Filter NTP traffic by packet size?

Edward Roels edwardroels at
Thu Feb 20 20:41:27 UTC 2014

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 are
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?

More information about the NANOG mailing list