[PATCH] samba-tool dbcheck: handle missing objectClass

[Felix Botner]

> Felix:  Any news on testing this with a genuinely corrupted network?

Unfortunately not, but I think your analysis is correct with regard to the USN 
ordering related attribute skip. We had a case in our internal testing 
environments which looked like a replication issue related to USN ordering, 
for details see https://forge.univention.org/bugzilla/show_bug.cgi?id=33904 .

> Are all the failing installations all pure Samba?

Yes.

> Does this only happen on domains with additional objectclasses, or also
> on domains with stock schema?  Is the corruption across ALL DCs, or just
> some?

This happened in a domain without schema extensions. The corruption may vary 
between DCs, but I'm not 100% sure about this. Cannot confirm this currently.

> Looking at the bug:
> https://bugzilla.samba.org/show_bug.cgi?id=10398
> and the code, I think I've found a possible cause.
> 
> get_nc_changes_build_object() has this code in it:
> 
>                 /* if the attribute has not changed, and it is not the
>                    instanceType then don't include it */
>                 if (md.ctr.ctr1.array[i].local_usn < highest_usn &&
>                     extended_op != DRSUAPI_EXOP_REPL_SECRET &&
>                     md.ctr.ctr1.array[i].attid !=
> DRSUAPI_ATTID_instanceType) continue;
> 
> The purpose of this chunk is to avoid re-sending changes that the client
> already has.  This would be particularly important for jpegPhoto, for
> example.
> 
> However, if the highest USN the client thought it had seen was actually
> higher than the one for which it had seen all objects, then a 'new'
> object could be downloaded without all it's attributes.
> 
> I think that is how this corruption happens.  While disturbing, this
> also suggests that the fix is to force re-replication, not to delete the
> object, as on at least one DC, the whole correct object exists. 

Yes, this would pretty much explain the behavior we have seen.

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Felix Botner

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