From: Scott, Tyson C (tyson.scott@hp.com)
Date: Tue Mar 02 2004 - 03:01:38 GMT-3
David,
My bad. I was not thinking about which port I was talking about. It
would need to be port 21. That is how it establishes the tcp connection
so you would either have a reflexive access list to allow this in or use
the established as below but not with port 20. Sorry. You will need it
to establish port 21 between the two and then it, the client, negotiates
some higher port than 1023. But yes it does use port 21. How would it
ever know that it is creating an ftp session? The server would have to
think any tcp connection above 1023 is an ftp session coming in without
some basis to start from. Sorry for confusing the two ports.
-----Original Message-----
From: Adel Abushaev [mailto:adel@netmasterclass.net]
Sent: Monday, March 01, 2004 9:04 PM
To: Scott, Tyson C; David Hurtado; nomad@gfoyle.org;
kwchen@netvigator.com; ccielab@groupstudy.com
Subject: Re: Passive FTP Examples in Pratical Studies Vol. 2.
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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