Geeky Bob

A few years ago I wrote the following blog, wherein I described how the WebDAV functionality in IIS 6.0 worked with files that are Compound Document format:

IIS 6.0 WebDAV and Compound Document Format Files

As I explained in that blog post, WebDAV needs somewhere to store "properties" for files that are uploaded to the server, and WebDAV uses the compound document format to accomplish this according to the following implementation logic:

• If the file is already in the compound document file format, IIS simply adds the WebDAV properties to the existing file. This data will not be used by the application that created the file - it will only be used by WebDAV. However, the file size will increase because WebDAV properties are added to the compound document.
• For other files, WebDAV stores a compound document in an NTFS alternate data stream that is attached to the file. You will never see this additional data from any directory listing, and the file size doesn't change because it's in an alternate data stream.

I recently had a customer contact me in order to ask if there was a way to disable this functionality since he didn't want his files modified in order to store the WebDAV properties. Unfortunately there is no built-in option for IIS that will disable this functionality, but there are a few workarounds.

Workaround #1 - Change the File Type

First and foremost - you can change your file type to something other than the compound document format. For example, if you are uploading files that were created in Microsoft Office, if you can upload your files in the newer Office Open XML formats, then you will not run into this problem. By way of explanation, older Microsoft Office files are in compound document format, whereas files that are that are created with Microsoft Office 2010 and later are in a zipped, XML-based file format. These files will have extensions like *.DOCX for Microsoft Word documents, *.XLSX for Microsoft Excel spreadsheets, and *.PPTX for Microsoft PowerPoint presentations.

Workaround #2 - Wrap Compound Document Files in a Separate File Type

If you are using a file that must be in compound document format, like a setup package in Microsoft Installer (*.MSI) format, you can upload the file in a *.ZIP file, or you can wrap the setup package inside a self-extracting executable by using a technology like Microsoft's IExpress Wizard (which ships as a built-in utility with most versions of Windows).

Workaround #3 - Block WebDAV Properties

If you absolutely cannot change your document from compound document format, I have a completely unsupported workaround that I can suggest. Since the problem arises when properties are added to a file, you can find a way to intercept the WebDAV commands that try to set properties. The actual HTTP verb that is used is PROPPATCH, so if you can find a way to keep this command from being used, then you can prevent files from being modified. Unfortunately you cannot simply suppress PROPPATCH commands by using a security tool like Microsoft's UrlScan to block the command, because this will cause many WebDAV clients to fail.

Instead, what I did as a workaround was to write an example ISAPI filter for IIS 6.0 that intercepts incoming PROPPATCH commands and always sends a successful (e.g. "200 OK") response to the WebDAV client, but in reality the filter does nothing with the properties and ends the request processing. This tricks a WebDAV client into thinking that it succeeded, and it prevents your files in compound document format from being modified. However, this also means that no WebDAV properties will ever be stored with your files; but if that's acceptable to you, (and it usually should be), then you can use this workaround.

With that in mind, here's the C++ code for my example ISAPI filter, and please remember that this is a completely unsupported workaround that is intended for use only when you cannot repackage your files to use something other than the compound document format.

#define _WIN32_WINNT 0x0400

#include <windows.h>
#include <httpfilt.h>

#define STRSAFE_LIB
#include <strsafe.h>

#define BUFFER_SIZE 2048

const char xmlpart1[] = "<?xml version=\"1.0\"?>"
  "<a:multistatus xmlns:a=\"DAV:\">"
  "<a:response>"
  "<a:href>";

const char xmlpart2[] = "</a:href>"
  "<a:propstat>"
  "<a:status>HTTP/1.1 200 OK</a:status>"
  "</a:propstat>"
  "</a:response>"
  "</a:multistatus>";

BOOL WINAPI GetFilterVersion(PHTTP_FILTER_VERSION pVer)
{
  HRESULT hr = S_OK;
  // Set the filter's version.
  pVer->dwFilterVersion = HTTP_FILTER_REVISION;
  // Set the filter's description.
  hr = StringCchCopyEx(
    pVer->lpszFilterDesc,256,"PROPPATCH",
    NULL,NULL,STRSAFE_IGNORE_NULLS);
  if (FAILED(hr)) return FALSE;
  // Set the filter's flags.
  pVer->dwFlags = SF_NOTIFY_ORDER_HIGH | SF_NOTIFY_PREPROC_HEADERS;
  return TRUE;
}

