What do you want your ISP to block today?

Sat Aug 30 08:28:11 UTC 2003

On zaterdag, aug 30, 2003, at 09:54 Europe/Amsterdam, Ray Wong wrote:

>> What would be great though is a system where there is an automatic
>> check to see if there is any return traffic for what a customer sends
>> out. If someone keeps sending traffic to the same destination without
>> anything coming back, 99% chance that this is a denial of service

> Eh?  Have you ever run a mailing list?

No, haven't had the pleasure.

> The majority of subscribers NEVER post.  Those who do, post prior to 
> the large quantity of traffic originates.

So? SMTP uses TCP, TCP generates incoming ACKs for outgoing data, so no 
problems there.

Christopher L. Morrow's mention of asymmetric routing for multihomed 
customers is more to the point, but if we can solve this for all those 
single homed dial, cable and ADSL end-users and not for multihomed 
networks, I'll be very happy.

>> attack. If someone sends traffic to very many destinations and in more
>> than 50 or 75 % of the cases nothing comes back or just an ICMP port
>> unreachable or TCP RST, 99% chance that this is a scan of some sort.

> Sure, and I scan my systems from outside all the time. I'm looking for
> validation that my system has NOT started listening on ports I don't
> run services on.  It's called external monitoring, and is rather useful
> in letting me get a good night's sleep.

So which do you prefer: nobody gets to scan your systems from the 
outside (including you) or everyone gets to scan your systems from the 
outside (including you).

> but I'd still need a way to verify my sites can be reached from other
> places.

They have something for that now. It's called "ping".

> If you want to know how TCP is working to a destination, you
> have to use TCP to test it.

As I mentioned above: this will not impact TCP at all because TCP 
generates return traffic. I'm sure there are one or two UDP 
applications out there that don't generate return traffic, but I don't 
know any. The only problem (except asymmetric routing when multihomed) 
would be tunnels, but you can simply enable RIP or something else on 
the tunnel to make sure it's used in both directions. Multicast doesn't 
generate return traffic so this would only apply to unicast 
destinations.

> Scans by themselves certainly aren't inherently dangerous.

It should be possible to have a host generate special "return traffic" 
that makes sure that stuff that would otherwise be blocked is allowed 
through.