[SSSD-users] Re: does sssd support using a Microsoft read-only Domain Controller (RODC)?

[Sumit Bose]

On Wed, Apr 18, 2018 at 01:51:38PM -0400, James Ralston wrote:
> On Tue, Apr 17, 2018 at 2:27 AM, Sumit Bose <sb...@redhat.com> wrote:
> 
> > On Mon, Apr 16, 2018 at 04:28:59PM -0400, James Ralston wrote:
> >
> > > Has anyone figured out how to make sssd utilize a Microsoft
> > > read-only Domain Controller (RODC)?
> > >
> > > But no matter how we create the computer account object, or how we
> > > export the Kerberos keytab, sssd cannot use the resulting keytab
> > > file to authenticate to the RODC: when sssd sends the AS-REQ, the
> > > RODC always replies with KRB5KDC_ERR_PREAUTH_FAILED.
> >
> > If 'kinit -k' works, SSSD should work as well.
> 
> Thanks; that's good to know.
> 
> > Can you send the SSSD logs with debug_level=9, most important would
> > be the domain log and the ldap_child.log files.
> >
> > For comparison it would be good to see the output of
> >
> >     KRB5_TRACE=/dev/stdout kinit -k ....
> >
> > as well.
> 
> OK, I will look into that.  (KRB5_TRACE especially seems like it will
> be useful; I wasn't aware of that.)
> 
> I think part of why we're struggling here is because the behavior of
> sssd doesn't seem to match its documentation.
> 
> Specifically…
> 
> For RWDCs, when I use Samba "net ads join" to create the computer
> account in AD and write the /etc/krb5.keytab file, it always creates
> multiple entries:
> 
> Keytab name: FILE:/etc/krb5.keytab
> KVNO Principal
> ---- 
> --------------------------------------------------------------------------
>    2 host/myhost.example....@example.org (des-cbc-crc)
>    2 host/myhost.example....@example.org (des-cbc-md5)
>    2 host/myhost.example....@example.org (aes128-cts-hmac-sha1-96)
>    2 host/myhost.example....@example.org (aes256-cts-hmac-sha1-96)
>    2 host/myhost.example....@example.org (arcfour-hmac)
>    2 host/myh...@example.org (des-cbc-crc)
>    2 host/myh...@example.org (des-cbc-md5)
>    2 host/myh...@example.org (aes128-cts-hmac-sha1-96)
>    2 host/myh...@example.org (aes256-cts-hmac-sha1-96)
>    2 host/myh...@example.org (arcfour-hmac)
>    2 MYHOST$@EXAMPLE.ORG (des-cbc-crc)
>    2 MYHOST$@EXAMPLE.ORG (des-cbc-md5)
>    2 MYHOST$@EXAMPLE.ORG (aes128-cts-hmac-sha1-96)
>    2 MYHOST$@EXAMPLE.ORG (aes256-cts-hmac-sha1-96)
>    2 MYHOST$@EXAMPLE.ORG (arcfour-hmac)
> 
> Using "adcli" does much the same thing, but additionally creates
> RestrictedKrbHost SPNs:
> 
> Keytab name: FILE:/etc/krb5.keytab
> KVNO Principal
> ---- 
> --------------------------------------------------------------------------
>    2 MYCLIENT$@EXAMPLE.ORG (aes256-cts-hmac-sha1-96)
>    2 MYCLIENT$@EXAMPLE.ORG (aes128-cts-hmac-sha1-96)
>    2 MYCLIENT$@EXAMPLE.ORG (des3-cbc-sha1)
>    2 MYCLIENT$@EXAMPLE.ORG (arcfour-hmac)
>    2 MYCLIENT$@EXAMPLE.ORG (des-cbc-md5)
>    2 MYCLIENT$@EXAMPLE.ORG (des-cbc-crc)
>    2 host/myclient.example....@example.org (aes256-cts-hmac-sha1-96)
>    2 host/myclient.example....@example.org (aes128-cts-hmac-sha1-96)
>    2 host/myclient.example....@example.org (des3-cbc-sha1)
>    2 host/myclient.example....@example.org (arcfour-hmac)
>    2 host/myclient.example....@example.org (des-cbc-md5)
>    2 host/myclient.example....@example.org (des-cbc-crc)
>    2 host/mycli...@example.org (aes256-cts-hmac-sha1-96)
>    2 host/mycli...@example.org (aes128-cts-hmac-sha1-96)
>    2 host/mycli...@example.org (des3-cbc-sha1)
>    2 host/mycli...@example.org (arcfour-hmac)
>    2 host/mycli...@example.org (des-cbc-md5)
>    2 host/mycli...@example.org (des-cbc-crc)
>    2 RestrictedKrbHost/mycli...@example.org (aes256-cts-hmac-sha1-96)
>    2 RestrictedKrbHost/mycli...@example.org (aes128-cts-hmac-sha1-96)
>    2 RestrictedKrbHost/mycli...@example.org (des3-cbc-sha1)
>    2 RestrictedKrbHost/mycli...@example.org (arcfour-hmac)
>    2 RestrictedKrbHost/mycli...@example.org (des-cbc-md5)
>    2 RestrictedKrbHost/mycli...@example.org (des-cbc-crc)
>    2 RestrictedKrbHost/myclient.example....@example.org
> (aes256-cts-hmac-sha1-96)
>    2 RestrictedKrbHost/myclient.example....@example.org
> (aes128-cts-hmac-sha1-96)
>    2 RestrictedKrbHost/myclient.example....@example.org (des3-cbc-sha1)
>    2 RestrictedKrbHost/myclient.example....@example.org (arcfour-hmac)
>    2 RestrictedKrbHost/myclient.example....@example.org (des-cbc-md5)
>    2 RestrictedKrbHost/myclient.example....@example.org (des-cbc-crc)
> 
> For adcli, this is the corresponding computer object in AD:
> 
>     sAMAccountName: MYCLIENT$
>     sAMAccountType: 805306369
>     dNSHostName: myclient.example.org
>     userPrincipalName: host/myclient.example....@example.org

