On Wed, 06 Nov 2013 14:00:15 +0100
Jeroen Massar <jeroen@massar.ch> allegedly wrote:

> On 2013-11-06 13:47 , mick wrote:
> > On Wed, 06 Nov 2013 14:00:09 +0200
> > Lars Noodén <lars.nooden@gmail.com> allegedly wrote:
> >
> >> On 11/06/2013 01:26 PM, mick wrote:
> >>> I disagree. Dropping all traffic other than that which is
> >>> explicitly required is IMHO a better practice. (And how do you
> >>> know in advance which ports get attacked?)
> >>
> >> Using reject instead of drop simplifies troubleshooting.
> >>
> >> http://www.chiark.greenend.org.uk/~peterb/network/drop-vs-reject
> >>
> >> Drop tends to get in the way.
> >
> > Again, I disagree. But I recognise that this can be a religious
> > decision. My default policy is to drop rather than reject. I know
> > that strict adherence to standards implies we should “REJECT” with a
> > helpful ICMP error message.
>
> Configure your host with DROP, do an nmap, then configure it with
> REJECT thus for Linux:
>
> IPv4: -j REJECT --reject-with icmp-port-unreachable"
> IPv6: -j REJECT --reject-with icmp6-port-unreachable"
>
> Now repeat that nmap; indeed, for the DROP it is shown that these
> ports are filtered, for REJECT the ports are just 'closed'.
>
> Hence, the adversary did not learn anything in the REJECT case
> (services apparently are not there), but in the DROP case they
> learned that you have a firewall configured and that those services
> are likely there...

Not true. Since my default is to drop for ALL ports not expicitly open
and receiving traffic, the adversary has learned nothing about what
other services may or may not be there.

I have no need to say politely to anyone connecting to any random port
on my server, "Sorry, nothing here, you can close your connection". The
only legitimate connections inbound to my server are those for which I
advertise a service.

>
> As you say it is one of those 'religious' decisions, but in this, the
> facts show what should be preferred for multiple reasons ;)

I also prefer vi to emacs :-)

> > But, doing that can mean that
> > incoming packets with a spoofed source address can get replies sent
> > back to that (innocent) source address. DDOS bots exploit this
> > behaviour.
>
> As there is no amplification (only a portion of the incoming packet is
> included) this is not used; there are much better sources of attack.
>

I agree. DNS amplification is much more dangerous and useful to an
adversary. But that does not mean that no adversary will attempt to
use ICMP replies in an attack.

Mick
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 Mick Morgan
 gpg fingerprint: FC23 3338 F664 5E66 876B  72C0 0A1F E60B 5BAD D312
 http://baldric.net

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