RE: Odd Data Transfer Issue

["John Fox" <foxj@xxxxxxxxxxxxx>]

   Hi Scott

   thanks for clarifying.

   Sent from my mobile phone.

   -------- Original message --------

   From: Scott Klement

   Date:06/05/2014 5:15 PM (GMT-05:00)

   To: HTTPAPI and FTPAPI Projects

   Subject: Re: Odd Data Transfer Issue

   John,
   An ephemeral port number (in the higher range... usually MUCH higher
   than 1023) is chosen in both passive and active connections.
   The difference is who originates the connection, and who receives the
   connection.  In an active FTP session, the server makes a connection
   back to the client.  In a passive session, the client makes the
   connection to the server.
   You state that port 20 is used for active FTP.  This is true, but what
   you didn't explain is that port 20 in that case is the _originating_
   port number.  When we talk about ports, we almost never discuss
   originating ports, we always discuss the target port number...   so
   this
   might be misleading.
   In every TCP connection, there are always two ports involved, one for
   each computer involved in the conversation.  Whichever side originates
   the connection (typically the "client" side) has a port number, and the
   application that is being connected to (typicall the "server" side) has
   a port number (which is the target port)
   In the vast majority of applications, the originating port number is
   chosen from the pool of free, unused, port numbers by the operating
   system.  So the originating port is "random", it's different for each
   connection.  The target port number is usually a well-known port number
   that's used in all situations for a given application. For this reason,
   when we set up firewalls, proxies, etc, we typically talk about the
   target port.  We say that HTTP runs on 80, Telnet on 23, SMTP on 25,
   FTP
   (command channel) on 21, etc.   That's the target port.
   When you say that an FTP data connection is made on port 20, that's
   actually the originating port number, not the target.  It does not
   connect _to_ port 20.  It connects _from_ port 20.  This distinction is
   critical when setting up a firewall, because firewalls often do not
   have
   the ability to open/block the originating port, just the destination
   port.  (Though, some do -- especially the better ones -- but even in
   those cases, it's a separate field.)
   -SK
   On 6/5/2014 1:25 PM, John Fox wrote:
   > FTP communications uses 2 ports.  The command port (port 21) is used
   to
   > send the FTP commands from the client to the server.  The data port
   is
   > opened between the client and the server when there is a need to
   > transfer data between the two systems.  This is port 20 if you are
   using
   > Active connection or it could be a port number greater than port 1023
   if
   > you are using passive mode.  This web page does a descent job of
   > explaining things:
   >
   > http://www.slacksite.com/other/ftp.html
   >
   > The error message you sent seems to indicate that a connection for
   the
   > data port could not be established in order to transfer the data.
   >
   > This kind of problem typically happens if there is a firewall between
   > the 2 servers that is not properly configured, or there is a change
   to
   > the FTP server and one is expecting PASV while the other is expecting
   > Active connections.
   >
   > I recommend that you troubleshoot along these lines for a correction
   to
   > your problem.
   >
   > -----Original Message-----
   > From: ftpapi-bounces@xxxxxxxxxxxxxxxxxxxxxx
   > [mailto:ftpapi-bounces@xxxxxxxxxxxxxxxxxxxxxx] On Behalf Of Brian
   > Sent: Thursday, June 05, 2014 1:57 PM
   > To: HTTPAPI and FTPAPI Projects
   > Subject: Odd Data Transfer Issue
   >
   > Hello List,
   >
   > We've suddenly begun to experience an issue moving files between our
   > iSeries and another one.  I do not believe this is an FTPAPI issue at
   > all because we are using it to move files to and from our machine and
   > other iSeries without issue.  I am just hoping someone may have ideas
   on
   > how to locate the root cause of this or get more info from FTPAPI.
   >
   > We are using put/get/append at different times to move iSeries
   database
   > files around between our iSeries and another.  We are using binary
   mode
   > *ON and trim mode *OFF.  This had been working just fine a week ago
   and
   > suddenly this week we are getting the error message in FTPAPI of "Not
   > able to open data connection.". We are not getting error on the same
   > file each time and not every time we connect and move files around.
   >
   > I know this is pretty vague but I am hoping someone may have some
   > insight or thoughts on this.
   >
   > Thanks much in advance.
   >
   >
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