Adarsh's blog : FtpWebRequest

WebException message vs. Response.StatusDescription on FtpWebRequest

adarshk — Wed, 04 May 2005 09:38:00 GMT

Recently one of the reader asked an interesting question, he was using FtpWebRequest with MakeDirectory method, if directory already exist then application received the WebException with message."The remote server returned an error: (550) File unavailable (e.g., file not found, no access).". This exception message appears to be very generic and not helping the user to figure out what causes this request to fail. The reason is exception messages are generally set by framework implementation, because they also need to be localized on different language versions.

In such case if FtpWebRequest user want to diagnose and figure out why request actually failed, he could access the actual response message from server using ((FtpWebResponse)e.Response).StatusDescription property. Your actual code would look like as below

try
{
   . . . . . . . .
   WebResponse response = request.GetResponse();
. . . . . . . . . .
}
catch(WebException e)
{
        String status = ((FtpWebResponse)e.Response).StatusDescription;
}

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Using FtpWebRequest to do FTP over SSL

adarshk — Fri, 22 Apr 2005 23:27:00 GMT

Last few weeks we were busy to get Whidbey Beta-2 bits ready for release.

If you are looking for some API where your application could talk to a FTP server, which supports SSL. FtpWebRequest under System.Net namespace is your solution. Here I will just point to SSL specific features of the class

Enabling FtpWebrequest to use Ssl is pretty simple, you just need to set EnableSsl flag before calling GetResponse() or GetRequestStream() on the FtpWebRequest object.

FtpWebRequest request = WebRequest.Create(ftp://myftpserver/dir/filename);
request.Method = WebRequestMethods.Ftp.DownloadFile;
request.EnableSsl = true; // Here you enabled request to use ssl instead of clear text
WebResponse response = request.GetResponse();

Some people asked me why FtpWebRequest support "ftps:" protocol based uri similar to "https:", the reason is there is no standard "ftps" scheme specified (yet) and ftp-over-ssl mechanism actually does not demand dedicated port for ssl, you could do it on the same server port on which you are doing regular clear text ftp. It depends on server configuration choice to force the SSL or allow both.

Once you start doing Ftp over SSL there are two important things you will need to know

Validating Server Certificate

If you were old WebRequest user, you might already know about using ServicePointManager.CertificatePolicy for https server certificate validation. In whidbey you will notice the compiler warning saying ServicePointManager.CertificatePolicy is obsolete and replaced with ServicePointManager.ServerCertificateValidationCallback which is delegate of type RemoteCertificateValidationDelegate. New delegate provide better programming model with all certificate errors reported in a single callback and you will also get instance of X509Chain object, which allow you to make decision on certificate chain.

ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(myCertificateValidation);

Actual method will look as below
  public bool myCertificateValidation(Object sender,
      X509Certificate cert,
      X509Chain chain,
      SslPolicyErrors Errors)
{ return (certificate.GetName() == "my_trusted_name"); }; //Just an example, not real world scenaio

:) Another additional advantage you can take with delegate is from anonymous method support of C# 2.0, especially if you have very simple 1-2 line certificateplicy to implement, see follwing example.

ServicePointManager.ServerCertificateValidationCallback = delegate(Object obj, X509Certificate certificate, X509Chain chain, SslPolicyErrors errors)
{ return (certificate.GetName() == "my_trusted_name"); }; //Just an example, not real world scenaio

Using Client Certificate

Using Client certificate based authentication when connecting to FTP-SSL is no different then existing HttpWebRequest. You just need to assign appropriate X509Certificate instance to the request object before making GetResponse() or GetRequestStream() call.

This posting is provided "AS IS" with no warranties, and confers no rights

Resuming broken file download with FtpWebRequest class

adarshk — Wed, 01 Dec 2004 23:04:00 GMT

This post is valid for .Netframeworks 2.0

When we are downloading a large file from ftp site and connection got broken in between, on next attempt you would be interested in downloading the rest of the file content instead of full file. FtpWebRequest class have a nice way to meet this requirement. You could use the FtpWebRequest.ContentOffset property to specify the starting position for file to download. See code example below

Following is the code for downloading the file

public static void ResumeFtpFileDownload(Uri sourceUri, string destinationFile)
{
        FileInfo file = new FileInfo(destinationFile);
        FileStream localfileStream ;
        FtpWebRequest request = WebRequest.Create(sourceUri) as FtpWebRequest;
        request.Method = WebRequestMethods.Ftp.DownloadFile;
        if (file.Exists)
        {
            request.ContentOffset = file.Length;
            localfileStream = new FileStream(destinationFile, FileMode.Append, FileAccess.Write);
        }
        else
        {
               localfileStream = new FileStream(destinationFile, FileMode.Create, FileAccess.Write);
        }
        WebResponse response = request.GetResponse();
        Stream responseStream = response.GetResponseStream();
        byte[] buffer = new byte[1024];
        int bytesRead = responseStream.Read(buffer, 0, 1024);
        while (bytesRead != 0)
        {
               localfileStream.Write(buffer, 0, bytesRead);
               bytesRead = responseStream.Read(buffer, 0, 1024);
        }
        localfileStream.Close();
        responseStream .Close();
}

