Rewriting server variable in IIS 7.5 - url

Question

I have a rewrite rule, which changes a server variable with the value of a subdomain.
This works on subdomain.mydomain.nl/somethinghere but not on subdomain.mydomain.nl
<rule name="Change code" enabled="true" patternSyntax="ECMAScript" stopProcessing="false">
<match url=".*" ignoreCase="true" />
<conditions logicalGrouping="MatchAll" trackAllCaptures="true">
<add input="{SERVER_NAME}" pattern="(www\.)?(\w+)\.mydomain\.nl" />
<add input="{SERVER_NAME}" pattern="^www.mydomain.nl.*" negate="true" />
<add input="{SERVER_NAME}" pattern="^mydomain.nl.*" negate="true" />
</conditions>
<serverVariables>
<set name="MYVARIABLE" value="{C:2}" />
</serverVariables>
<action type="None" />
</rule>
I have tested 2 urls:
1: subdomain.mydomain.nl/somethinghere
2: subdomain.mydomain.nl
I retrieve the variable in PHP with the following code:
echo $_SERVER['MYVARIABLE'];
In case of URL 1, the output of this is "subdomain".
In case of URL 2, the output of this is "".
The output of URL 1 is correct, but the output of URL 2 should be "subdomain" too.
I have run a trace of both requests, and they both show that the rule is being matched and executed.
Can anyone help me?

Answer 1

When you set a custom server variable, you should start it with HTTP_. When you add your own header, it should start with HTTP_X_ to add a host header starting with an X.
To be honest, I can't really explain why it works without HTTP_ in some scenarios, but with HTTP_ it works in all scenarios and that's also how it's documented.
<rules>
<rule name="Change code" enabled="true" patternSyntax="ECMAScript" stopProcessing="false">
<match url=".*" ignoreCase="true" />
<conditions logicalGrouping="MatchAll" trackAllCaptures="true">
<add input="{SERVER_NAME}" pattern="(www\.)?(\w+)\.testsite\.nl" />
<add input="{SERVER_NAME}" pattern="^www\.testsite\.nl$" negate="true" />
<add input="{SERVER_NAME}" pattern="^testsite\.nl$" negate="true" />
</conditions>
<serverVariables>
<set name="HTTP_X_MYVARIABLE" value="{C:2}" />
</serverVariables>
<action type="None" />
</rule>
</rules>
You can now get the subdomain name with echo $_SERVER["HTTP_X_MYVARIABLE"];.
I've also cleaned up your conditional regular expressions to escape the .s and also added a $ to make it truly match the exact domain names.