> we run our servers in ~30 datacenters around the world and we
> recently came across a strange issue in Mexico. From time to time
> (like twice a day), the {1,2,3,4}.centos.ntp.org domains returns just
> one IP instead of usual 4. Moreover, that IP does not provide NTP
> service or is completely down.
The example you cite gives an address of 132.248.30.3. This address is
actually xalli.cie.unam.mx.
My best guess - given the very limited information I have - is that you
are dealing with a provider intercepting your DNS requests and
returning forged responses, probably on the assumption that you are a
mass-market end user trying to look at Web pages. If cie.unam.mx is
related to the provider you get your Mexican connectivity from, my
estimate of the chance that this is the explanation goes up
substantially.
> The issue does not appear at any of our other DCs, so BGP and/or DNS
> GeoBalancing may be involved.
Provider interception of your DNS requests would also explain it.
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