Vadim, The case for ratio-based techniques is stronger as a means for a NOC to detect a strange situation and investigate it than as a means to automatically shut down an interface. Note that, given your 'opposite direction' idea, I could shut down service on campus 'A' by [1] logging into any host on campus 'A', [2] launching an attack that might not be harmful in itself but which would trigger the auto shutdown you advocate, and then [3] sitting back and watch all of campus 'A' get shut down with the presumptive blame focused on them. It's still a denial of service attack. The problem is not with detecting the ratio imbalance, but with simple deterministic response to it. That determinism could be used by an attacker. In sum, I like the idea of detecting the problem and rapidly tracing it, but I'm skeptical about a totally automated response to it given our current low level of experience with it. -- Guy At 05:58 PM 9/17/96 -0700, you wrote:
Regis Donovan <regisdo@microsoft.com> wrote:
um... maybe i'm missing the clue here, but if the router vendors add something that shuts down an interface if the SYN/SYN-ACK/ACK ratio becomes too bad make it *easier* for me if i'm doing a denial of service attack on a host?
No, you took the "anti-SYN" shut-off in opposite direction.
ISPs could install the asymmetry shut-off (why stop at SYNs / SYN-ACK pairs?) enforcing rough balance of SYNs coming from customer and SYN-ACKs coming back to customer. If the traffic is legitimate, the balance will hold. Any attempt to flood by that customer (intentional, or unintentional, by a broken software) will cause massive disbalance.
The equivalent filter on victim's side won't see those SYNs and SYN-ACKs, simply because thet are going in opposite direction.
instead of denying service to a given host, all i have to do is drive the router into alarm mode so it shuts off the interface and then i get to deny service to an entire segment and everything downstream from that segment...
Yes, the defense may be multi-staged. I.e. if a local ISP does not enable anti-flooding defenses on its own customer links, it'll risk backbone ISP shutting its entire operation.
BTW, telcos use the statistical traffic analysis (bit-density monitors is the most trivial example) to isolate troubles for years.
--vadim