From: Adel Abushaev (adel@netmasterclass.net)
Date: Mon Mar 01 2004 - 23:03:44 GMT-3
Scott, et al,
In passive mode when client is ready to send a file the following occurs:
1. client sends PASV
2. server responds 227 entering passive mode, followed by IP Address and
port number on which server is listening for client
to connect
3. client sends STOR with file name
4. server confirms it by answering 150 Opening [Binary|Ascii] mode data
connection for filename
... the actual transfer goes between client and server over negotiated ports
5. client closes data stream and server acknowledges it by saying 226
Transfer complete over command port.
client will use next available port from kernel, server also does the same.
you may verify it with "/usr/sbin/tcpdump -X -v -s 1500 tcp port 21" on
unix, or with windump with the same parameters on windows platforms.
HTH,
Adel Abouchaev
CCIE# 12037, MCSE
http://www.netmasterclass.net
----- Original Message -----
From: "Scott, Tyson C" <tyson.scott@hp.com>
To: "David Hurtado" <deivi78@hotmail.com>; <nomad@gfoyle.org>;
<kwchen@netvigator.com>; <ccielab@groupstudy.com>
Sent: Monday, March 01, 2004 7:23 PM
Subject: RE: Passive FTP Examples in Pratical Studies Vol. 2.
> If it doesn't use port 20 how does it setup the FTP session?
>
>
>
> Regards,
>
>
>
> Tyson Scott
>
> Agilent Problem Management Team
>
> Managed Network Services
>
> Phone: 313-583-5812
>
> Pager: 877-997-0811
>
>
>
> -----Original Message-----
> From: David Hurtado [mailto:deivi78@hotmail.com]
> Sent: Monday, March 01, 2004 11:53 AM
> To: Scott, Tyson C; nomad@gfoyle.org; kwchen@netvigator.com;
> ccielab@groupstudy.com
> Subject: RE: Passive FTP Examples in Pratical Studies Vol. 2.
>
>
>
> Hello everybody,
>
> I don't think that the following inbound ACL would be correct to allow the
> communication between FTP passive client and server (supposing that the
server
> is outside the interface and its IP address is X.X.X.X)
>
> 1. permit tcp host x.x.x.x any ftp
>
> 2. permit tcp host x.x.x.x any gt 1023 established
>
> In FTP passive mode, the client never uses port 20 nor port 21, so the
first
> statement would be useless. Please, tell me if i'm wrong.
>
> Thanks for the help
>
>
>
>
>
>
> >From: "Scott, Tyson C"
>
> >Reply-To: "Scott, Tyson C"
>
> >To: "Jonathan Hays" , "William Chen" ,
>
> >Subject: RE: Passive FTP Examples in Pratical Studies Vol. 2.
>
> >Date: Fri, 27 Feb 2004 08:11:16 -0600
>
> >
>
> >But if this was going the opposite way it would be correct but hosts
>
> >would need to be reversed in if this was an inbound list towards the
>
> >hosts. And it should not be ip tcp for sure it should just be tcp
>
> >
>
> >1. permit tcp host x.x.x.x any ftp
>
> >2. permit tcp host x.x.x.x any gt 1023 established
>
> >
>
> >Regards,
>
> >
>
> >Tyson Scott
>
> >Agilent Problem Management Team
>
> >Managed Network Services
>
> >Phone: 313-583-5812
>
> >Pager: 877-997-0811
>
> >
>
> >
>
> >-----Original Message-----
>
> >From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
>
> >Jonathan Hays
>
> >Sent: Thursday, February 26, 2004 8:04 PM
>
> >To: 'William Chen'; ccielab@groupstudy.com
>
> >Subject: RE: Passive FTP Examples in Pratical Studies Vol. 2.
>
> >
>
> >you wrote:
>
> > >-----Original Message-----
>
> > >From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
>
> > >Behalf Of William Chen
>
> > >Sent: Thursday, February 26, 2004 7:14 PM
>
> > >To: ccielab@groupstudy.com
>
> > >Subject: Passive FTP Examples in Pratical Studies Vol. 2.
>
> > >
>
> > >
>
> > >Dear all,
>
> > >
>
> > > In all the examples of passive FTP in the book of
>
> > >Ptractical Studies Vol.
>
> > >2. It states that to match the passive FTP traffic by ACL to a
>
> > >server, the
>
> > >ACL should be:
>
> > >
>
> > > 1. permit ip tcp any host x.x.x.x ftp
>
> > > 2. permit ip tcp any host x.x.x.x gt 1023 established
>
> > >
>
> > > I wonder why established is used in the statement 2, since
>
> > >in passive
>
> > >FTP, the data channel should be initiated by the client to
>
> > >server at a port
>
> > >greater than 1023.
>
> > >
>
> > >Best Regards,
>
> > >William Chen
>
> >= = =
>
> >The book is wrong. This is clearly a typo.
>
> >
>
> >Step 2 states "FTP clients will connect to the FTP server using passive
>
> >FTP sessions" but the solution (shown on both pages 421 and 423) is for
>
> >*active* FTP:
>
> >
>
> >access-list 103 permit tcp any host 10.54.148.15 eq ftp
>
> >access-list 103 permit tcp any host 10.54.148.15 gt 1023 established
>
> >
>
> >This ACL is applied to the Priority list on Cancun, outbound toward the
>
> >FTP server. The first ACL allows initiation of the FTP Control (port 21)
>
> >TCP connection from the client to the server. In the second, erroneous
>
> >ACL the keyword "established" allows traffic that was initiated from the
>
> >server. This is incorrect since in passive FTP all TCP connections are
>
> >initiated by the client and this ACL is outbound toward the server. The
>
> >second ACL would be correct if the keyword "established" were removed so
>
> >we can allow client TCP SYN packets toward the FTP server.
>
> >
>
> >See Priscilla Oppenheimer's excellent treatment of the details of FTP at
>
> >
>
> >http://www.troubleshootingnetworks.com/ftpinfo.html
>
> >
>
> >Jonathan
>
> >
>
> >_______________________________________________________________________
>
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