router syn/syn-ack/ack alarming...

Wed Sep 18 13:35:47 UTC 1996

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 at 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
>