Recommended Number of NTP Servers

[Richard Laager]

On 11/4/19 12:35 PM, Achim Gratz via devel wrote:
> Richard Laager via devel writes:
>> The default for maxclock is 10. This includes the "pool" entries, of
>> which the stock Debian configuration has four:
>> pool 0.debian.pool.ntp.org iburst
>> pool 1.debian.pool.ntp.org iburst
>> pool 2.debian.pool.ntp.org iburst
>> pool 3.debian.pool.ntp.org iburst
>>
>> As a result, the default is to use 6 pool servers.
> 
> That is a cargo-culted configuration from an earlier (buggy) pool
> implementation in NTP classic that didn't mobilize enough pool servers
> when only a single pool line was present.  With recent NTPd you really
> only need one pool ine
Each of these names (N.debian.pool.ntp.org) resolves to only 4 IPs.* The
four of them resolve to (mostly) non-overlapping IPs. In other words, if
I resolve only one name, I get 4 IPs, but if I resolve all four names, I
get 15 IPs. If I wait 150 seconds for the TTL to expire, I do get more
IPs. But using all four names definitely gets me IPs a lot faster, which
makes a difference for initial convergence.

* I'm ignoring IPv4 vs IPv6 here and considering only one address
family, which is relevant to IPv4-only hosts. Though apparently only one
of the four names is returning AAAA records.

-- 
Richard