Question

Below are 2 code snippets used to check if a folder exists in a SharePoint document library.
The PROPFIND method seems to work, while the other method, using HEAD results in a 401.

Can someone please tell me why? Don’t get distracted by the credentials, I’ve set it to the same in both examples, and it works fine….

Here is the code that works:

// Create the web request object
var oReq = (HttpWebRequest)WebRequest.Create(url);

// Set the needed properties
oReq.Method = "PROPFIND";
oReq.Credentials = this.wsLists.Credentials; // Use same credentials as wsLists. 
oReq.AllowAutoRedirect = true;
oReq.UserAgent = "Microsoft-WebDAV-MiniRedir/6.1.7600";

// Enumerate through top level only, increasing the depth will find children.
oReq.Headers["Depth"] = "0";
oReq.Headers["translate"] = "f";
var oRequest = new StreamWriter(oReq.GetRequestStream());
oRequest.WriteLine();
oRequest.Close();
var oResponse = new StreamReader(oReq.GetResponse().GetResponseStream());
string sResponse = oResponse.ReadToEnd();
oResponse.Close();

and here is the offending code:

private bool MossResourceExists(string url)
{
    var request = (HttpWebRequest)WebRequest.Create(url);
    request.Method = "HEAD";

    // Create a new CredentialCache object and fill it with the network
    // credentials required to access the server.
    var myCredentialCache = new CredentialCache();
    if (!string.IsNullOrEmpty(this.Domain ))
    {
        myCredentialCache.Add(new Uri(url),
       "NTLM",
       new NetworkCredential(this.Username , this.Password , this.Domain )
       );
    }
    else
    {
        myCredentialCache.Add(new Uri(url),
       "NTLM",
       new NetworkCredential(this.Username , this.Password )
       );
    }

    request.Credentials = myCredentialCache;

    try
    {
        request.GetResponse();
        return true;
    }
    catch (WebException ex)
    {
        var errorResponse = ex.Response as HttpWebResponse;

        if (errorResponse != null)
            if (errorResponse.StatusCode == HttpStatusCode.NotFound)
                return false;
            else
                throw new Exception("Error checking if URL exists:" + url + ";Status Code:" + errorResponse.StatusCode + ";Error Message:" + ex.Message ) ;
    }
    return true;
}

Answer 1

My two cents:

I think it has to do with the way WebDAV works:

• the first request is always sent anonymous, because WebDAV is a “challenge / response” protocol, That first request without auth headers is necessary; the response from WebDAV contains a nonce to validate the next request, helping deflect against, for example, replay attacks. (from answer to this question, see links in answer for more info).

• Is the site you are trying to access in your “Local Intranet” zone in IE? If not, the following might give some more info on your issue and a possible woraround:

Understanding Why it Happens

When you use Internet Explorer to
access the WebDAV site, Internet
Explorer uses Windows HTTP Services
(WinHTTP). WinHTTP sends user
credentials only in response to
requests that occur on a local
intranet site during an authenticated
logon process. However, WinHTTP does
not check the security zone settings
in Internet Explorer to determine
whether a Web site is a local intranet
site. Instead, WinHTTP depends on the
proxy settings in Internet Explorer to
determine whether a Web site is a
local intranet site.

If the Automatically detect settings
option is not enabled, any
auto-configuration script that is
defined will not be processed. WinHTTP
will not identify the WebDAV site as a
local intranet site. Therefore,
WinHTTP will send out a request
without user credentials, and you will
be prompted to type user credentials.

So as you can see, this problem only
seem to appear on an Extranet site and
not on an Intranet site. Unfortunately
many of my customers run geophysical
solutions all over the country with no
proxy’s in between.
From here.