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Re: DOS?

  • From: Jack Bates
  • Date: Sat Jan 25 21:18:38 2003

From: "Iljitsch van Beijnum"

> Are you saying that I shouldn't believe Cisco's own documentation?
> Obviously, it's going to take _some_ CPU cycles, but I would expect the
> box to remain operational.
Actually, Cisco's documentation is not always accurate, and it heavily
depends on IOS version, train, feature set, and hardware.

> > One thing to keep in mind is that the S-train
> > platforms are different in handling logging than the normal trains...
> Ok, I've been working with Cisco equipment for 8 years now and I can
> configure them in my sleep, but all the version/image/train/feature set
> is still voodoo to me. Obviously, the router caches the information it
> wants to log for a while and then counts hits against the cache until it
> actually logs. This should work very well, and it does as per my tests
> on a heavily loaded 4500 router. So why would one type of IOS do this
> right and another version that isn't immediately recognizable by the
> version number as inferior do it wrong?
As stated above, it depends on the code. When logging high volume, I
recommend turning off all logging facilities except the one you plan to use.
Multiple logging facilities will create a multiple effect on the CPU for
some trains and versions. ie. logging to console and syslog and running a
term mon is a very, very bad thing under heavy logging. This also depends on
what you are logging. Narrow the scope as much as possible, ie, log only a
narrow customer selection at a time, then try the next.

> > possible and happily saturate it :( (Don't log on like a 7500 for
> > if the packet rates are over like 5kpps...)
> I think today's events show that CPU-based routers have no business
> handling anything more than 1 x 100 Mbps in and 1 x 100 Mbps out. If a
> box has 40 FE interfaces or 4 GE interfaces, at some point you'll see 4
> Gbps coming in so the box must be able to handle it to some usable
> degree.
Actually, you wouldn't expect to see 4 Gbps comming in. That would be full
saturation, which would imply serious performance degregation. Most networks
that I've dealt with stick to a 70-80% saturation rule. In addition, many of
the problems concerning this traffic weren't throughput issues. Each router
has a bandwidth limitation and a pps limitation. The worst DDOS I've had to
deal with didn't even show as a bandwidth spike on my circuits but exceeded
the pps of the router. Luckily, such attacks are easily dealt with using
access-lists as the router is optimized to block more pps than it is
designed to switch. This worm had both. The packets were small and the
bandwidth utilization was high. Blocking the packets would lower cpu
utilization to a manageable degree while the bandwidth usage on each
infected circuit was localized to that circuit. Depending on the type of
circuit depended on how well it dealt with the loading as different L2
protocols handle saturation differently. ATM is the ideal medium as the
latency remains lower than FE or GE at peak saturation. One's responsibility
is only to the edge of their controllable network, though. If you can't shut
off the ethernet port to an infected server, the customer is responsible for
that equipment. Ideally, you have one customer per each circuit that you

Jack Bates
Network Engineer
BrightNet Oklahoma

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