Re: [chrony-users] hostnames vs. IP address in chrony.conf

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Bill

It should work the same way as every other piece of network-enabled software in the world, by implicitly picking up DNS changes the next time the domain's TTL expires, rather than doing so never.

Regards
Tom



----- Reply message -----
From: "Bill Unruh" <unruh@xxxxxxxxxxxxxx>
To: <chrony-users@xxxxxxxxxxxxxxxxxxxx>
Subject: [chrony-users] hostnames vs. IP address in chrony.conf
Date: Tue, Oct 30, 2012 20:46


On Tue, 30 Oct 2012, John.Florian@xxxxxxxx wrote:

> Bill Unruh <unruh@xxxxxxxxxxxxxx> wrote on 10/30/2012 15:45:14:
>>
>> On Tue, 30 Oct 2012, Tomalak Geret'kal wrote:
>>
>>> On 30/10/2012 19:21, Bill Unruh wrote:
>>>>  On Tue, 30 Oct 2012, Tomalak Geret'kal wrote:
>>>>
>>>>>  Could chronyd not be made to pay attention to the TTL of the IPs
> it
>>>>>  resolves?
>>>>>  That would /truly/ be "using IP to make that association".
>>>>
>>>>  What is the "TTL of the IPs it resolves"?
>>> Let me be clearer, then.
>>>
>>> Every domain name has a TTL, short for "time to live". When chrony
>> resolves a
>>> domain name to its IP, could it not be made to abide by this TTL and
>>> re-resolve the domain name when it expires? This is the way that
>> every other
>>> domain resolver in the world works, and is the fundamental
> underpinning of
>>> the Domain Name System.
>>
>> So you want to saddle chrony with a whole bunch of resolver software as
> well?
>> It is complex enough as it is ( remember that every addition to the code
>> doubles the probability of bugs.)
>> Remember KISS.
>
> I don't follow you here.  Surely there isn't a resolver of any kind in
> chronyd is there?  I would expect nothing more than a call to
> getaddrinfo() or getnameinfo() -- I know far more about DNS resolution
> than the C library -- to be present in chronyd.  All of the actual DNS
> querying, /etc/host examination, caching and preference handling should
> come automatically.  To me, that would be KISS; like let something else
> handle this.

Agreed except that the suggestion was that chrony keep track of the time to
live of the addresses ( which as far as I know is not part of the getaddrinfo
returned info) in order to tell it when it should requery the IP address for a
hostname. The problem is that the change in the IP address need not occur only
after that time expires. You can, and the suggestions was that the person did,
change the IP address at any  time. How is chrony to know that. The time to
live certainly does not give that info ( and even if it did, chrony would not
have that information).

So chrony goes out and gets the IP addreess for a hostname. It then uses that
addresses both to send out queries and to record the history of responses to
the ntp packets. That is all it has. Now, exactly what protocol should chrony
follow in order to be able to catch that the IP address of the server has
changed? time to live does not seem suitable. Does it wait until the one
packet has failed to return? (that would again seem to load things down with
dns queries on poor networks, which would seem to make things worse). Does it
scrap the history of a IP address as soon as it cannot connect? How many times
should it not connect before it scraps the history? Etc. There are a lot of
issues here.

Now I think that the latest chrony does have some heuristic for reconnection
and re querying the dns, but cannot remember it well enough right now to say
for sure, or know what it is.


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