Re: FTP - Entering passive mode

[Scott Klement <klemscot@xxxxxxxxxxxx>]

On Tue, 25 Jun 2002, Bartell, Aaron L. (TC) wrote:
>
> What does it mean when I am FTP'ing from my AS/400 to another FTP server and
> I get the following line in my FTP output?
>
> 227 Entering Passive Mode (188,25,6,33,13,111).
> So far I have only
> been able to find sites that give documentation like the following:
>
> Entering Passive Mode (h1,h2,h3,h4,p1,p2).

It means that the FTP client has told the server to enter passive mode.
What you're seeing is the server's reply stating that it is entering
passive mode, and specifying the IP address & port number that it will
use for the transfer.

>From this info, you can tell that the FTP server's IP address is
188.25.6.33  (the h1-h4 mean "host 1-4" meaning the address of the host)
and that the connection will be on port 3439. (13*256 + 111 = 3439, the
p1-p2 standard for "port 1-2")

FTP operates in two modes.   "Standard" or "Passive".  In Standard mode,
when a file transfer is requested, the client sends a PORT command to the
server to tell the server an IP address & port.  The server then connects
back to the client on that IP and port and sends the raw data from the
file through that port. When the port closes, the transfer is complete.
In "Passive" mode, the FTP client requests that the server go into passive
mode, and then the server sends an IP address & port to the client (In
that 227 message you noted) and then the client connects to the server to
transmit the file.

The only difference between passive & standard is which end opens up a
port, and which end connects to that port.  The main reason for passive is
that a lot of companies allow outgoing connections through their firewall
on any port, but restrict incoming connections to specific ports.  With
passive, all the connections are outgoing.

> Also, if someone could point out a site that contains all of the return
> codes like 227 and what they mean that would be great.
>
>
> Which doesn't tell me much.

For full, technical, documentation on all Internet standards, including
FTP, you need to read the "Request For Comments" (RFC) documents which are
defined by the Internet Engineering Task Force.  To get started, do this:

        1) Go to http://www.rfc-editor.org
        2) click "RFC Search"
        3) In the search box, type "FTP" and click the Search Button.

The results shown indicate that the standard for FTP is RFC0959 (also
referred to as STD0009).  And that it's updated by RFC2228, RFC2640, and
RFC2773.   Which means that you need to read those 4 documents to
understand the FTP standard.

I'm not certain why you need to know all of this, however, unless you're
planning to create your own FTP client or server?