain is submitted to the service and available for preloading then it will never be connected using HTTP. Supporting browsers will automatically substitute HTTPS if the HTTP scheme is entered into the address bar.
Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
The HTTP Strict-Transport-Security response header is used to inform the client that it needs to use HTTPS, rather than HTTP, for a set time.