301 redirect from /page.html to /page

For global solution you can follow this link. Within Kentico you need to enable extensions for .html under Settings -> URL & SEO -> Friendly URL Extensions if not already to add them as Alias. Check this link to do this within Kentico.

Assuming you have the proper web.config (<modules runAllManagedModulesForAllRequests="true">) it sounds like you're only missing adding the ".html" extension here:

Settings>URL & SEO>Friendly URL Extensions

Brenden - adding '.html' to Settings>URL & SEO>Friendly URL Extensions does the opposite - it forces the friendly URL to redirect from:

http://mysite/page

to

http://mysite/page.html

Sorry about that, you need to include a blank at the beginning as well so your value should be

;.html;

The blank at the beginning will cover the extensionless URL setup.

I sussed that and added this:

Friendly URL Extensions ";.html;.aspx"

This now works as expected with a .aspx extension added but not with .html

If you want extensionless URLs you need to make sure you have

<modules runAllManagedModulesForAllRequests="true">

under the

<system.webServer>

node.

I'd already added this and extensionless URLs are already working for .aspx

http://mysite.com/page.aspx

redirects as expected to:

http://mysite.com/page

But I get a 404 on the following:

http://mysite.com/page.html

In your alias on the page, do not include the period.
Your extension should read html not .html

It looks like the runAllManagedModulesForAllRequests attribute was not added into the correct place. It has to be added here.