DWORD WINAPI HttpFilterProc(
  PHTTP_FILTER_CONTEXT pfc,
  DWORD NotificationType,
  LPVOID pvNotification )
{
  // Verify the correct notification.
  if ( NotificationType == SF_NOTIFY_PREPROC_HEADERS)
  {
    PHTTP_FILTER_PREPROC_HEADERS pHeaders;
    HRESULT hr = S_OK;
  
    bool fSecure = false;

    char szServerName[BUFFER_SIZE] = "";
    char szSecure[2] = "";
    char szResponseXML[BUFFER_SIZE] = "";
    char szResponseURL[BUFFER_SIZE] = "";
    char szRequestURL[BUFFER_SIZE] = "";
    char szMethod[BUFFER_SIZE] = "";

    DWORD dwBuffSize = 0;

    pHeaders = (PHTTP_FILTER_PREPROC_HEADERS) pvNotification;

    // Get the method of the request
    dwBuffSize = BUFFER_SIZE-1;
    // Exit with an error status if a failure occured.
    if (!pfc->GetServerVariable(
      pfc, "HTTP_METHOD", szMethod, &dwBuffSize))
      return SF_STATUS_REQ_ERROR;

    if (strcmp(szMethod, "PROPPATCH") == 0)
    {
      // Send the HTTP status to the client.
      if (!pfc->ServerSupportFunction(
        pfc, SF_REQ_SEND_RESPONSE_HEADER,"207 Multi-Status", 0, 0))
        return SF_STATUS_REQ_ERROR;

      // Get the URL of the request.
      dwBuffSize = BUFFER_SIZE-1;
      if (!pfc->GetServerVariable(
        pfc, "URL", szRequestURL, &dwBuffSize))
        return SF_STATUS_REQ_ERROR;
        
      // Determine if request was sent over secure port.
      dwBuffSize = 2;
      if (!pfc->GetServerVariable(
        pfc, "SERVER_PORT_SECURE", szSecure, &dwBuffSize))
        return SF_STATUS_REQ_ERROR;
      fSecure = (szSecure[0] == '1');
        
      // Get the server name.
      dwBuffSize = BUFFER_SIZE-1;
      if (!pfc->GetServerVariable(
        pfc, "SERVER_NAME", szServerName, &dwBuffSize))
        return SF_STATUS_REQ_ERROR;
        
      // Set the response URL.
      hr = StringCchPrintf(
        szResponseURL,BUFFER_SIZE-1, "http%s://%s/%s",
        (fSecure ? "s" : ""), szServerName, &szRequestURL[1]);
      // Exit with an error status if a failure occurs.
      if (FAILED(hr)) return SF_STATUS_REQ_ERROR;

      // Set the response body.
      hr = StringCchPrintf(
        szResponseXML,BUFFER_SIZE-1, "%s%s%s",
        xmlpart1, szResponseURL, xmlpart2);
      // Exit with an error status if a failure occurs.
      if (FAILED(hr)) return SF_STATUS_REQ_ERROR;

      // Write the response body to the client.
      dwBuffSize = strlen(szResponseXML);
      if (!pfc->WriteClient(
        pfc, szResponseXML, &dwBuffSize, 0))
        return SF_STATUS_REQ_ERROR;

      // Flag the request as completed.
      return SF_STATUS_REQ_FINISHED;
    }
  }
    
  return SF_STATUS_REQ_NEXT_NOTIFICATION;
}

I hope this helps. ;-]

Note: This blog was originally posted at http://blogs.msdn.com/robert_mcmurray/

We recently ran into a situation where a customer thought that they were seeing file corruption when they were transferring files from a Windows 7 client to their IIS 6.0 server using WebDAV. More specifically, the file sizes were increasing for several specific file types, and for obvious reasons the checksums for these files would not match for verification. Needless to say this situation caused a great deal of alarm on the WebDAV team when we heard about it - file corruption issues are simply unacceptable.

To alleviate any fears, I should tell you right up front that no corruption was actually taking place, and the increase in file size was easily explained once we discovered what was really going on. All of that being said, I thought that a detailed explanation of the scenario would make a great blog entry in case anyone else runs into the situation.

The Customer Scenario

First of all, the customer was copying installation files using a batch file over WebDAV; more specifically the batch file was copying a collection of MSI and MST files. After the batch file copied the files to the destination server it would call the command-line comp utility to compare the files. Each MSI and MST file that was copied would increase by a small number of bytes so the comparison would fail. The customer computed checksums for the files to troubleshoot the issue and found that the checksums for the files on the source and destination did not match. Armed with this knowledge the customer contacted Microsoft for support, and eventually I got involved and I explained what the situation was.

The Architecture Explanation

Windows has a type of file format called a Compound Document, and many Windows applications make use of this file format. For example, several Microsoft Office file formats prior to Office 2007 used a compound document format to store information.

