IIS and Azure Files

Our friends in Azure have just announced the GA of Azure File Storage, which allows fully managed file shares in the cloud. https://azure.microsoft.com/en-us/blog/azure-file-storage-now-generally-available/

One common question is "can I host my IIS web content in the cloud using Azure Files”? The answer is yes you can! :)

In general, Web Apps is the recommended and best way to create and deploy Web Applications in Azure.

We also recognize that customers may choose to host their web servers on IaaS VMs. Until now, for these scenarios, the web content always had to be hosted locally on the VM. With Azure File Storage, the web content can now be stored independently of the web server.

One advantage of using Azure File Storage in this way is if you have a ‘farm’ of web servers accessing content in a centralized location (previously, you would have to copy the content across all the VMs and keep them sync).

This blog entry walks you through the step by step of how to configure IIS to work with Azure Files.

Basics / Prerequisites

This walkthrough assumes that you already have an Azure storage account and that the Azure Files share is already setup (https://azure.microsoft.com/en-us/documentation/articles/storage-dotnet-how-to-use-files/)

For the purpose of this walkthrough we have already setup a share called ‘webshare1’ and a storage account as follows

Account Name youraccnt123
File Endpoint https://youraccnt123.file.core.windows.net
Account Key 1 someVeryLongStorageAccountKeyxxxxyyyyyzzzz1111222233334444555566667777888899990000AAABB==

Whereby the share would be accessed from the command line as follows, net use <drive-letter>: \\<storage-account-name>.file.core.windows.net\<share-name> /u:<storage-account-name> <storage-account-key> e.g.

net use * \\ youraccnt123.file.core.windows.net\webshare1 /u: youraccnt123 someVeryLongStorageAccountKeyxxxxyyyyyzzzz1111222233334444555566667777888899990000AAABB==

IIS Setup

Creating local user

Azure Files does not currently support Windows Authentication, which means on the Web Server (e.g. the Azure VM hosting IIS that will be accessing the Azure file share) we will need to create a local user that maps to the storage account user.

From Computer Management UI: 'Local Users and Groups' -> select Users -> right click -> 'New User'

In this case I have ‘User cannot change password’ and ‘Password never expires’ checked, but this is entirely your decision. You can also script the creation of the local user e.g. using PowerShell. By default, Windows has a restriction of 20 characters for the user name. Technically this limit can be changed in Active Directory but for the purpose of integrating IIS with Azure Files we should aim to keep it at 20. This means your Azure storage account name should also be a max of 20 chars.

Add user to IIS_IUSRS group

Add the newly created local user to the local IIS_IUSRS group.

From Computer Management UI: 'Local Users and Groups' -> select Groups -> IIS_IUSRS

You can also accomplish this via command line e.g. net localgroup IIS_IUSRS youraccnt123 /add

Shared Configuration

IIS has a ‘shared centralized global configuration’ feature, whereby multiple servers in a Web Farm can share the same configuration settings by hosting the files on an UNC share. Some more information on shared configuration here: http://www.iis.net/learn/manage/managing-your-configuration-settings/shared-configuration_264

Similarly, with Azure Files, we can use the same shared configuration files on the Azure share. If you are not using Shared Configuration, then you can omit this section, otherwise read on.

Exporting files and keys

The first step of setting up shared configuration is to export the configuration file and keys. In the UI, open the Shared Configuration option under Management, select ‘Export Configuration’ (far right in Actions pane).

One way is to export the files locally and then you copy to the Azure file share. Another way is to set the physical path to point to your Azure file share and set credentials using your newly created local user, that maps to the storage account.

If you are writing to the remote share and get an error “Cannot write to the specified path. Make sure that the path and credentials are valid” (and assuming you gave the correct credentials) then you could be hitting a bug that has been fixed on newer OS releases such as Windows 10 and Windows Server 2016. If you hit this issue then export locally, then copy to the Azure file share.

At this point we should have a directory on the Azure file share (e.g. \\youraccnt123.file.core.windows.net\webshare1\sharedconfig) that contains the following files:

  • administration.config
  • applicationHost.config
  • configEncKey.key

Enabling Shared Configuration

Once the configuration files are exported, you can enable shared configuration and point the physical path to the Azure file share (in UNC format), using the local user/password that we created above.

