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Re: New Internet-draft on DDOS defense...
----- Original Message ----- From: Vipul Shah <firstname.lastname@example.org> > > I'd like to bring your attention to a recent Internet-draft. The URL is: > > http://www.ietf.org/internet-drafts/draft-vshah-ddos-smurf-00.txt > > This draft proposes a specific (simple) change to RFC1122 which would > help reduce the use of Smurf amplification in DDOS attacks. This is > augments ingress filtering; it is designed specifically for the case > where the attacker (source) is using broadcast on the local LAN as > part of a DDOS attack. This is a case where ingress filtering does > not help. The proposal suggests that hosts not respond to ICMP Echo broadcasts if the source address is not within the same subnet as the workstation. The rational is that even with "no ip directed-broadcast" (or it's equivalent on non-Cisco routers), smurf attacks can still be launched by a local machine on the local subnet (provided that there are no filters in place to prevent forged source-addresses from that subnet). Such an attack would only be useful where the aggregate bandwidth to the Internet from the subnet of the compromised host is signifigantly larger than the aggregate bandwidth to the Internet from the compromised host itself. In the traditional case of a simple shared media ethernet, this is obviously not the case -- rather than launching a "local smurf" attack to generate 10Mbps worth of flooding, the attacker could simply have the local machine generate 10 Mbps worth of flooding. In modern networks, a switch of some sort is likely to be involved, so there is some potential for amplification. However, given that the individual devices in such an environment are likely to be attached with a minimum of 100 Mbps Ethernet (and, assuming they are running a reasonable IP stack, they should then be able to generate a minimum of, say, 80Mbps of flooding), the cases where a "local smurf" would be beneficial to an attacker are limited to sites with switched ethernet and OC-3 or better connectivity. Experience has shown that such sites are not generally problematic smurf amplifiers. (OC-3 is actually just a low end number. Given that any such site is likely to have a lot of other traffic competing for bandwidth, I think 25% is a good high end number for maximum amplication effect you'd get. You'll need OC-12 or higher for any serious level of amplification.) I think the exposure that you seek to eliminate here is not an exposure that is large enough to justify changing the behavior of host IP stacks. -- Brett