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smcauley

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The Kerberos client received a KRB_AP_ERR_MODIFIED error from the server

Hi Experts,

Periodically our PDC issues EventID 4 "The Kerberos client received a KRB_AP_ERR_MODIFIED error from the server **** The target name used was ldap/****.***.com.au/***.com.au@***.COM.AU. This indicates that the target server failed to decrypt the ticket provided by the client. This can occur when the target server principal name (SPN) is registered on an account other than the account the target service is using. Ensure that the target SPN is only registered on the account used by the server. This error can also happen if the target service account password is different than what is configured on the Kerberos Key Distribution Center for that target service. Ensure that the service on the server and the KDC are both configured to use the same password. If the server name is not fully qualified, and the target domain (***.COM.AU) is different from the client domain (***.COM.AU), check if there are identically named server accounts in these two domains, or use the fully-qualified name to identify the server."

We can then see this error replicated on the other domain controllers and more importantly to our Exchange Servers. At that point we can no longer authenitcate Outlook clients and email essentially stops working. From what I've read this is related to the machine password being updated and not replicating properly. I've done some digging as to why that would be and checked all DNS settings which seem to be fine. No issues reported with AD replication either. I think restarting the KCC service resolves the issue but we'd like to understand the root cause.
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Amit
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smcauley

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I'm not sure how this information helps me. I'm not using a specific service account for Kerberos and the target name error is ldap/ and the name of my PDC. I don't claim to know much about this but unregistering SPNs for domain controllers doesn't seem like a good idea.
Please run the setspn -Q command form that article. -Q is just a query
I have run it but I'm not sure how to interpet the results.

setspn -Q ldap/LFGAD1.lfg.com.au
Checking domain DC=lfg,DC=com,DC=au
CN=LFGAD1,OU=Domain Controllers,DC=lfg,DC=com,DC=au
        ldap/LFGAD1.lfg.com.au/AuthLite.lfg.com.au
        RPC/16f32469-101f-4415-b8d3-4b41fbdfedbf._msdcs.lfg.com.au
        HOST/LFGAD1/LFG
        HOST/LFGAD1.lfg.com.au/LFG
        GC/LFGAD1.lfg.com.au/lfg.com.au
        exchangeAB/LFGAD1.lfg.com.au
        HOST/LFGAD1.lfg.com.au/lfg.com.au
        exchangeAB/LFGAD1
        ldap/LFGAD1/LFG
        ldap/16f32469-101f-4415-b8d3-4b41fbdfedbf._msdcs.lfg.com.au
        ldap/LFGAD1.lfg.com.au/LFG
        ldap/LFGAD1
        ldap/LFGAD1.lfg.com.au
        ldap/LFGAD1.lfg.com.au/DomainDnsZones.lfg.com.au
        ldap/LFGAD1.lfg.com.au/ForestDnsZones.lfg.com.au
        ldap/LFGAD1.lfg.com.au/lfg.com.au
        DNS/LFGAD1.lfg.com.au
        WSMAN/LFGAD1
        WSMAN/LFGAD1.lfg.com.au
        E3514235-4B06-11D1-AB04-00C04FC2DCD2/16f32469-101f-4415-b8d3-4b41fbdfedbf/lfg.com.au
        Dfsr-12F9A27C-BF97-4787-9364-D31B6C55EB04/LFGAD1.lfg.com.au
        TERMSRV/LFGAD1
        TERMSRV/LFGAD1.lfg.com.au
        RestrictedKrbHost/LFGAD1
        HOST/LFGAD1
        RestrictedKrbHost/LFGAD1.lfg.com.au
        HOST/LFGAD1.lfg.com.au

Existing SPN found!
you say that "no issues with replication are reported", but what do repadmin and dcdiag say about replication health?
Hi Michael, apologies about the delay in coming back to you, I've been on leave. DCDIAG reports every test as "passed test".

repadmin /replsummary reports no fails. /showrepl shows all connections as successful.

I ended up logging a case with MS who had me temporarily configure security policy at the domain controller OU level to not accept computer password updates:-
a.  Navigate to:  Computer Configuration\Policies\Windows Settings\Security Settings\Local Policy\Security Options
b. Enable:  Domain Controller: Refuse machine account password changes

While this has stopped the issue from occuring it didn't deal with the route cause. I'm unsure whether to consider this a 'fix" or not.
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