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C# FTP client, based on the WinInet.dll API

, 16 Sep 2003 190K 2.3K 43
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The WinInet.dll API is able to tunnel through firewalls - this simple FTP client will support file transfer even when your firewall has only port 21open!

Introduction

When I needed to add automated file transfer to and from a remote FTP server, I found any number of excellent, clear, well structured and beautifully documented examples; both commercial and private - and all useless to me! Our Internet access is over a rather 'well throttled' firewall that seems to only have port 21 left open. The classic sockets approach to implementing an FTP client will open a 'command channel' over port 21 (default FTP port), allowing the client to sign-on etc., but as soon as any data is required to flow a second, 'data channel' must be opened (default port 20). Active or passive, I could not establish a second channel through our firewall.

I would have given up at this stage and started to look for a new employer, when it occurred to me that the Windows Explorer was perfectly able to exchange data with all and any FTP server I pointed it at... this runs over exactly the same firewall, so something strange and not just a bit magical is happening here. It turns out this bit of magic is to be found in the WinInet.dll. I am still not quite sure how the WinInet.dll works, it seems to sweet talk the firewall into opening just a small chink in its armor - but the important thing is that it works!

In order to use the FTP features of WinInet.dll, I needed to encapsulate the API in C#. This was mostly pain free, except for the marshaling of string parameters - it is documented, but you have to know what to look for. Having finished this, I figured I could offer it to the C# community; maybe I will save somebody many frustrating hours trying to bore through his corporate firewall, but also because I am quite sure I am going to get a bunch of really helpful feedback... for example, why does a successful file deletion return error code 2 = file not found? Ah, the mysteries of API programming.

I have included a very minimal console application to test the basic functionality - the default URL is ftp://ftp.microsoft.com/, users are invited to replace this with their own target.

License

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About the Author

Peter Beedell
Web Developer
Germany Germany
As a Brit who sacrificed warm beer baked beans for Bayerisch lager and Bratwurst about 12 years ago, I feel European and proud of it!

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Comments and Discussions

 
QuestionUnknown error code: 1008 Pin
dba@tom30-Aug-11 23:04
memberdba@tom30-Aug-11 23:04 
RantInternetOpen Bug Pin
jbuehler25-Jan-10 15:05
memberjbuehler25-Jan-10 15:05 
GeneralMy vote of 2 Pin
John Simmons / outlaw programmer8-Jan-09 9:26
mvpJohn Simmons / outlaw programmer8-Jan-09 9:26 
GeneralRe: My vote of 2 Pin
Elmue11-Jan-09 5:23
memberElmue11-Jan-09 5:23 
GeneralA comfortable FTP class in .NET Pin
Elmue27-Aug-08 12:22
memberElmue27-Aug-08 12:22 
GeneralRe: A comfortable FTP class in .NET Pin
John Simmons / outlaw programmer8-Jan-09 9:27
mvpJohn Simmons / outlaw programmer8-Jan-09 9:27 
GeneralRe: A comfortable FTP class in .NET Pin
Elmue11-Jan-09 5:12
memberElmue11-Jan-09 5:12 
QuestionPossibility to use QUOTE RCMD SBMJOB CMD()? Pin
paoloden6-Jan-07 0:58
memberpaoloden6-Jan-07 0:58 
Generalget every file of a directory Pin
wilco9623-Feb-06 7:22
memberwilco9623-Feb-06 7:22 
funFTPFirstFile() works quit well, but does anybody has an idea how to get every file of a directory and not only the first one?
AnswerRe: get every file of a directory Pin
antgao9-Nov-06 4:21
memberantgao9-Nov-06 4:21 
GeneralRe: get every file of a directory Pin
gan.gary15-Sep-08 23:39
membergan.gary15-Sep-08 23:39 
Generalimportant change required for using in .Net 2.0... Pin
Tim_Mackey8-Feb-06 2:55
memberTim_Mackey8-Feb-06 2:55 
GeneralRe: important change required for using in .Net 2.0... Pin
shareeef8-Dec-06 8:52
membershareeef8-Dec-06 8:52 
GeneralRe: important change required for using in .Net 2.0... Pin
Victortto16-Oct-07 13:08
memberVictortto16-Oct-07 13:08 
GeneralRe: important change required for using in .Net 2.0... Pin
Triple-S21-Oct-07 0:52
memberTriple-S21-Oct-07 0:52 
RantRe: important change required for using in .Net 2.0... Pin
HAIXJ28-May-08 1:09
memberHAIXJ28-May-08 1:09 
GeneralBug Pin
elbertlev8-Jan-06 6:52
memberelbertlev8-Jan-06 6:52 
QuestionHow to put it in passive mode Pin
adamloving28-Sep-05 7:56
sussadamloving28-Sep-05 7:56 
GeneralfunIConnect failed, error value: 3 Pin
jklpq23-Mar-05 3:14
memberjklpq23-Mar-05 3:14 
GeneralRe: funIConnect failed, error value: 3 Pin
DCChristopher29-Sep-05 5:20
memberDCChristopher29-Sep-05 5:20 
GeneralRe: funIConnect failed, error value: 3 Pin
Member 291684726-Feb-08 12:03
memberMember 291684726-Feb-08 12:03 
GeneralRe: funIConnect failed, error value: 3 Pin
jasonsmithwick12-Jan-09 8:30
memberjasonsmithwick12-Jan-09 8:30 
GeneralConnection Timeout Problem Pin
Rattan25-Nov-04 19:41
memberRattan25-Nov-04 19:41 
GeneralOverwrite locked file Pin
kholland1522-Nov-04 4:35
memberkholland1522-Nov-04 4:35 
GeneralFree, open-source pure C# FTP component Pin
Hans Andersen11-Nov-04 9:15
sussHans Andersen11-Nov-04 9:15 

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