From: lingwei cao (caolw@xxxxxxxxxxxx)
Date: Thu May 10 2001 - 05:25:51 GMT-3
In FTP passive Mode
1.FTP client opens command channel to ftp server and requests "passive"
mode:
Server (21) <-----(PASV)----Client(port x,x>1023)
2.FTP Server allocates port for the data channel and transmits the port
number z(random,>1023) to
use for data tranmission.
Server(21)--------(z,random port number)--->client(port x)
3.FTP client open the data channel on the specified port
Server(port z)<--------(Data channel)----Client(port y,y is other
random port,>1023)
best regards
----- Original Message -----
From: "Zeng Puyang" <zbridge98@yahoo.com>
To: "ccielab" <ccielab@groupstudy.com>; "lingwei cao" <caolw@fosco.com.cn>
Sent: Thursday, May 10, 2001 4:14 PM
Subject: Re: Ftp port question
> Thank you very much. It's much helpful. But I still don't understand the
following sentance:
>
> > Passive FTP Operation
> > 7 The client receives this information and opens a data channel to the
> > server assigned port.
> > The server receives the data and sends an "OK" (ack).
> After get reply of PASV, client will open a tcp connection from
> client tcp 20 ----------> server tcp x (x>1023)
> or client tcp y (y>1023, choice by client) ---> server tcp x
>
> Thanks
>
> Zeng
>
> ----- Original Message -----
> From: "lingwei cao" <caolw@fosco.com.cn>
> To: "Zeng Puyang" <zbridge98@yahoo.com>; <ccielab@groupstudy.com>
> Sent: Thursday, May 10, 2001 3:07 PM
> Subject: Re: Ftp port question
>
>
> > Active FTP Operation
> > The active mode of operation is less secure than the passive mode. This
mode
> > of operation complicates the construction of firewalls, because the
firewall
> > must anticipate the connection from the FTP server back to the client
> > program. The steps of this mode of operation are discussed below and are
> > shown in Figure 1.
> > 7 The client opens a control channel (port 21) to the server and tells
the
> > server the port number to respond on. This port number is a randomly
> > determined port greater than 1023.
> > 7 The server receives this information and sends the client an
> > acknowledgement "OK" (ack). The client and server exchange commands on
this
> > control connection.
> > 7 When the user requests a directory listing or initiates the sending or
> > receiving of a file, the client software sends a "PORT" command that
> > includes a port number > 1023 that the client wishes the server to use
for
> > the data connection.
> > 7 The server then opens a data connection from port 20 to the client's
port
> > number, as provided to it in the "PORT" command.
> >
> > Passive FTP Operation
> > This mode of operation is assumed to be more secure because all the
> > connections are being initiated from the client, so there is less chance
> > that the connection will be compromised. The reason it is called passive
is
> > that the server performs a "passive open." The steps of this mode of
> > operation are discussed below and are shown in Figure 2.
> > 7 In passive FTP, the client opens a control connection on port 21 to
the
> > server, and then requests passive mode through the use of the "PASV"
> > command.
> > 7 The server agrees to this mode, and then selects a random port number
> > (>1023). It supplies this port number to the client for data transfer.
> > 7 The client receives this information and opens a data channel to the
> > server assigned port.
> > The server receives the data and sends an "OK" (ack).
> >
> > Is this helpful?
> > ----- Original Message -----
> > From: "Zeng Puyang" <zbridge98@yahoo.com>
> > To: "ccielab" <ccielab@groupstudy.com>
> > Sent: Wednesday, May 02, 2001 11:53 AM
> > Subject: Ftp port question
> >
> >
> > > hi, group:
> > >
> > > I am a little confused by the ftp ports, and hope you can help me to
make
> > this clear:
> > >
> > > A: ftp client B: ftp server
> > >
> > > Standard mode:
> > > A tcp >1023 -----> B tcp 21
> > > A tcp 20 <----- B tcp X (X>1023,choiced by B)
> > >
> > > Passive mode:
> > > A tcp >1023 -----> B tcp 21
> > > PASV ----->
> > > <----- portY (Y>1023)
> > > A tcp 20 -----> B tcp Y
> > >
> > > Is this correct?
> > >
> > > Thanks for your help
> > >
> > > Zeng
> > > **Please read:http://www.groupstudy.com/list/posting.html
> ?(9?!"z7*.-z^{&!"?(T']-k,jXm6YT(%I(h!J&
**Please read:http://www.groupstudy.com/list/posting.html
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