Reading remote file using ASP.NET impersonation

Question

I want to read a pdf stored on a remote server. I have been provided with a username/password which has read access rights.

I am using ASP.NET impersonation given in this url https://support.microsoft.com/kb/306158

I am logging everything to a logFile just to help with debugging.

 StreamWriter sw = new StreamWriter(Server.MapPath("~/log/logFile.txt"), true);
 sw.WriteLine("Just before Impersonation");

 if(impersonateValidUser("username", "domain", "password"))
 {
    try
     {
          byte[] bytes = File.ReadAllBytes(documentName);
          sw.WriteLine("Bytes read!!");
          undoImpersonation();
     }
    catch(Exception ex)
    {
       sw.WriteLine(ex.Message + "\n" + ex.StackTrace);
       return;
 }
 else
 {
    sw.WriteLine("Impersonation Failed");
            return;
 }

In my log file, I just see the "Just before Impersonation". Neither of the messages of the try nor catch block is written to the log file. Surprisingly, I do not see the impersonation failed message.

Just wondering if anyone has previous experience with this kind of behavior? Is there any extra requirement to access file on a remote machine? I know that the remote machine does have advapi32.dll and kernel32.dll

Answer 1

We had problems using the MSDN sample as well and if I recall correctly, it had to do with handles getting closed prematurely.

We ended up rewriting it in the following way, which has worked very well for us:

    private void DoLogin()
    {
        var token = LogonAsUser(userName, domain, password);
        if (!IntPtr.Equals(token, IntPtr.Zero))
        {
            WindowsImpersonationContext impersonatedUser = null;
            try
            {
                var newIdentity = new WindowsIdentity(token);

                impersonatedUser = newIdentity.Impersonate();

                // Do impersonated work here
            }
            finally
            {
                if (impersonatedUser != null)
                {
                    impersonatedUser.Undo();
                }
                LogonAsUserEnd(token);
            }
        }
    }

    private IntPtr LogonAsUser(String userName, String domain, String password)
    {
        IntPtr token = IntPtr.Zero;

        if (LogonUserA(userName, domain, password, LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT, ref token) != 0)
        {
            return token;
        }
        else
        {
            return IntPtr.Zero;
        }
    }

    private void LogonAsUserEnd(IntPtr token) {
        if (!IntPtr.Equals(token, IntPtr.Zero))
        {
            CloseHandle(token);
        }

    }

One other side note: we defined LogonUserA as returning a bool, not an int, which could also be part of the issue you are encountering.