On Sun, Oct 24, 2010 at 8:34 AM, Brandon Kim <brandon....@brandontek.com> wrote: > > Hey guys: > > I wanted to open up this question regarding NTP server. I recalled someone > had created a posting of this quite awhile back. > >From a service provider/ISP standpoint, does anyone think that having a > >local NTP server is really necessary? > > I've asked some of my fellow engineers at work and many of them gives me the > same response, "Can't we just use free ones out on the internet?"
Depends on how much you trust other people. NTP can potentially be used as a DoS vector by your upstream clocks, if you're not running your own. I've seen 50,000 servers panic in the blink of an eye when the NTP source issued a leap second, and the kernel wasn't patched to handle it properly; and that's a forward leap second. Nobody's tested reverse leap seconds yet; who knows what would happen to your hosts if your upstream NTP servers decided to issue a reverse leap second towards you? Granted, if you choose enough diverse upstream clocks, that becomes more difficult for someone to exploit; but it's not impossible, and you can't count on keeping your upstream clock sources secret, given the bidirectional communication that can take place between NTP servers. *shrug* It's cheap enough to run your own clock sources, once you're above a certain size, and it's one less potential attack vector from the outside; why wouldn't you want to secure your edge against it? Matt