This posting is provided "AS IS" with no warranties, and confers no rights

Sample code for parsing FtpwebRequest response for ListDirectoryDetails

adarshk — Thu, 16 Sep 2004 03:33:00 GMT

This posting is valid for .Net frameworks 2.0 (Currently released as Whidbey Beta1)

ResponseStream of FtpWebResponse provides the raw data bytes to the user, some of you had asked that it would be more useful to provide methods which return list of directory and files on ListDirectory request to the server. Current .Net frameworks doesn't support this, so here is some sample code I had written for parsing, in general I found it works very well against most of the server. But just to make sure it is not extensively tested, so treat only as sample, its not guaranteed to work against every server.

using System;
using System.IO;
using System.Net;
using System.Collections;
using System.Collections.Generic;
using System.Text.RegularExpressions;

   public struct FileStruct
   {
        public string Flags;
        public string Owner;
        public string Group;
        public bool IsDirectory;
        public DateTime CreateTime;
        public string Name;
    }
   public enum FileListStyle{
        UnixStyle,
        WindowsStyle,
        Unknown
    }

public class ParseListDirectory
{
       public static void Main(string[] args)
       {
              if(args.Length < 1)
              {
                  Console.WriteLine("\n Usage FTPListDirParser <uriString>");
                  return;
              }
              try
              {
                     FtpWebRequest ftpclientRequest = WebRequest.Create(args[0]) as FtpWebRequest;
                     ftpclientRequest.Method = FtpMethods.ListDirectoryDetails;
                     ftpclientRequest.Proxy = null;
                     FtpWebResponse response = ftpclientRequest.GetResponse() as FtpWebResponse;
                     StreamReader sr = new StreamReader(response.GetResponseStream(), System.Text.Encoding.ASCII);
                     string Datastring = sr.ReadToEnd();
                     response.Close();

                     FileStruct[] list = (new ParseListDirectory()).GetList(Datastring);
                     Console.WriteLine ("------------After Parsing-----------");
                     foreach(FileStruct thisstruct in list)
                     {
                            if(thisstruct.IsDirectory)
                                Console.WriteLine("<DIR> "+thisstruct.Name+","+thisstruct.Owner+","+thisstruct.Flags+","+thisstruct.CreateTime);
                            else
                                  Console.WriteLine(thisstruct.Name+","+thisstruct.Owner+","+thisstruct.Flags+","+thisstruct.CreateTime);
                     }
           }
           catch(Exception e)
           {
                  Console.WriteLine(e);
           }
    }

private FileStruct[] GetList(string datastring)
{
  List<FileStruct> myListArray = new List<FileStruct>();
  string[] dataRecords = datastring.Split('\n');
  FileListStyle _directoryListStyle = GuessFileListStyle(dataRecords);
  foreach (string s in dataRecords)
  {
   if (_directoryListStyle != FileListStyle.Unknown && s != "")
   {
    FileStruct f = new FileStruct();
    f.Name = "..";
    switch (_directoryListStyle)
    {
     case FileListStyle.UnixStyle:
      f = ParseFileStructFromUnixStyleRecord(s);
      break;
     case FileListStyle.WindowsStyle:
      f = ParseFileStructFromWindowsStyleRecord(s);
      break;
    }
    if (!(f.Name == "." || f.Name == ".."))
    {
     myListArray.Add(f);
    }
   }
  }
  return myListArray.ToArray(); ;
}

private FileStruct ParseFileStructFromWindowsStyleRecord(string Record)
{
  ///Assuming the record style as
  /// 02-03-04 07:46PM       <DIR>          Append
  FileStruct f = new FileStruct();
  string processstr = Record.Trim();
  string dateStr = processstr.Substring(0,8);
  processstr = (processstr.Substring(8, processstr.Length - 8)).Trim();
  string timeStr = processstr.Substring(0, 7);
  processstr = (processstr.Substring(7, processstr.Length - 7)).Trim();
  f.CreateTime = DateTime.Parse(dateStr + " " + timeStr);
  if (processstr.Substring(0,5) == "<DIR>")
  {
   f.IsDirectory = true;
   processstr = (processstr.Substring(5, processstr.Length - 5)).Trim();
  }
  else
  {
   string[] strs = processstr.Split(new char[] { ' ' }, true);
   processstr = strs[1].Trim();
   f.IsDirectory = false;
  }
  f.Name = processstr; //Rest is name
  return f;
}

