web.config redirect non-www to www

For a safer rule that works for both Match Any and Match All situations, you can use the Rewrite Map solution. It’s a perfectly good solution with the only drawback being the ever so slight extra effort to set it up since you need to create a rewrite map before you create the rule. In other words, if you choose to use this as your sole method of handling the protocol, you’ll be safe.

You can create a Rewrite Map called MapProtocol, you can use {MapProtocol:{HTTPS}} for the protocol within any rule action.

<rewrite>
  <rules>
    <rule name="Redirect to www" stopProcessing="true">
      <match url="(.*)" />
      <conditions trackAllCaptures="false">
        <add input="{HTTP_HOST}" pattern="^example.com$" />
      </conditions>
      <action type="Redirect"
        url="{MapProtocol:{HTTPS}}://www.example.com/{R:1}" />
    </rule>
  </rules>
  <rewriteMaps>
    <rewriteMap name="MapProtocol">
      <add key="on" value="https" />
      <add key="off" value="http" />
    </rewriteMap>
  </rewriteMaps>
</rewrite>

Reference

Since 2018, consider to use everytime https (SSL) !

So I use redirect to www and to https together.

<rule name="Redirect to www" stopProcessing="true">
    <match url="(.*)" />
    <conditions trackAllCaptures="false">
        <add input="{HTTP_HOST}" pattern="^example.com$" />
    </conditions>
    <action type="Redirect" url="https://www.example.com/{R:1}" redirectType="Permanent" />
</rule>

<rule name="HTTP/S to HTTPS Redirect" enabled="true" stopProcessing="true">
    <match url="(.*)" />
    <conditions logicalGrouping="MatchAny">
        <add input="{SERVER_PORT_SECURE}" pattern="^0$" />
    </conditions>
    <action type="Redirect" url="https://{HTTP_HOST}{REQUEST_URI}" redirectType="Permanent" />
</rule>

more info here : https://forums.realmacsoftware.com/t/effective-july-2018-google-s-chrome-browser-will-mark-non-https-sites-as-not-secure/18597

Other answers here are unnecessary long, here's the rule i use (just copy and paste) :

<rule name="ensurewww" stopProcessing="true">
  <match url=".*" />
  <conditions>
    <add input="{CACHE_URL}" pattern="^(.+)://(?!www)(.*)" />
  </conditions>
  <action type="Redirect" url="{C:1}://www.{C:2}" redirectType="Permanent" />
</rule>

This will redirect to the www version while preserving the HTTP and HTTPS protocol

Hope this helps.