Consider the following scenario:

You configure Host to Client Redirection as specified here:  https://docs.citrix.com/en-us/xenapp-and-xendesktop/7-15-ltsr/general-content-redirection/host-to-client-redirection.html

You confirm that Host to Client Redirection is working properly.  Redirected URLs are opening on the client successfully.  If your local browser supports tabs, redirected URLs open in tabs on the client's local browser.

You configure the "ValidSites" registry key under HKLM\Software\Wow6432Node\Citrix\SFTA in order to redirect some URLs but not others.

You may observed one or both of the following behaviors:

1. If you have a browser other than Internet Explorer set as the default on the VDA, URLs that are *not* redirected will open in Internet Explorer instead of the default browser.
2. URLs that are not redirected will open in new Internet Explorer windows and not in existing browser window tabs.

This behavior is by design.  When the VDA is installed, in addition to other registry keys created to manage Host to Client Redirection, the following keys are created as well:

HKEY_CLASSES_ROOT\http\shell\open\command_backup
Default - REG_SZ = "c:\program files\internet explorer\iexplore.exe" %1

HKEY_CLASSES_ROOT\https\shell\open\command_backup
Default - REG_SZ = "c:\program files\internet explorer\iexplore.exe" %1

In the event the Host to Client Redirection code does not redirect a URL (either due to it not being on the ValidSites list or some error such as the client being unable to reach the URL) Host to Client Redirection will pass the URL to the command_backup key indicated above.  This will open a new instance of Internet Explorer for the URL the same way as if you had run the command in a Run prompt in Windows.  For Internet Explorer this results in a new window for each URL passed.

These keys can also be used to specify the browser to be used on the VDA for opening URLs that are not being redirected to client. Modify the key value to include the executable path of the browser in quotes. To use Google Chrome browser, for example:

HKEY_CLASSES_ROOT\http\shell\open\command_backup
Default - REG_SZ = "C:\Program Files (x86)\Google\Chrome\Application\chrome.exe" %1

HKEY_CLASSES_ROOT\https\shell\open\command_backup
Default - REG_SZ = "C:\Program Files (x86)\Google\Chrome\Application\chrome.exe" %1
 

Caution! Using Registry Editor incorrectly can cause serious problems that might require you to reinstall your operating system. Citrix cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk. Be sure to back up the registry before you edit it.

This article provides an overview of how Host to Client Redirection works with Tabs in browsers