Hello,

first let me introduce our setup:

- FreeIPA 4.6.5 (I know it's a bit old already) masters CentOS 7
- FreeIPA 4.6.6 client CentOS 7
- Windows Server 2016 DCs
- Netapp Filer NFS server

There's a two-way trust between the AD and IPA domains which works nicely. User 
accounts exist in the AD domain and can be used on IPA members as well. The 
Netapp has a computer account in AD. IPA clients mount NFSv4 shares using krb5p 
encryption.

The problem:

After installing the latest Windows updates on the DCs (kb4586830) the Kerberos 
authentication to the file server started failing. We were able to identify it 
as a Kerberos problem by trying to mount without Kerberos, which worked but of 
course nothing was accessible. After trying a bunch of different things and 
reading a lot of logs, we finally uninstalled the update on the DCs and 
everything worked again. There's not a whole lot of error messages to go on 
even though log/debug levels were set to the highest. The mounting client will 
simply say "mount.nfs: access denied by server while mounting". On the DC I was 
a able to find a Failure Code 0x3C for the Kerberos ticket request. 0x3C is a 
generic error, according to 
https://docs.microsoft.com/en-us/windows/security/threat-protection/auditing/event-4769.
 None of the possible causes listed by Microsoft apply to our situation.

Since uninstalling the update on the DCs made the problem go away, I guess it's 
safe to assume that Microsoft changed something. The update notes don't really 
mention anything useful, but after some googling I found 
https://msrc.microsoft.com/update-guide/en-US/vulnerability/CVE-2020-17049 
which seems like something that could have caused this. Is there some settings 
in the IPA that could be changed to comply with the changes made by Microsoft?

Thanks!
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