Pre-auth encrypted timestamp clock skew

[Greg Hudson]

On 08/23/2017 05:23 AM, Benjamin N Hall wrote:
> We recently started testing pre-authentication for certain principals, and
> I'm seeing an odd issue whereby kinit fails with "Clock skew too great
> while getting initial credentials", despite all three KDCs and the client
> systems' time being within one second of each other.
[...]
> The part that seems interesting to me is that the pre-auth encrypted
> timestamp appears to be off by way more than 5 minutes from the other
> timestamps.

The encrypted timestamp is unexpectedly for 1234 seconds in the future.
The only explanation I can think of is that you had a pre-existing
ccache with a recorded time offset of roughly 20 minutes.  For various
reasons I'm not sure that's the right explanation.  If you can
investigate with a debugger, I would be very interested in knowing the
actual cause.

The client code in 1.12 and later will use the timestamp sent by the KDC
in its PREAUTH_REQUIRED error (assuming kdc_timesync is set to true,
which it is by default), which would likely suppress this problem.