Did you call adcli with the --user-principal option?

I would expect that without this option the userPrincipalName attribute
will not be set and 'MYCLIENT$@EXAMPLE.ORG' would be the default user
principal.

>     servicePrincipalName: RestrictedKrbHost/myclient.example.org
>     servicePrincipalName: RestrictedKrbHost/MYCLIENT
>     servicePrincipalName: host/myclient.example.org
>     servicePrincipalName: host/MYCLIENT
> 
> If use Samba to join the host to AD, the account is the same, minus
> the RestrictedKrbHost SPNs.
> 
> According to the sssd-ldap(5) man page, ldap_sasl_authid defaults to:
> 
>     host/hostname@REALM

Unfortunately there is a special behavior of the AD provider which is
not documented in the man page which would use MYCLIENT$@EXAMPLE.ORG as
default, see below ...

> 
> However, this is ambiguous.  Does this mean:
> 
>     host/shorthostname@REALM
> 
> …or does it mean:
> 
>     host/fqdn@REALM

Whatever you have written in userPrincipalName. With AD you can only get
a TGT for the principal in userPrincipalName or if this is not set for
sAMAccountName@AD.REALM (MYCLIENT$@EXAMPLE.ORG in the example above).
And this is what is needed to authenticate against Active Directory.

For the principals listed in servicePrincipalName you can get a service
ticket but not a TGT.

> 
> I'm not sure.
> 
> But at least for AD, this doesn't seem to be what sssd is doing.  We
> don't set ldap_sasl_authid, but for hosts joined to a RWDC with a
> keytab like the examples above, we can see in packet captures that
> sssd defaults to using SHORTHOSTNAME$:

As said above this is expected for the SDDD AD provider because by
default the user principal used by AD for hosts is
SHORTHOSTNAME$@AD.REALM.

