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Re: Ideas on solving the file transfer problem

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On Fri, 20 Jan 2012 03:44:33 +0000, Rui Maciel <rui.maciel@gmail.com>
wrote:

>Robert Wessel wrote:
>
>> On Thu, 19 Jan 2012 21:22:13 +0000, Rui Maciel <rui.maciel@gmail.com>
>>
>>>Domestic users tend to control their own firewalls.  Would they be
>>>affected by this?
>> 
>> 
>> Well, sometimes they do.  But doesn't making such a change move this
>> out of the range of easy solution for the vast majority of users?
>> 
>> But FTP ports are commonly blocked by (residential) ISPs, along with
>> SMTP and a few others.  You can often ask to get them unblocked.
>
>This isn't a problem.  Port 21 may be the default port for FTP, but FTP 
>servers do provide a way to configure.  Regarding the client, it's possible 
>to provide URLs to a FTP resources that specify a non-default port.
>
>
>>>DNS wouldn't be a problem, as domain names aren't exactly necessary.  NAT
>>>wouldn't be a bigger problem than it already is.
>> 
>>
>> Given the two channel nature of FTP, NAT is a bigger problem than
>> usual.  Most firewalls that support FTP have specific code to deal
>> with the data connection.  PASV mode helps, but isn't usually the
>> default, and is often disabled on FTP servers.
>
>Firewalls can be configured by the end-user to open the necessary ports.  As 
>a last resort, it is also possible to use ports which are already opened but 
>attributed to other services. 


As I said... *sometimes* end users can configure their firewalls.  And
sometimes they can hack around them by using other ports.

But as regards FTP, there are NAT/firewall issues in both active and
PASV modes.  The command channel needs to be dealt with by the
NAT/firewall on the server end, which at least means setting up a
forwarding, and possibly a translation rule.  But at least that can be
static.

In "normal" active mode FTP, the client end will have to accept an
incoming connection on dynamically selected port.  This requires a
dynamic change to the firewall/NAT rules, which most firewalls
accomplish by actively monitoring the command channel looking for the
PORT command from the client to the server, and then temporarily
opening the appropriate incoming hole.  The server end just needs
outgoing access on port 20.  Unfortunately, on the client side, many
basic NAT/firewall devices do not handled the reverse data channel
well.

FTP PASV mode tries to fix that by making the client open the data
channel to the server, instead of the other way around.  This requires
little from the NAT/firewall at the client other than letting outbound
connections through.  Unfortunately this moves the problem to the FTP
server's firewall/NAT, which now has to understand when to open a
random incoming port to the FTP server.  Basically the same problem as
at the client side.

Now that required configuration is usually OK for "real" FTP servers,
since it's not end-users setting them up, and needing to get things
like static IP addresses (or dynamic DNS) going is usually not a
particular difficulty for the people who *are* setting them up.

So while FTP is OK for the client, the server requires a fair bit of
pain that most end users are ill-equipped to shoulder.

And honestly, why go through all that work dealing with the second
data channel - do the file transfer via HTTP.  And while that still
has NAT/firewall issues at the server end, it has none of FTPs
additional issues.

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Thread

Ideas on solving the file transfer problem Rui Maciel <rui.maciel@gmail.com> - 2012-01-15 17:17 +0000
  Re: Ideas on solving the file transfer problem "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2012-01-15 18:35 +0100
    Re: Ideas on solving the file transfer problem Rui Maciel <rui.maciel@gmail.com> - 2012-01-15 18:07 +0000
      Re: Ideas on solving the file transfer problem "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2012-01-15 20:19 +0100
        Re: Ideas on solving the file transfer problem Rui Maciel <rui.maciel@gmail.com> - 2012-01-15 19:42 +0000
  Re: Ideas on solving the file transfer problem Willem <willem@toad.stack.nl> - 2012-01-15 19:15 +0000
    Re: Ideas on solving the file transfer problem Rui Maciel <rui.maciel@gmail.com> - 2012-01-15 20:10 +0000
  Re: Ideas on solving the file transfer problem Leif Roar Moldskred <leifm@dimnakorr.com> - 2012-01-18 09:16 -0600
    Re: Ideas on solving the file transfer problem Rui Maciel <rui.maciel@gmail.com> - 2012-01-19 16:26 +0000
      Re: Ideas on solving the file transfer problem Robert Wessel <robertwessel2@yahoo.com> - 2012-01-19 14:59 -0600
        Re: Ideas on solving the file transfer problem Rui Maciel <rui.maciel@gmail.com> - 2012-01-19 21:22 +0000
          Re: Ideas on solving the file transfer problem Robert Wessel <robertwessel2@yahoo.com> - 2012-01-19 15:54 -0600
            Re: Ideas on solving the file transfer problem root <NoEMail@home.org> - 2012-01-19 22:20 +0000
              Re: Ideas on solving the file transfer problem Robert Wessel <robertwessel2@yahoo.com> - 2012-01-19 17:43 -0600
                Re: Ideas on solving the file transfer problem Rui Maciel <rui.maciel@gmail.com> - 2012-01-20 04:22 +0000
                Re: Ideas on solving the file transfer problem Robert Wessel <robertwessel2@yahoo.com> - 2012-01-19 23:43 -0600
                Re: Ideas on solving the file transfer problem Rui Maciel <rui.maciel@gmail.com> - 2012-01-20 12:59 +0000
                Re: Ideas on solving the file transfer problem Geoff <geoff@invalid.invalid> - 2012-01-20 02:41 -0800
                Re: Ideas on solving the file transfer problem Rui Maciel <rui.maciel@gmail.com> - 2012-01-20 12:45 +0000
                Re: Ideas on solving the file transfer problem Geoff <geoff@invalid.invalid> - 2012-01-20 15:41 -0800
                Re: Ideas on solving the file transfer problem root <NoEMail@home.org> - 2012-01-21 00:18 +0000
                Re: Ideas on solving the file transfer problem Geoff <geoff@invalid.invalid> - 2012-01-20 16:41 -0800
                Re: Ideas on solving the file transfer problem Robert Wessel <robertwessel2@yahoo.com> - 2012-01-20 18:44 -0600
                Re: Ideas on solving the file transfer problem Geoff <geoff@invalid.invalid> - 2012-01-20 15:43 -0800
                Re: Ideas on solving the file transfer problem Robert Wessel <robertwessel2@yahoo.com> - 2012-01-20 18:34 -0600
                Re: Ideas on solving the file transfer problem "Chris Uppal" <chris.uppal@metagnostic.REMOVE-THIS.org> - 2012-01-21 10:21 +0000
              Re: Ideas on solving the file transfer problem Rui Maciel <rui.maciel@gmail.com> - 2012-01-20 03:59 +0000
            Re: Ideas on solving the file transfer problem Rui Maciel <rui.maciel@gmail.com> - 2012-01-20 03:44 +0000
              Re: Ideas on solving the file transfer problem Robert Wessel <robertwessel2@yahoo.com> - 2012-01-19 23:41 -0600
    Re: Ideas on solving the file transfer problem Ian Collins <ian-news@hotmail.com> - 2012-01-20 14:07 +1300
      Re: Ideas on solving the file transfer problem blp@cs.stanford.edu (Ben Pfaff) - 2012-01-19 18:20 -0800
      Re: Ideas on solving the file transfer problem Leif Roar Moldskred <leifm@dimnakorr.com> - 2012-01-19 20:33 -0600
        Re: Ideas on solving the file transfer problem Rui Maciel <rui.maciel@gmail.com> - 2012-01-20 04:08 +0000

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