Re: FTP Proxy

[pablo gietz <pablo.gietz () nuevobersa com ar>]

Fernando

The outside FPT server pertains to other company . I do not sure if I’ve 
expressed correctly my problem. I think this must be a very common 
problem “ connecting with FTP clients to the outside world through a 
firewall”.

In our case, the fact of having a departmental firewall and a proxy in 
the middle may difficult the things. So because we are newbies with 
linux, iptables, etc. we are a little lost.

Thanks aniway



Fernando Gont wrote:

> 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
>
> .

--
Pablo A. C. Gietz
Jefe de Seguridad Informática
Nuevo Banco de Entre Ríos S.A.
Te.: 0343 - 4201351
Fax: 0343 - 4201329



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