IIS with ARR and URL Rewrite

Background

This configuration makes IIS on Windows Server 2025 act as a secure reverse proxy in front of several internal HTTPS services running on different localhost ports.

It solves these problems:

• It exposes multiple backend applications through one public IIS site on standard HTTPS port 443, instead of requiring users to access 8443, 8450, 8446 or 8445 directly.

• It routes requests by path, so /access, /saml, /oidc , /forms , /pwdreset and /portal each go to the correct internal service. Requests to / will be routed to /portal.

• It preserves the original public host header, which many applications need for URL generation, redirects, SAML, cookies, and validation logic.

• It adds X-Forwarded-Proto: https and X-Forwarded-Port: 443, so backend apps know the original client connection was HTTPS on the public endpoint, even though IIS is proxying the request internally.

• It allows IIS/ARR to connect to backend HTTPS services even if those internal certificates are self-signed, expired, or have hostname mismatches.

In practical terms, this is useful when you want users to access one external URL such as https://yourserver/access while the real applications stay bound internally to https://localhost:8443, https://localhost:8450, and https://localhost:8445.

The main issue it solves is publishing internal web apps securely behind a single IIS entry point without changing the applications to listen directly on the public interface.

Pre-reqs

Windows 2025 with Admin privileges

Installation and Configuration

Install the required modules

• IIS

• URL Rewrite

• Application Request Routing (ARR)

Official downloads:

• URL Rewrite

• ARR

Enable reverse proxy in ARR

In IIS Manager:

• Select the server

• Open Application Request Routing Cache

• Click Server Proxy Settings

• Check Enable proxy

Preserve the original Host header

Run as Administrator:

%windir%\system32\inetsrv\appcmd.exe set config -section:system.webServer/proxy /preserveHostHeader:"True" /commit:apphost

Add X-Forwarded-Proto and X-Forwarded-Port

In IIS, on the site used (such as Default Web Site):

• Open URL Rewrite

• Click View Server Variables...

• Add:

  • HTTP_X_FORWARDED_PROTO

  • HTTP_X_FORWARDED_PORT

• Create web.config for the site used (C:\inetpub\wwwroot for Default Web Site) with this content:

<configuration>
  <system.webServer>
    <rewrite>
      <rules>
		<rule name="Proxy root" stopProcessing="true">
          <match url="^(/.*)?$" />
          <serverVariables>
            <set name="HTTP_X_FORWARDED_PROTO" value="https" />
            <set name="HTTP_X_FORWARDED_PORT" value="443" />
          </serverVariables>
          <action type="Rewrite" url="https://localhost:8445/{R:0}" />
        </rule>
		
        <rule name="Proxy access" stopProcessing="true">
          <match url="^access(/.*)?$" />
          <serverVariables>
            <set name="HTTP_X_FORWARDED_PROTO" value="https" />
            <set name="HTTP_X_FORWARDED_PORT" value="443" />
          </serverVariables>
          <action type="Rewrite" url="https://localhost:8443/{R:0}" />
        </rule>

        <rule name="Proxy saml" stopProcessing="true">
          <match url="^saml(/.*)?$" />
          <serverVariables>
            <set name="HTTP_X_FORWARDED_PROTO" value="https" />
            <set name="HTTP_X_FORWARDED_PORT" value="443" />
          </serverVariables>
          <action type="Rewrite" url="https://localhost:8443/{R:0}" />
        </rule>
        
        <rule name="Proxy oidc" stopProcessing="true">
          <match url="^oidc(/.*)?$" />
          <serverVariables>
            <set name="HTTP_X_FORWARDED_PROTO" value="https" />
            <set name="HTTP_X_FORWARDED_PORT" value="443" />
          </serverVariables>
          <action type="Rewrite" url="https://localhost:8443/{R:0}" />

        <rule name="Proxy forms" stopProcessing="true">
          <match url="^forms(/.*)?$" />
          <serverVariables>
            <set name="HTTP_X_FORWARDED_PROTO" value="https" />
            <set name="HTTP_X_FORWARDED_PORT" value="443" />
          </serverVariables>
          <action type="Rewrite" url="https://localhost:8450/{R:0}" />
        </rule>

        <rule name="Proxy portal" stopProcessing="true">
          <match url="^portal(/.*)?$" />
          <serverVariables>
            <set name="HTTP_X_FORWARDED_PROTO" value="https" />
            <set name="HTTP_X_FORWARDED_PORT" value="443" />
          </serverVariables>
          <action type="Rewrite" url="https://localhost:8445/{R:0}" />
        </rule>
        
        <rule name="Proxy pwdreset" stopProcessing="true">
          <match url="^pwdreset(/.*)?$" />
          <serverVariables>
            <set name="HTTP_X_FORWARDED_PROTO" value="https" />
            <set name="HTTP_X_FORWARDED_PORT" value="443" />
          </serverVariables>
          <action type="Rewrite" url="https://localhost:8446/{R:0}" />
        </rule>
      </rules>
    </rewrite>
  </system.webServer>
</configuration>

OPTIONAL - Disable SSL validation against the backend

If you want IIS to ignore CN/name mismatch, expired certificates, and unknown CA errors, run this as Administrator:

reg add "HKLM\SOFTWARE\Microsoft\IIS Extensions\Application Request Routing\Parameters" /v SecureConnectionIgnoreFlags /t REG_DWORD /d 13056 /f

Restart IIS

Issue iisreset as administrator in order to restart the IIS.

iisreset

Verify configuration

Go to one of the url patterns previously configured and verify the service is reachable

Example: https://servername.company.org/portal