Expected behaviour: Authentication is proxied to a writable DC
Problem: Even when an RODC is online, it fails to authenticate users whose passwords are denied from being replicated (e.g. Domain Admins). Samba (winbind) correctly contacts a writable samba DC, but the writable DC not only denies the single object replication request later, but also the initial authentication attempt. This affects simple binds to ldap and gssapi binds (the initial kerberos tickets can't be retrieved).
The writable DC logs:
../source4/rpc_server/drsuapi/getncchanges.c:1205: DRSUAPI_EXOP_REPL_SECRET extended op on <GUID=951c8b5a-5156-4555-89b2-9e9f67da3f5c>;CN=Administrator,OU=People,OU=Domain Users,DC=xxx,DC=xxx,DC=xxx
../source4/rpc_server/drsuapi/getncchanges.c:1358: Denied single object with secret replication for CN=Administrator,OU=People,OU=Domain Users,DC=xxx,DC=xxx,DC=xxx by RODC CN=DC03,OU=Domain Controllers,DC=xxx,DC=xxx,DC=xxx
1131f6ad-9c07-11d1-f79f-00c04fc2dcd2 refused for security token on DC=xxx,DC=xxx,DC=xxx
Using a service ticket obtained from a writable DC works.
Kerberos RODC forwarding isn't implemented (yet) and LDAP simple binds rely on having the password stored locally (I'm not sure what the case is on Windows, but I would be somewhat surprised if this worked).
There should be tests of NTLM gssapi working.
The replication failing is entirely what you would expect (and will always trigger in case you've moved the user from the denied replication list).
(In reply to Garming Sam from comment #1)
As Andrew has corrected on the list, it's only the bad password forwarding that isn't complete with Kerberos. In the good case, and with the user not being preloaded, this should work.
I don't believe there are any tests of the simple bind working in Samba.