Filter NTP traffic by packet size?

Dobbins, Roland rdobbins at
Fri Feb 21 02:55:06 UTC 2014

On Feb 21, 2014, at 3:41 AM, Edward Roels <edwardroels at> wrote:

> From my brief testing it seems 90 bytes for IPv4 and 110 bytes for IPv6 are typical for a client to successfully synchronize to an NTP server.

Correct.  90 bytes = 76 bytes + Ethernet framing.

Filtering out packets this size from UDP/anything to UDP/123 allows time-sync requests and responses to work, but squelches both the level-6/-7 commands used to trigger amplification as well as amplified attack traffic.

Operators are using this size-based filtering to effect without breaking the world.  

Be sure to pilot this first, and understand whether packet-size classification on your hardware of choice includes framing or not.

Also, note that this filtering should be utilized to mitigate attacks, not as a permanent policy.  

Roland Dobbins <rdobbins at> // <>

	  Luck is the residue of opportunity and design.

		       -- John Milton

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