How can I check if traffic to a URL is allowed by the proxy?

Question

This question may sound weird but let's say I have asked to open traffic to cdn.mycompanycloud.com because there's an app to deploy that requires it. This URL does not reply to http requests by the way.

How can I check the URL has been allowed prior to installing the application?

nslookup would only prompt the DNS server so I would certainly always get a reply. ping the URL would actually ping the IP behing the DNS.

Any idea?

Thanks

Answer 1

If it's an https:// URL, it has to reply to HTTP requests, otherwise it wouldn't be an https:// URL.

Indeed, your example does respond – with an HTTP 404 "Not Found":

$ curl -I https://autologon.microsoftazuread-sso.com/

HTTP/1.1 404 Not Found
Cache-Control: private
Content-Length: 0
...

The response is zero-length, causing most web browsers to substitute their own error pages, but it's still a valid response and you can check for it. (Windows 10 comes with curl.exe, by the way.)

However, there are several "live" URLs at this domain, such as:

https://autologon.microsoftazuread-sso.com/YOUR_DOMAIN/winauth/trust/2005/windowstransport

When a GET request is made, it responds with HTTP 200 "OK", showing a pretty HTML-based error page saying "The endpoint only accepts POST requests". For example:

PS> Invoke-WebRequest https://autologon.microsoftazuread-sso.com/outlook.com/winauth/trust/2005/windowstransport

StatusCode        : 200
StatusDescription : OK
Content           : <!-- Copyright (C) Microsoft Corporation. All rights reserved. -->
                    ...

(PowerShell can be used to check URLs which return 404, but it's harder.)