public FileListStyle GuessFileListStyle(string[] recordList)
{
  foreach (string s in recordList)
  {
   if(s.Length > 10
    && Regex.IsMatch(s.Substring(0,10),"(-|d)(-|r)(-|w)(-|x)(-|r)(-|w)(-|x)(-|r)(-|w)(-|x)"))
   {
    return FileListStyle.UnixStyle;
   }
   else if (s.Length > 8
    && Regex.IsMatch(s.Substring(0, 8), "[0-9][0-9]-[0-9][0-9]-[0-9][0-9]"))
   {
    return FileListStyle.WindowsStyle;
   }
  }
  return FileListStyle.Unknown;
}

private FileStruct ParseFileStructFromUnixStyleRecord(string Record)
{
  ///Assuming record style as
  /// dr-xr-xr-x   1 owner    group               0 Nov 25 2002 bussys
  FileStruct f= new FileStruct();
  string processstr = Record.Trim();
  f.Flags = processstr.Substring(0,9);
  f.IsDirectory = (f.Flags[0] == 'd');
  processstr = (processstr.Substring(11)).Trim();
  _cutSubstringFromStringWithTrim(ref processstr,' ',0);   //skip one part
  f.Owner = _cutSubstringFromStringWithTrim(ref processstr,' ',0);
  f.Group = _cutSubstringFromStringWithTrim(ref processstr,' ',0);
  _cutSubstringFromStringWithTrim(ref processstr,' ',0);   //skip one part
  f.CreateTime = DateTime.Parse(_cutSubstringFromStringWithTrim(ref processstr,' ',8));
  f.Name = processstr;   //Rest of the part is name
  return f;
}

     private string _cutSubstringFromStringWithTrim(ref string s, char c, int startIndex)
     {
            int pos1 = s.IndexOf(c, startIndex);
           string retString = s.Substring(0,pos1);
           s = (s.Substring(pos1)).Trim();
           return retString;
       }
}
This posting is provided "AS IS" with no warranties, and confers no rights

Understanding the basic of FtpWebRequest programming model on .Net frameworks

adarshk — Mon, 13 Sep 2004 23:00:00 GMT

A long standing complain from user of .Net frameworks was that it doesn't have support for popular ftp protocol. This concern is going to addressed in .Net frameworks 2.0. Recently released Whidbey Beta-1 contains FtpWebRequest, FtpWebResponse classes to support FTP protocol.

WebRequest/ WebResponse Model

.Net frameworks support for FTP protocolo is implemented on top of WebRequest/WebResponse programming model, which was part of previous versions of .net frameworks. Here are some key differences, later part of the document also provide some code samples to address these differences

a) Uri Based programming model

Request/response programming model request is based on full Uri, so FtpWebRequest user would use Uri as compare to traditional filename and directory name based programming. Your target Uri could indicate directory name or filename depending on the ftp method

FTPMethod	Uri
AppendFile, DownloadFile ,DeleteFile, GetDateTimeStamp, UploadFile	Uri is file name like ftp://myftpserver/dir1/dir2/test.gif
ListDirectory, ListDirectoryDetails, RemoveDirectory, UploadFileWithUniqueName	Uri indicates directory name like ftp://myftpserver/dir1/dir2

b) All Ftp Connection Management is hidden to users and internal for users of FtpWebRequest and FtpWebResponse, each FtpWebRequest object is independent Ftp request, User should need write code for only Ftp operation he is doing, example to download file

FtpWebRequest request = WebRequest.Create( "ftp://myftpserver/dir1/dir2/test.htmf");

request.Credentials = new NetworkCredential("username","pass");

WebResponse response = request.GetResponse();

Stream dataStream = response.GetResponseStream();

// Now you can read stream containing the file data

c) Upoloading and downloading the data follows the standard Stream based pattern for .Net frameworks and allow user to use other stream based classes like StreamReader and StreamWriter on top of it as shown below

StreamReader sr = new StreamReader(responseStream, Encoding.ASCII);
string ss = sr.ReadToEnd();

d) Some advanced feature for Ftp Requests like Ftp pver SSL and Ftp requests through http proxy tunneling are very simple to program,

To go via HttpProxy you can just set Proxy property on WebRequest Object like below

request.Proxy = new WebProxy("http://myproxy"); // go through http proxy

request.EnableSSL = true; // Your web request would use SSL channel to talk to ftp server

Note on using Http Proxy on FTPWebRequest: Http proxy is only supported for limited number of ftp methods (mainly to download file only), so if you have IE settings for proxy on your machine you need to explicitly set FtpWebRequest to not use proxy like below

request.Proxy = GlobalProxySelection.GetEmptyWebProxy();

Keep looking at this weblog for more net class library 2.0 features

This posting is provided "AS IS" with no warranties, and confers no rights