Mailinglist Archive: opensuse-security (564 mails)

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Re: [slug] Re: ftp/firewall security
  • From: MaD dUCK <madduck@xxxxxxxxxxx>
  • Date: Thu, 4 Jan 2001 22:21:57 +0100
  • Message-id: <20010104222157.C23120@xxxxxxxxxxx>
also sprach gabriel rosenkoetter (on Thu, 04 Jan 2001 03:16:04PM -0500):
> You can't. Passive ftp and firewalls don't mix, and never have. Any
> intelligent ftp client (ncftp included, last I checked) will attempt
> to use passive, and fall back to active if it can't get through.

well, as far as i can tell the problem is beyond ftp. it's about
stateless firewalls. because if i ssh into allspice for instance, my
ssh client talks to port 22 on allspice which then spawns into a
dialogue between some port x on allspice and a port above 1024 on my
machine. so in a stateless firewall, that just has to be open. the
port 1024+ aspect i mean...

> > 99% of all ftp traffic nowadays is passive, so the data transfer
> > happens from port 21 of the server to port x {x | x >= 1024} of the
> > client.
> Um... where'd you come up with that figure?

no idea. i thought i read that somewhere...

> > more important in this situation, it is very possible that some client
> > program tries to establish a connection to a server with the backward
> > connect (server -> client) being something like x -> 5021.
> ... or 6xxx (X).

except that ports 6000/6001 could be enabled specifically if i wanted
X. the same applies to port 4000 (icq). the point is that X and icq
have specific ports but the client connections to common
spawn-services use the next available port...

> > (i DENY packets rather than to REJECT them).
> Why? Short of a signature suggesting a DoS, it's common courtesy to
> reject. Oh, wait, you lack state. Ne'mind. Get a real (stateful)
> firewall, then. ;^>

i am insulted. the reason is: there are windoze clients on the
networks that i am in and what i found is that if you REJECT, windoze
generates a whole lot more traffic than if you simply DENY and let it
poll every 15 seconds. it's weird, i know, but that's what tcpdump and
my switch LEDs indicate...

> > so while it is perfectly understandable to me how and why and what
> > ports under 1024 i have to block and open to secure the machine, the
> > ports above 1024 are a mystery. a lot of networks i have worked
> > in/with/for had firewall policies that allowed anything above 1024,
> > but as philip pointed out, this is asking for trojans.
> No, no, no. This is painfully wrong-headed. You're only asking for
> trojans if there's a way to break security on ports where you
> actually run servers. If not, then having these ports open does you
> no harm. If so, then a cracker will find a way to trojan you with
> the ports you have open anyway. (Trust me, it's not hard to write a
> daemon that behaves like an sshd except with certain input, when it
> gives you a root shell instead, and substitute that for your
> currently-running sshd.)

yes, you have a point. to be honest, until i started that discussion
on suse-security, i did not think that having ports 1024+ open would
be a problem if the rest of the system was secure...

> Every workable (note I don't say *good*) ipchains setup I've ever
> seen just allowed ports over 1024. If you want to keep rpc or X
> traffic inside, you're going to have to randomly decided to block
> a swath of those, or trust that blocking the ports the server
> daemons actually run on will be enough.

that's what i am doing now...

> Or install yourself lots of proxies (no kind of fun).


> > and do you guys know of free, open-source statefull firewalls for
> > linux?
> I'm pretty sure BSD ipf/ipnat will build on Linux (it does on
> Solaris). Not that I've tried.

does ipf do stateful in a good way? because the newer ipchains do it
to (context based), but it's still simple...


[greetings from the heart of the sun]# echo madduck@!#:1:s@\
heisenberg may have been here.

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