Filter NTP traffic by packet size?

Phil Bedard bedard.phil at
Fri Feb 21 00:37:13 UTC 2014

On 2/20/14, 3:41 PM, "Edward Roels" <edwardroels at> wrote:

>Curious if anyone else thinks filtering out NTP packets above a certain
>packet size is a good or terrible idea.
>From my brief testing it seems 90 bytes for IPv4 and 110 bytes for IPv6
>typical for a client to successfully synchronize to an NTP server.
>If I query a server for it's list of peers (ntpq -np <ip>) I've seen
>packets as large as 522 bytes in a single packet in response to a 54 byte
>query.  I'll admit I'm not 100% clear of the what is happening
>protocol-wise when I perform this query.  I see there are multiple packets
>back forth between me and the server depending on the number of peers it
>Would I be breaking something important if I started to filter NTP packets
>> 200 bytes into my network?

We are filtering a range of packet sizes for UDP/123 at the edge and it
has definitely helped thwart some of the NTP attacks.  I hate to do
blanket ACLs blocking traffic but multi-Gbps of attack traffic (not
counting the reflected traffic) is hard to ignore and it's worth the risk
of blocking a minute amount of legitimate traffic.


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