Martin wrote in to point to VU #568372. It contains a description of a vulnerability (CVE-2009-3563) in the ntpd.org reference implementation of ntpd, which will sound very familiar for any dog owner seeing his pet chase it's own tail. Basically all that's needed is a single spoofed packet to set of ntp daemons to start endlessly sending messages to themselves or to each-other.

Filtering in the short term is a possible workaround, but upgrading your ntp software to at least version 4.2.4p8 is a far better long term strategy.

Note that this software is often embedded in various devices and operating systems, so upgrading it might take a bit of effort in tracking it all down.

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Swa Frantzen -- Section 66