This article will explain three methods on how to change the “applicationHost.config” file in App Service.
Imagine a scenario where you want to change a particular setting, however, this cannot be changed at the “web.config” level.
For this scenario I would like to allow four Server Variables to be rewritten. By default, this can only be done at “applicationHost.config” level.
In a Virtual Machine this setting would look like this in the “applicationHost.config” file:
<add name="HTTP_X_UNPROXIED_URL" />
<add name="HTTP_X_ORIGINAL_ACCEPT_ENCODING" />
<add name="HTTP_X_ORIGINAL_HOST" />
<add name="HTTP_ACCEPT_ENCODING" />
The issue is that in an App Service the “applicationHost.config” file is read-only.
File is located at “C:\local\Config\applicationHost.config”.
What we can do is to create a transform file to, you guessed it, transform the actual “applicationHost.config” file.
There are three ways to do this:
Go to “C:\home\SiteExtensions” (crete /SiteExtensions folder if not present) and create a folder within this directory with the desire name of your extension.
In my case I will create a folder name “/MyPrivateExtension” inside “/SiteExtensions”.
Navigate to “/MyPrivateExtension” and create/upload a file called “applicationHost.xdt”. You can use “touch” command.
Past the desire property to transform and with the appropriate syntax.
For our purposes the correct syntax would be the following:
<add name="HTTP_X_UNPROXIED_URL" xdt:Locator="Match(name)" xdt:Transform="InsertIfMissing" />
<add name="HTTP_X_ORIGINAL_ACCEPT_ENCODING" xdt:Locator="Match(name)" xdt:Transform="InsertIfMissing" />
<add name="HTTP_X_ORIGINAL_HOST" xdt:Locator="Match(name)" xdt:Transform="InsertIfMissing" />
<add name="HTTP_ACCEPT_ENCODING" xdt:Locator="Match(name)" xdt:Transform="InsertIfMissing" />
Note: At the end of this article, I will leave some sample for you to understand syntax.
Save it and restart your App Service
This file should have transformed the actual “applicationHost.config”.
Let us check:
Top Level Private Extension
<Removed previous “applicationHost.xdt” and restarted App Service>
Basically, we will create the exact same file (“applicationHost.xdt”), but within “C:\home\site” folder.
Paste content and save file.
Restart App Service.
This should have transformed the actual “applicationHost.config”.
IIS Manager Extension
In Kudu go to Site Extension > Gallery and search for “iis manager” and install it.
Then add “/iismanager” to your Kudu URL. “https://<YOURAPPSERVICE.scm.azurewebsites.net/iismanager”
Make changes, as you normally would in the “applicationHost.config”, save and restart it.
This should have created a top level “applicationHost.xdt” that will modify actual “applicationHost.config”.
Changes in “applicationHost.config”:
If you restarted the App Service but still not seeing changes in “applicationHost.config” file, I would suggest restarting App Service again just to be sure. The other thing that you could do is to check transform logs located in “C:\home\LogFiles\Transform”. These logs can tell you why changes did not apply.