Filter NTP traffic by packet size?

Thu Feb 20 22:12:15 UTC 2014

On Thu, Feb 20, 2014 at 1:03 PM, Jared Mauch <jared at puck.nether.net> wrote:
>
>  On Feb 20, 2014, at 3:51 PM, John Weekes <jw at nuclearfallout.net> wrote:
> > On 2/20/2014 12:41 PM, Edward Roels 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?
> >
> > If your equipment supports this, and you're seeing reflected NTP
> attacks, then it is an effective stopgap to block nearly all of the inbound
> attack traffic to affected hosts. Some still comes through from NTP servers
> running on nonstandard ports, but not much.
>

Also, don't forget to ask those sending the attack traffic to trace where
the spoofed packets ingressed their networks.

 > Standard IPv4 NTP response packets are 76 bytes (plus any link-level
> headers), based on my testing. I have been internally filtering packets of
> other sizes against attack targets for some time now with no ill-effect.
>
> You can filter packets that are 440 bytes in size and it will do a lot to
> help the problem, but make sure you conjoin these with protocol udp and
> port=123 rules to avoid collateral damage.
>

Preferably just source-port 123.

You may also want to look at filtering UDP/80 outright as well, as that is
> commonly used as an "I'm going to attack port 80" by attackers that don't
> quite understand the difference between UDP and TCP.
>

Please don't filter UDP/80.  It's used by QUIC (
http://en.wikipedia.org/wiki/QUIC).

Damian