On 2/20/14, 3:41 PM, "Edward Roels" <edwardro...@gmail.com> 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 >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 >has? > > >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. Phil