The UI will also prompt you for the password for the encryption key (you used when exporting the keys). You may encounter a couple of message boxes with the error “object reference not set to an instance of an object”. You can dismiss them both by clicking on OK and ignore the error as the changes will actually be saved. The message box is a UI bug we will look into fixing in future versions.

Configuring Application Pools

Create a new Application Pool using the identity of the local user that maps to the storage account.

To do this in the IIS management UI:

  • Create a new Application Pool (right click on Application Pools -> Add New Application Pool) and give it a name e.g. AzFilePool
  • Select the newly created Application Pool -> Advanced Settings (far right pane under Actions) -> Identity -> Custom Account -> Set -> Enter user/password credentials of the local user (that maps to the storage account).

Changing physical path to point to Azure file share

In IIS configuration, there are multiple places where traditionally you would point to a local or UNC share, you should be able to point to the Azure file share.

Whether you are configuring Web Site, Application or FTP physical path, the steps are very similar.

  • Physical path to point to the Azure file share in UNC share – e.g. \\youraccnt123.file.core.windows.net\webshare1
    • Note – must be UNC format! Using mapped drive (e.g. Z:\) will not work!!! (mapped drive is only for the local user session)
  • In ‘Connect as’, specify the user name/password of the local user that we created above (where user name/password is the same as the storage account/key)
    • If there is a ‘Test Settings’ option then ‘Authentication’ should pass (ignore the ‘Authorization’ error).

Example of Application setting:

Receiving URL Too Long error?

If you are seeing HTTP 414 ‘URL Too Long’ errors and assuming that your URL really is not too long, could indicate a misconfiguration issue (the ‘URL too long’ error in this case is a bit misleading).

You may see this if you are accessing static content (such as a html page or image) AND you are using anonymous authentication AND you didn’t configure an appropriate anonymous identity (e.g. ‘Connect As’ for the application is set to Application User/Pass Thru, but anonymous authentication setting is set to IUSR).

In this case you can either set ‘Connect As’ to the specific local user (that maps to the storage account), or you need to configure anonymous authentication user identity with the local user (that maps to the storage account).

Setting up FTP application

In the same way that IIS can operate as a FTP server by referencing content on an UNC share, we can similarly point to an Azure file share.

Doing this from the Inetmgr UI is a little bit different than for configuring web applications.

First we set the physical path.

But from this UI pane there is no way to specify credentials. We need to go back through Basic Settings and then ‘Connect as’.

For FTP Site to work with Azure Files, will need to specify ‘Connect as’ credentials, even if you are running under a custom Application Pool (unlike with Web Application, where ‘pass through’ would succeed).

Considerations for ASP.NET

CAS (Code Access Security) Permissions

If you have custom .NET assemblies on the Azure File share that are used by the ASP.NET application then you will need to configure .NET security policy to trust the Azure file share.

caspol -m -ag 1 -url file://\\youraccnt123.file.core.windows.net\yourshare\* FullTrust

Impersonation/Delegation Scenarios

Azure Files does not currently support windows authentication nor ACLs on file shares. Therefore, any Asp.Net scenarios involving impersonation to access file shares will not work, as all access to the file share will be via the storage account user.

Considerations for deploying large ASP.NET applications

Azure Regions

Have your IIS VMs in the same Azure region as your Storage.

Otherwise, having your IIS VMs in one region and your Azure File share content in a different geographic region will undoubtedly increase latency and impact performance, particularly for large websites with lots of content. Of course this applies to all web content, not just ASP.NET applications.


If you have a very large ASP.NET web application in the order of thousands of aspx files then you should consider precompiling your application. If you do not precompile (or this is not an option for you) then ASP.NET will automatically compile your code the 1st time a resource is requested.

For large applications hosted on any kind of network share (including Azure file shares), this could have a potential performance impact during startup since ASP.NET will need to access the .aspx and source files in order to perform the compilation. If you have a ‘web farm’ configuration with multiple servers, then the performance impact could become more prominent as each server would have to repeat the compilation step.