A compound document file is somewhat analogous to a file-based database, or in some situations like a mini file system that is hosted inside another file system. In the case of an MST or MSI file these are both true: MST and MSI files store information in various database-style tables with rows and columns, and they also store files for installation.

With that in mind, here's a behind-the-scenes view of WebDAV in IIS 6.0:

The WebDAV protocol extension allows you to store information in "properties", and copying files over the WebDAV redirector stores several properties about a file when it sends the file to the server. If you were to examine a protocol trace for the WebDAV traffic between a Windows 7 client and an IIS server, you will see the PUT command for the document followed by several PROPPATCH commands for the properties.

IIS needs a way to store the properties for a file in a way where they will remain associated with the file in question, so the big question is - where do you store properties?

In IIS 7 we have a simple property provider that stores the properties in a file named "properties.dav," but for IIS 5.0 and IIS 6.0 WebDAV code we chose to write the properties in the compound document file format because there are lots of APIs for doing so. Here's the way that it works in IIS 5 and IIS 6.0:

• If the file is already in the compound document file format, IIS simply adds the WebDAV properties to the existing file. This data will not be used by the application that created the file - it will only be used by WebDAV. This is exactly what the customer was seeing - the file size was increasing because the WebDAV properties were being added to the compound document.
• For other files, WebDAV stores a compound document in an NTFS alternate data stream that is attached to the file. You will never see this additional data from any directory listing, and the file size doesn't change because it's in an alternate data stream.

So believe it or not, no harm is done by modifying a compound document file to store the WebDAV properties. Each application that wants to pull information from a compound document file simply asks for the data that it wants, so adding additional data to a compound document file in this scenario was essentially expected behavior. I know that this may seem counter-intuitive, but it's actually by design. ;-]

The Resolution

Once I was able to explain what was actually taking place, the customer was able to verify that their MST and MSI files still worked exactly as expected. Once again, no harm was done by adding the WebDAV properties to the compound document files.

You needn't take my word for this, you can easily verify this yourself. Here's a simple test: Word 2003 documents (*.DOC not *.DOCX) are in the compound document file format. So if you were to create a Word 2003 document and then copy that document to an IIS 6.0 server over WebDAV, you'll notice that the file size increases by several bytes. That being said, if you open the document in Word, you will see no corruption - the file contains the same data that you had originally entered.

I hope this helps. ;-]

Note: I originally wrote the following script for a friend, but as every good programmer often does, I kept the script around because I realized that it could come in handy. I've found myself using the script quite often with several of the servers that I manage, so I thought that I'd share it here.

When managing a large web server with dozens of web sites, it's hard to keep track of all the host headers that you have configured in your settings. With that in mind, I wrote the following script that lists the host headers that are assigned on an IIS web server. To use the example script, copy the script into notepad or some other text editor, save it to your server as "HostHeaders.vbs", and then double-click the script to run it. The script will create a text file named "HostHeaders.txt" that contains all the host headers listed by site for your server.

Option Explicit
On Error Resume Next

Dim objBaseNode, objChildNode
Dim objBindings, intBindings
Dim objFSO, objFile, strOutput

' get a base object
Set objBaseNode = GetObject("IIS://LOCALHOST/W3SVC")
Set objFSO = WScript.CreateObject("Scripting.FileSystemObject")
Set objFile = objFSO.CreateTextFile("HostHeaders.txt")

' check if we have an error ...
If (Err.Number <> 0) Then

    ' ... and output the error.
    strOutput = "Error " & Hex(Err.Number) & "("
    strOutput = strOutput & Err.Description & ") occurred."

' ... otherwise, continue processing.
Else

    ' loop through the child nodes
    For Each objChildNode In objBaseNode

        ' is this node for a web site?
        If objChildNode.class = "IIsWebServer" Then

            ' get the name of the node
            strOutput = strOutput & "LM/W3SVC/" & _
                objChildNode.Name

            ' get the server comment
            strOutput = strOutput & " (" & _
                objChildNode.ServerComment & ")" & vbCrLf
            ' get the bindings
            objBindings = objChildNode.ServerBindings
            ' loop through the bindings
            For intBindings = 0 To UBound(objBindings)
                strOutput = strOutput & vbTab & _
                    Chr(34) & objBindings(intBindings) & _
                    Chr(34) & vbCrLf
            Next
        End If
    ' try not to be a CPU hog
    Wscript.Sleep 10
    Next
End If
objFile.Write strOutput
objFile.Close
Set objBaseNode = Nothing
Set objFSO = Nothing

If you feel adventurous, you could easily modify the script to return the text in a tab-separated or comma-separated format.

Enjoy!