On Sat, 4 Feb 2017 10:09:11 +0000, David Woolley <david@ex.djwhome.demon.invalid> wrote:
>On 03/02/17 23:10, Robert Scott wrote: >> But with other servers, the first >> response came back good, the second response came back LI=3, the third >> and fourth responses never came back at all. (recvfrom() has to be >> aborted). >> >> Is this possibly a defense mechanism against a DOS attack? > > >See <https://www.eecis.udel.edu/~mills/ntp/html/rate.html>, in >particular the section on Kiss-of-Death. Your code should be >recognizing these and raising an alarm. My code does parse for the Kiss-of-Death codes. And when it gets it, it will not query that host again. But my code checks for LI=3 first, and if it gets that, it does not check further because my response to LI=3 is the same as my response to KOD. I do not query that host again. The only reason I was querying those hosts again is because of the bug in my code that prevented my looping through all four hosts. So I thought I was accessing a different host when in fact I was accessing the same host four times. Since I fixed that bug, I don't think I will be getting LI=3 very often. But if I do, I will treat it the same as the KOD. By the way, the code I am writing is not part of a NTP algorithm to adjust a system clock for time. It is for a one-time frequency calibration of an oscillator. I take a time snapshot at the beginning and at the end of an approximately six hour period during which I am counting cycles from the oscillator in question. I hope to achieve a frequency accuracy of 5 PPM. Once that measurement is made, I store it for subsequent use in my app. Unless the hardware changes, there is no need to do the calibration again. -Robert Scott Hopkins, MN _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
