I recently got this error when updating my rewrite rules in my system.webServer node of my web.config. The error message stated “HTTP Error 500.50 – URL Rewrite Module Error. The server variable “SERVER_PORT” is not allowed to be set. Add the server variable name to the allowed server variable list.”

Here is how you add a server variable using IIS

STEP 1 :

In your IIS with your website selected, select URL Rewrite option step1

STEP 2

Next select the View Server Variable option step2

STEP 3

Add server variable as shown below step3

Alternately you could also add through your web.config like below under <system.webServer></system.webServer> node.

<allowedServerVariables xdt:Transform="Insert">
    <add name="SERVER_PORT" />
    <add name="REMOTE_ADDR" />
</allowedServerVariables>