UDP DoS mitigation?

Rick Ernst ernst at easystreet.com
Fri Dec 12 20:47:36 UTC 2008

Replying to my own since there are currently about a dozen responses.

- Hardware/ASIC routers are a consistent response.  We are currently
  evaluating Juniper for other reasons, but I'll add DoS mitigation to
- Upstream involvement: We get transit from 701, 1239, etc.  I've had
  mixed results getting timely responses from our upstreams.  It's useful
  for long-term issues, but I need as much local and timely  control as I
  can get.
- I'm not having a problem with pipe bandwidth, but high pps.
- uRPF and RTBH helped internally, but anything passing through that
  upstream connection was impacted.
- This instance was a DoS, not DDoS.  Single source and destination, but
  the source (assuming no spoofing) was in Italy.  Turning off netflow
  seemed to help, but the attack itself stopped at about the same time.

Also, thanks for the offers of individual help in mitigation, although I'd
be concerned that "Hey, can somebody block traffic {from} or {to}?" would
be an interesting experiment in a socially-engineered DoS.

Finally, there were some suggestions "S/RTBH".  RTBH I get, but my
Google-fu is weak on S/RTBH.  Details?


On Fri, December 12, 2008 10:15, Rick Ernst wrote:
> We've had an increasing rate of DoS attacks that spew tens-of-thousands of
> small UDP packets to a destination on our network.  We are getting roughly
> 2x our entire normal pps across all providers through one interface, or
> about 4x normal through the individual interface.  The Cisco
> 7206VXR/NPE-G1 CPU melts (>95% load vs 15% average, 20% normal peak) when
> this hits.
> I'm using CEF and ip-route-cache flow on the outside interface.  Unicast
> RPF is also enabled on the interface.  Unicast RPF in conjunction with a
> BGP black-hole generator handles TCP attacks fairly well.
> Two questions:
> - Are there any knobs I should be turning in the Cisco config to help with
> mitigate this?
> - Are there any platforms that deal with high PPS/small packet more
> gracefully?
> We are looking at a network refresh and aren't locked into Cisco as a
> vendor (although our current IP network consists entirely of Cisco gear).
> Our current aggregate (all providers, in- plus out-bound) bandwidth is
> ~500Mbs, but projected growth is 1Gbs within the year.
> Thanks,
> Rick

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