FreeBSD Security Advisory – If ntpd receives a mode 7 (MODE_PRIVATE) request or error response from a source address not listed in either a ‘restrict noquery’ or a ‘restrict ignore’ section it will log the even and send a mode 7 error response. If an attacker can spoof such a packet from a source IP of an affected ntpd to the same or a different affected ntpd, the host(s) will endlessly send error responses to each other and log each event, consuming network bandwidth, CPU and possibly disk space.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/84917/FreeBSD-SA-10-02.ntpd.txt
Source: https://packetstormsecurity.com/files/84917/FreeBSD-Security-Advisory-ntpd-Denial-Of-Service.html