Change Host Header with IIS URLRewrite

I have requests coming to my webserver at

http://www.example.com/blog

I want to Rewrite that to another server listening on

http://blog.example.com

Here is my rewrite rule:

<rule name="blogredirect1" stopProcessing="true">
          <match url="^blog(.*)" />
          <action type="Rewrite" url="http://blog.example.com{R:1}"  />
</rule>

What ends up happening is the rewrite sends the request to the second servers IP at blog.mysite.com but the request header's host is still www.mysite.com.

How do I make sure that the redirected request HOST is set to blog.mysite.com (which is set in the redirect rule)

May I know how did you check the HOST header?

It is recommended to check {HTTP_HOST} variable instead of view the HTTP header directly. Because if you view the request header, you will always see www.mysite.com.

You could fetch {HTTP_HOST} request variable from backend server blog.mysite.com.

However, if you mean page in blog.mysite.com also displayed {HTTP_HOST} as www.mysite.com, then please check whether you have set system.webServer/proxy/preserveHostHeader to true?

By the way, IIS supports to rewrite HTTP_HOST manually. You could modify your rule like this:

<rule name="blogredirect1" stopProcessing="true">
  <match url="^blog(.*)" />
  <action type="Rewrite" url="http://blog.example.com{R:1}" />
  <serverVariables>
    <set name="HTTP_HOST" value="blog.example.com" />
  </serverVariables>
</rule>

Please remember to Allow Server Variable {HTTP_HOST} in URL rewrite.

https://learn.microsoft.com/en-us/iis/extensions/url-rewrite-module/setting-http-request-headers-and-iis-server-variables

You could also set this in applicationhost.config <location path="sitename"> section.

<location path="Default Web Site">
  <system.webServer>
    <rewrite>
      <allowedServerVariables>
        <add name="HTTP_HOST" />
      </allowedServerVariables>
    </rewrite>
  </system.webServer>
</location>