How to set up reverse-proxy to an external server in IIS

If I go to http://www.trees.com/ficus/macrophylla using the browser, then will get

If I go to http://www.trees.com/ will also get the following

Using SSL Request to trees.com

Clicking "Click here to ignore the mismatch...", will then get

In the configuration,

we can see TLS 1.0, 1.1, 1.2 and 1.3 are supported. Yet, green color for TLS 1.2 and 1.3.

We can configure PowerShell to use TLS 1.3

[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls13

And confirm it will be using it

[Net.ServicePointManager]::SecurityProtocol

In PowerShell (as Administrator), if one uses Invoke-WebRequest

Invoke-WebRequest -Uri trees.com/ficus/macrophylla

then will get

and if one uses

Invoke-WebRequest -Uri trees.com

then will get

So far so good. But if we want to test it for CORS from the https://figs.wild.com.au,

(Invoke-WebRequest -Uri 'http://trees.com' -Headers @{ "Origin" = "https://figs.wild.com.au" }).Headers

we get

Key                           Value
---                           -----
Transfer-Encoding             chunked
X-Adblock-Key                 MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBAL/3/SrV7P8AsTHMFSpPmYbyv2PkACHwmG9Z+1IFZq3vA54IN7pQcGnhgNo+8SN9r/KtUWCb9OPqTfWM1N4w/EUCAwEAAQ==_FamzgofQ7ugTniHINrZ7yp35i/Nqkt7q/gZsgPGyvhOwIQhj04Bd9+/nir6OLAFDPB56kU4m0GgS7SvEoFqRbQ==
Access-Control-Allow-Origin   *
Access-Control-Allow-Methods  *
Access-Control-Request-Method *
Access-Control-Allow-Headers  *
Access-Control-Max-Age        86400
X-UA-Compatible               IE=Edge,chrome=1
X-Request-Id                  556905ec3cb435a1168cc1b28d70875f
X-Runtime                     0.048014
X-Rack-Cache                  miss
Cache-Control                 max-age=0, private, must-revalidate
Content-Type                  text/html; charset=utf-8
Date                          Mon, 20 Jul 2020 09:40:37 GMT
ETag                          "8e51e434b70033ee6a90cb7397af53f9"
Set-Cookie                    _digiadmin2_session=BAh7B0kiD3Nlc3Npb25faWQGOgZFVEkiJTNmOWRlMDA5NjRiZWZlMzgyZTRmN2NlOWIzZmQxZjIzBjsAVEkiEF9jc3JmX3Rva2VuBjsARkkiMVFOckhMdElRMWc1cGZBcGl5OGQ1WkVNeXo3elpobWRwc2QyR0djTFlNUEE9BjsARg%3D%3D--e55261be794bb9f95ee407c73a3e2b315ef...
Server                        nginx/1.10.1

Notice that Access-Control-Allow-Origin has the value asterisk (*) which means that any domain is allowed. Then, if we use the following command

Invoke-WebRequest -Uri 'http://trees.com' -Headers @{ "Origin" = "https://figs.wild.com.au" }

we will get the following result

In other words, it's allowing cross-origin request and not blocking like you're mentioning in the question. Could be that you're giving fictitious URLs too just for the sake of the explanation.

In regards to the question and considering the comments, redirection to an external URL is possible in IIS as shown here.

<system.webServer>
  <rewrite>
    <rules>
      <rule name="External Redirect" stopProcessing="true">
        <match url="^VirtualDirectory" negate="true" />
        <conditions>
          <add input="{HTTP_HOST}" ignoreCase="true" negate="true" pattern="hostname"/>
          <!-- add this input condotion to make this redirect url not work with http://hostname/VirtualDirectory -->
        </conditions>
        <action type="Redirect" url="{your url}" redirectType="Found" />
      </rule>
    </rules>
  </rewrite>
</system.webServer>

Also, a simple redirect is possible using NGIX and is addressed, for instances, in this answer.

server {
    listen 80;
    server_name     example.com;
    return          301 http://www.example.com$request_uri;

server {
    listen 80;
    server_name     www.example.com;
    [...]

and in this answer

server {
  listen 80;
  server_name localhost;
  merge_slashes off;

  location /rdr {
    location /rdr/http:// {
      rewrite ^/rdr/(.*)$ $1 permanent;
    }
    rewrite ^/rdr/(.*)$ http://$1 permanent;
  }
}

Yet, what you want isn't to see the content of that page but to save that data anywhere and then redirect again. Where will that data be coming from then to feed the IFRAME?

Instead of doing this redirect > save data > redirect, I would suggest to do that separately. More specifically, you'd get the data from https://trees.com/ficus/macrophylla and save it in the location of https://figs.wild.com.au/trees/ficus/macrophylla and use what you want from that file for the IFRAMEs.

To get the content of the file in the location https://trees.com (without JS and CSS coming from other files) and save it in an html file, you'd do something like

from urllib.request import urlopen
html = urlopen("http://trees.com").read().decode('utf-8')
#print(html)
with open("test.html", "w") as file:
    file.write(html)

This will save the content in an HTML file named test present in the same location of this script.

(If CSS and JS is also required, do check this SO question).

If you don't want to go through that hustle, there's tools like HTTrack that allow to download complete websites. This way you wouldn't need to know the mapsite to then iterate over the possible variations.

I can see the convenience of what you want. Will investigate further and let you know if find that super-automated way to do this but would help to know "Where will that data be coming from then to feed the IFRAME?".