[ntp:questions] ntp pool servers disappear - more data

Jim Pennino jimp at gonzo.specsol.net
Fri Jun 25 23:45:09 UTC 2021


William Unruh <unruh at invalid.ca> wrote:
> On 2021-06-25, Jim Pennino <jimp at gonzo.specsol.net> wrote:A

<snip old stuff>

>> Or for $14/machine I could use a USB GPS, my machine with PPS GPS, and A
>> public server that does NOT request use of DNS which yields 3 sources of
>> time without using a pool or DNS lookups.
> 
> Not at all sure what you are suggesting. DNS is a way of translating
> names to IP addresses, which your machine MUST use to talk to a remote
> machine not on your network. The remote machine has nothing to do with
> this. Now some remote machines will as for the name associated with the
> IP address of machines sending the remote machine a query, to try to see
> if someone is spoofing the IP address, but as far as I know ntpd does
> not do that. Takes too much time and would make the time responses
> really bad. 

This is not quite correct.

If a program has an IP address, as in put the IP address in ntp.conf,
then the program already has the IP address and does NOT need to do a
DNS query ever.

Using a IP address for ntp pools is a bad idea as someone else has said.

However, there are lists of publicly available ntp servers which list
the owners preference for DNS usage. Some servers want you to use the
fully qualified domain name and some servers don't care if you use the
IP address.

>> Or for $28/machine I could use 2 USB GPS receivers and my machine with
>> PPS GPS, which also provides 3 sources of time without any network
>>e access at all.
> 
> Sure. The problem of course is that that $28 onlybuys you a pretty bad
> time source (pretty bad meaning milliseconds rather than microseconds or
> nanoseconds), which for most of man's history on this earth is
> absolutely astonishingly, and inconceivably good.

Except you are forgetting a few things:

1. I have a ntp server with a real PPS GPS attached which is good to
microseconds.
2. My actual real time requirement is in the 10s of millisecond range.
3. Any accuracy past the requirements of number 2 is purely out of
curiosity.

> Note that hanging all three off of one machine can lead to conflict
> between them as to interrupt processing, leading to degraded time
> performance. But again that is at the microsecond level, not milli or
> second level.

As each will go into a separate plug, that is HIGHLY unlikely to happen.

I never said anything about hanging three receivers on one machine,
as two receivers are more than sufficient for normal, i.e. WWIII isn't
happening, times.

> Of course if you machine is at the bottom of a mineshaft in mountains,
> gps receivers are pretty useless. Or in the basement of a highrise
> without windows. 

At one place I worked at where the computer room was in the basement and
they did care about accurate time, they bought a commercial ntp server
black box that cost several thousands of dollars and ran a cable to the
roof for the antenna.




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