How to force HTTPS using a web.config file

You need URL Rewrite module, preferably v2 (I have no v1 installed, so cannot guarantee that it will work there, but it should).

Here is an example of such web.config -- it will force HTTPS for ALL resources (using 301 Permanent Redirect):

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.webServer>
        <rewrite>
            <rules>
                <clear />
                <rule name="Redirect to https" stopProcessing="true">
                    <match url=".*" />
                    <conditions>
                        <add input="{HTTPS}" pattern="off" ignoreCase="true" />
                    </conditions>
                    <action type="Redirect" url="https://{HTTP_HOST}{REQUEST_URI}" redirectType="Permanent" appendQueryString="false" />
                </rule>
            </rules>
        </rewrite>
    </system.webServer>
</configuration>

P.S. This particular solution has nothing to do with ASP.NET/PHP or any other technology as it's done using URL rewriting module only -- it is processed at one of the initial/lower levels -- before request gets to the point where your code gets executed.

For those using ASP.NET MVC. You can use the RequireHttpsAttribute to force all responses to be HTTPS:

GlobalFilters.Filters.Add(new RequireHttpsAttribute());

Other things you may also want to do to help secure your site:

1. Force Anti-Forgery tokens to use SSL/TLS:

   AntiForgeryConfig.RequireSsl = true;
   

2. Require Cookies to require HTTPS by default by changing the Web.config file:

   <system.web>
       <httpCookies httpOnlyCookies="true" requireSSL="true" />
   </system.web>
   

3. Use the NWebSec.Owin NuGet package and add the following line of code to enable Strict Transport Security (HSTS) across the site. Don't forget to add the Preload directive below and submit your site to the HSTS Preload site. More information here and here. Note that if you are not using OWIN, there is a Web.config method you can read up on on the NWebSec site.

   // app is your OWIN IAppBuilder app in Startup.cs
   app.UseHsts(options => options.MaxAge(days: 720).Preload());
   

4. Use the NWebSec.Owin NuGet package and add the following line of code to enable Public Key Pinning (HPKP) across the site. More information here and here.

   // app is your OWIN IAppBuilder app in Startup.cs
   app.UseHpkp(options => options
       .Sha256Pins(
           "Base64 encoded SHA-256 hash of your first certificate e.g. cUPcTAZWKaASuYWhhneDttWpY3oBAkE3h2+soZS7sWs=",
           "Base64 encoded SHA-256 hash of your second backup certificate e.g. M8HztCzM3elUxkcjR2S5P4hhyBNf6lHkmjAHKhpGPWE=")
       .MaxAge(days: 30));
   

5. Include the https scheme in any URL's used. Content Security Policy (CSP) HTTP header and Subresource Integrity (SRI) do not play nice when you imit the scheme in some browsers. It is better to be explicit about HTTPS. e.g.

   <script src="https://ajax.aspnetcdn.com/ajax/bootstrap/3.3.4/bootstrap.min.js">
   </script>
   

6. Use the ASP.NET MVC Boilerplate Visual Studio project template to generate a project with all of this and much more built in. You can also view the code on GitHub.

To augment LazyOne's answer, here is an annotated version of the answer.

<rewrite>
  <rules>
     <clear />
     <rule name="Redirect all requests to https" stopProcessing="true">
       <match url="(.*)" />
         <conditions logicalGrouping="MatchAll">
           <add input="{HTTPS}" pattern="off" ignoreCase="true" />
         </conditions>
         <action 
            type="Redirect" url="https://{HTTP_HOST}{REQUEST_URI}" 
            redirectType="Permanent" appendQueryString="false" />
     </rule>
  </rules>
</rewrite>

Clear all the other rules that might already been defined on this server. Create a new rule, that we will name "Redirect all requests to https". After processing this rule, do not process any more rules! Match all incoming URLs. Then check whether all of these other conditions are true: HTTPS is turned OFF. Well, that's only one condition (but make sure it's true). If it is, send a 301 Permanent redirect back to the client at http://www.foobar.com/whatever?else=the#url-contains. Don't add the query string at the end of that, because it would duplicate the query string!

This is what the properties, attributes, and some of the values mean.

• clear removes all server rules that we might otherwise inherit.
• rule defines a rule.
  • name an arbitrary (though unique) name for the rule.
  • stopProcessing whether to forward the request immediately to the IIS request pipeline or first to process additional rules.
• match when to run this rule.
  • url a pattern against which to evaluate the URL
• conditions additional conditions about when to run this rule; conditions are processed only if there is first a match.
  • logicalGrouping whether all the conditions must be true (MatchAll) or any of the conditions must be true (MatchAny); similar to AND vs OR.
• add adds a condition that must be met.
  • input the input that a condition is evaluating; input can be server variables.
  • pattern the standard against which to evaluate the input.
  • ignoreCase whether capitalization matters or not.
• action what to do if the match and its conditions are all true.
  • type can generally be redirect (client-side) or rewrite (server-side).
  • url what to produce as a result of this rule; in this case, concatenate https:// with two server variables.
  • redirectType what HTTP redirect to use; this one is a 301 Permanent.
  • appendQueryString whether to add the query string at the end of the resultant url or not; in this case, we are setting it to false, because the {REQUEST_URI} already includes it.

The server variables are

• {HTTPS} which is either OFF or ON.
• {HTTP_HOST} is www.mysite.com, and
• {REQUEST_URI} includes the rest of the URI, e.g. /home?key=value
  • the browser handles the #fragment (see comment from LazyOne).

See also: https://www.iis.net/learn/extensions/url-rewrite-module/url-rewrite-module-configuration-reference