Roslyn Compiler

‘Roslyn’ is a new .NET compiler platform from Microsoft that can make a difference in reducing compilation time, particularly for large complex applications. The following blog explains more about Roslyn and ASP.NET:


The above blog refers to an earlier preview version of Roslyn, which is now officially released with version 1.0.0, though much of the blog content is still relevant. Visual Studio 2015 also comes with Roslyn built in, so creating a new web application with Visual Studio 2015 will automatically leverage Roslyn.

Every application and scenario is different, so you will need to experiment to see if this approach works better for you.


Feedback on Azure Storage can be posted on https://social.msdn.microsoft.com/Forums/azure/en-US/home?forum=windowsazuredata


  • The 'add someone to IIS_IUSRS' step gets called out a lot in various contexts, but worker processes get that group injected into their token, so it's not typically needed (unless someone's toggled manualGroupMembership). Thanks for the article!

  • Do you mind sharing some performance stats comparing it to local content hosting ?
    Also ASP.NET by default monitors the content location for bin and web.config changes and I am wondering how well does that work in this scenario ?

  • Binary and web.config monitoring works the same since Azure Files is SMB 3.0 compliant. If you modify web.config on the Azure share then this will trigger AppDomain (not worker process) recycle. Regarding performance, this is a difficult one to answer as there are many variables depending on the type of content (static vs dynamic), size of files, caching settings, # of files etc. In general, you're trading off a little performance overhead for the management convenience of having all the web content in one place (Azure Files). In our own testing we've seen perf stats 10% diff and up under heavy load, though every app is likely to be different.

  • Re: IIS_USRS: If the user (that maps to the storage account user) is not a member of IIS_IUSRS and you are using ASP.NET you'll get an error during compilation such as "The current identity (MACHINE\USER) does not have write access to 'C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files'". If you have temporary files set to a different location (such as Azure File share itself) then adding the user to IIS_IUSRS may not be necessary.

  • Kindly assist me with settings for creating a new website on a server running a default website.
    I receive this server error
    The following error was encountered:
    • Unable to forward this request at this time.

    This request could not be forwarded to the origin server or to any parent caches. The most likely cause for this error is that:
    •The cache administrator does not allow this cache to make direct connections to origin servers, and
    •All configured parent caches are currently unreachable.

  • Instructions do not work if you want to use content for SSL centralized storage. IIS refuses accept username/password combination. I beleive it's bug in UI. To workaround this situation, you need to prepend username in UI with bogus domain like blablah\username, this will force UI to accept it (it will through error message but will write it to registry). Then go manually to registry (HKLM\SOFTWARE\Microsoft\IIS\CentralCertProvider) and remove domain. After that it will start working. Details are here

  • thanks

  • Its really great.....thumbs up....

  • Just wanted to say a sincere thanks for this article. It contained all the bits of info I was missing in my setup attempt!

  • Thanks a lot for the article, really helpful.

    The "Connect As" didn't work for me, always responding with 500.19 "Cannot read configuration file".

    Instead I've configured the anonymous authentication user identity with the local user, and it works like a charm.

    Thanks a lot!

  • This is a great article and was really easy to follow. I've implemented in our testing of Web Roles using a shared IIS storage and it works very well.

    I was never able to get a shared IIS config to work. I think the coupling between Web Roles and IIS is too tight to be poking at that level.

    One thing I have noticed though, is that I see a serious performance hit when using the UNC to access the files. When the files are local to the Web Role/VM the page fly. When they are retrieved via the UNC, I see a 10x increase in time it takes to process the page.

    I've tried various Instance sizes, from Small to D1_v2 to D2_v2. It doesn't seem to make a difference.

    Any thoughts?

  • Hi Eric.
    10x does sound excessive. In my own testing I saw a small perf difference once the app was warmed up, but nowhere near 10x.
    Is that delay even after the app has been warmed up (gone through the one time compilation phase) and shows up on every single web access, or it is only on the first time cold start? On the first time, for large apps there could be a hit when the app is going through compilation phase.
    I've found using Process Monitor (https://technet.microsoft.com/en-us/sysinternals/processmonitor.aspx) very useful in debugging what files are being accessed.

  • Great article. I wish I found it sooner. Like Eric , we are having performance issues of same magnitude. Compare to our Colocated VM local share, azure files is 10-12 times slower. No logic is being processed from files on azure. Just static content read and write. Any help would be appreciated. We have been trying to do this for over 5 months now.

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