North American Network Operators Group

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DDOS attacks lately?

  • From: John O Comeau
  • Date: Sat Aug 19 20:29:40 2000

It's been a while since the big-name websites got DDOS'ed, but some large
data centers I know of still see hosts on their networks getting DDOS'ed
once or twice a day. It's not uncommon for those with only DS3 connections
to get them completely clogged for several minutes with attack traffic. If
it happens on the UUNet connection then it can get blocked quickly but
other NOCs often take a half hour or more to answer. And as discussed
earlier on this list, the major players will NOT maintain access lists on
their routers for their customers. (At least, that's what they tell their
DS3-and-smaller customers).

One thing I've recommended to my clients is to temporarily add a
(shorter prefix) route through one of their upstream providers who _will_
maintain access lists for them.

For example, is under attack. is the netblock being
advertised through teir 1 providers, and the DS3's to those major
providers is being swamped with attack traffic. Quickly modify the BGP
advertisement through ???.net to advertise as a route. Since
that is a shorter prefix, it becomes the preferred route to, and
since ???.net will maintain access lists providing, let's say, CAR for
UDP and ICMP, that's all that needs to be done to block that particular

That plan quickly falls apart if several hosts are getting attacked at
once, though if your total NORMAL inbound traffic is smaller than the
committed rate on ???.net, you can split all your netblocks into two
subnets and advertise those through ???.net for the duration of the
attacks. Or if it's an ACK attack, or something else that can't easily be
rate-limited or blocked, you're still screwed until you can reach the
provider and have the target null-routed.

I know this causes the global routing tables to increase temporarily, BUT
it keeps the datacenter's routes from flapping due to losing the BGP
sessions with the upstreams, which often happens if nothing is done about
the attack.

Another interesting point to note is that lately, most attacks have been
for the age-old purpose of taking over IRC channels by knocking out
the host on which the operator's bot is running. At least, none of my
clients have seen their websites getting attacked lately. Maybe the calm
before the storm?

Of course, there is also the option of stopping them at the source, which
is time-consuming and is usually done after it has been blocked via other
means. And for every compromised server or workstation (usually on .edu
domains) that gets fixed, two more take its place. Plus, we rarely get
helpful responses from non-US domains (primarily because we're unwilling
to make the international long-distance calls and find out we don't have
any languages in common).

It's looking like the only way to improve is to buy bigger connections;
but in light of the Yahoo attack (800Mbps was the last word I guess?) how
big is big enough? How many OC48 connections can _your_ data center

[email protected] aka John Otis Lene Comeau
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Disclaimer: Don't risk anything of value based on free advice.
"Anybody can do the difficult stuff. Call me when it's impossible."