[Freeipa-devel] NTP in FreeIPA

[Petr Spacek]

On 29.11.2016 09:11, Jan Cholasta wrote:
> On 28.11.2016 20:57, Rob Crittenden wrote:
>> David Kupka wrote:
>>> On 22/11/16 23:15, Gabe Alford wrote:
>>>> I would say that it is worth keeping in FreeIPA. I know myself and some
>>>> customers use its functionality by having the clients sync to the IPA
>>>> servers and have the servers sync to the NTP source. This way if the NTP
>>>> source ever gets disrupted for long periods of time (which has
>>>> happened in
>>>> my environment) the client time drifts with the authentication source.
>>>> This
>>>> is the way that AD often works and is configured.
>>>
>>> Hello Gabe,
>>> I agree that it's common practice to synchronize all nodes in network
>>> with single source in order to have the same time and save bandwidth.
>>> Also I understand that it's comfortable to let FreeIPA installer take
>>> care of it.
>>> But I don't think FreeIPA should do it IMO this is job for Ansible or
>>> similar tool. Also the problem is that in some situations FreeIPA
>>> installer makes it worse.
>>>
>>> Example:
>>>
>>> 1. Install FreeIPA server (ipa1.example.org)
>>> 2. Install FreeIPA client on all nodes in network
>>> 3. Install replica (ipa2.example.org) of FreeIPA server to increase
>>> redundancy
>>>
>>> Now all the clients have ipa1.example.org as the only server in
>>> /etc/ntp.conf. If the first FreeIPA server becomes unreachable all
>>> clients will be able to contact KDC on the other server thanks to DNS
>>> autodiscovery in libkrb5 but will be unable to synchronize time.
>>
>> Remember that the goal of IPA was to herd together a bunch of software
>> to make hard things easier. This included dealing with the 5-minute
>> Kerberos window so ntp was configured on the client and server (which is
>> less of any issue now).
>>
>> When making changes you have to ask yourself who are you making this
>> easier for: you or the user.
>>
>> Yes, getting NTP right is hard, but does it meet the 80/20 rule in terms
>> of success? I'd think so. I
>>
>> If someone wants to configure it using Ansible they can use the
>> --no-ntp. If they want to use different time servers they can pass in
>> --ntp-server. But by default IMHO it should do something sane to give a
>> good experience.
> 
> I think to do something sane is exactly the point of this, and the sanest
> thing we can do is to not touch NTP configuration at all:
> 
>   * if the NTP configuration obtained via DHCP works, we can't make it any
> better by touching it, only worse,
>   * if the default NTP configuration shipped with the distribution works, we
> again can't make it any better by touching it,
>   * if we are running inside container, time is synchronized by other means
> and we should not touch NTP configuration at all,
>   * if neither the default NTP configuration nor the NTP configuration
> obtained via DHCP works and we are not running inside container, we may
> attempt to fix the configuration, but it will not be permanent and will work
> only for this specific host.
> 
> I think the first 3 points cover 99% of real-life deployments, and yet we are
> optimized towards the remaining 1%, with the potential of breaking the
> configuration for the 99%. This is far from sane IMHO.

+1 for Honza's point.

Current NTP code is works only for initial setup and silently breaks
synchronization later on. Most importantly it breaks synchronization as soon
as admin removes old replicas and replaces them with new ones - there is no
mechanism to update the records in the client configuration (and SRV discovery
is not supported by clients).

I.e. when admin decommission replicas which were around at the time of client
installation, the NTP on client will silently break. This would not happen if
you did not touch it.

(This also implicitly means that IPA-configured NTP is broken on all clients
in topologies which were completely migrated from RHEL 6 to RHEL 7.)

Either DHCP or default distro config would solve the problem better.

Petr^2 Spacek


>> There don't seem to be a ton of NTP tickets and I don't recall a lot of
>> user's pressing for it to go away (the reverse, many times their
>> problems revolve around time not being synced). I wonder if a survey on
>> freeipa-users would be in order to see how hot an issue this really is.