> 
>     Kerberos AS-REQ
>         Pvno: 5
>         MSG Type: AS-REQ (10)
>         padata: PA-ENC-TIMESTAMP Unknown:149
>             Type: PA-ENC-TIMESTAMP (2)
>                 …
>             Type: Unknown (149)
>                 Value: <MISSING>
>         KDC_REQ_BODY
>             Padding: 0
>             KDCOptions: 00000010 (Renewable OK)
>             Client Name (Principal): MYCLIENT$
>                 Name-type: Principal (1)
>                 Name: MYCLIENT$
>             Realm: EXAMPLE.ORG
>             Server Name (Service and Instance): krbtgt/EXAMPLE.ORG
>                 Name-type: Service and Instance (2)
>                 Name: krbtgt
>                 Name: EXAMPLE.ORG
>             till: 2018-04-13 20:11:22 (UTC)
>             Nonce: 462670550
>             Encryption Types: …
>                 Encryption type: aes256-cts-hmac-sha1-96 (18)
>                 Encryption type: aes128-cts-hmac-sha1-96 (17)
>                 Encryption type: rc4-hmac (23)
>                 Encryption type: des-cbc-crc (1)
>                 Encryption type: des-cbc-md5 (3)
>                 Encryption type: des-cbc-md5-nt (20)
>                 Encryption type: Unknown (19)
>                 Encryption type: des3-cbc-sha1 (16)
>                 Encryption type: Unknown (25)
>                 Encryption type: Unknown (26)
>                 Encryption type: des-cbc-md4 (2)
> 
> This behavior is why we've been trying to export a Kerberos keytab
> file from AD that looks *exactly* like one that samba/adcli would
> create.  As we've discovered, that's not trivial.
> 
> But if in fact we don't need those additional entries—that just the
> host/myclient.example....@example.org entry in the keytab file is
> sufficient for sssd—then that would help us immensely.
> 
> So: if the only thing in the /etc/krb5.keytab file is this:
> 
> Keytab name: FILE:/etc/krb5.keytab
> KVNO Principal
> ---- 
> --------------------------------------------------------------------------
>    2 host/myclient.example....@example.org (aes256-cts-hmac-sha1-96)
>    2 host/myclient.example....@example.org (aes128-cts-hmac-sha1-96)
>    2 host/myclient.example....@example.org (des3-cbc-sha1)
>    2 host/myclient.example....@example.org (arcfour-hmac)
>    2 host/myclient.example....@example.org (des-cbc-md5)
>    2 host/myclient.example....@example.org (des-cbc-crc)
> 
> …and the corresponding machine account in AD is:
> 
>     sAMAccountName: MYCLIENT$
>     sAMAccountType: 805306369
>     dNSHostName: myclient.example.org
>     userPrincipalName: host/myclient.example....@example.org
>     servicePrincipalName: host/myclient.example.org
> 
> Should that be sufficient for sssd to work correctly?

Yes, but let's circle back to the beginning, joining the AD domain.

Was my assumption correct that you used the --user-principal of adcli?
If yes, is there a reason or were you just trying different options to
make SSSD work?

If you want to use --user-principal you have to set ldap_sasl_authid
explicitly in sssd.conf to the same value because the AD provider will
use a different default.

If you use adcli without additional options the userPrincipalName
attribute should not be created in the host object and the defaults of
the AD provider should work.

If you want to get rid of the RestrictedKrbHost entries in the keytab
using '--service-name=host' with adcli should work. The documentation
here is a bit unclear, it will not add more entries but override the
default 'host' and 'RestrictedKrbHost'.

As an alternative, after joining without additional options you can use
ktutil to remove unwanted entries from a keytab. Nevertheless the
RestrictedKrbHost should not do any harm.

HTH

bye,
Sumit

> 
> Thanks,
> James
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