RE: FTP Proxy

[Fernando Gont <fernando () gont com ar>]

At 11:19 29/01/2004 -0800, David Gillett wrote:

> > If the client is configured to do passive transfers, the
> > client will use the connection requests for both the control
> > and data connections.  That means, you won't need to allow
> > incoming connection requests to hosts inside your network.
> > I think it's the best option.
>   Which is "best" depends on whether you're looking from the
> client side or the server side, and what kind of border security
> you have at each end.
>   If you have stateful firewalls with FTP fixup, they can listen
> to the FTP control conversation and permit the requested data
> connections as needed -- and this is true regardless of which
> direction wants to open the data connection.

This requieres more processing in the firewall, though.
Because the PORT command must be "patched" in the stream, it may be the 
case that the firewall not only needs to recalculate TCP's checksum, but 
may have to "recalculate" the sequence numbers, too. (The "patched" PORT 
command might be longer or shorter than the original one).


>   If you rely on packet filters, either the client side or the
> server side has to allow arbitrary data connections to be opened.
> The only closure of this hole you can implement is that if the
> server opens the data connection ("active" mode), the source port
> number will be 20.  [In "Hacking Exposed", there's passing reference
> to doing a pen-test against a network that would permit any
> connection sourced from port 20; this is why it was configured that
> way.]

Sorry, I didn't understand that part where you said "this is why it was 
configured that way".


>   It isn't that passive mode is "better" than or "more secure" than
> (boy, have I heard that one claimed a lot of times!) active mode; it's
> that if you're not using stateful firewalls that know about FTP,
> passive mode dumps all the risk on the server instead of the clients.

It's probably more easy to configure the FTP server to use some specified 
port range (and thus allow incoming connections on only those ports) than 
configure *all* the clients that want to access your FTP site in a similar way.

BTW, the FTP server was external to his organization, so... why should 
*him* take the risk?

Best Regards,


--
Fernando Gont
e-mail: fernando () gont com ar || fgont () acm org



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