[Freeipa-devel] [RFC] Migrating existing environments to Trust

Sumit Bose sbose at redhat.com
Thu Apr 17 09:15:39 UTC 2014

On Wed, Apr 16, 2014 at 09:02:00PM -0400, Dmitri Pal wrote:
> On 04/15/2014 05:13 AM, Sumit Bose wrote:
> >Hi,
> >
> >I have started to write a design page for 'Migrating existing
> >environments to Trust'
> >http://www.freeipa.org/page/V3/Migrating_existing_environments_to_Trust
> >It shall cover https://fedorahosted.org/freeipa/ticket/3318 and
> >https://fedorahosted.org/freeipa/ticket/3979 .
> >
> >I came across several questions which need answers so that more details
> >can be added to the design. Besides that comments and suggestions are
> >welcome as well.
> >
> >For your convenience I added the test below as well.
> >
> >bye,
> >Sumit
> >
> >= Overview =
> >
> >This page illustrates how existing solutions which make AD users
> >available to Linux hosts can be migrated to FreeIPA with Trusts. This
> >includes migrations from the FreeIPA WinSync feature or environments
> >where the AD users where correlated to NIS users.
> >
> >The common problem here is that some if not all attributes needed by a
> >POSIX user or group must be overwritten or supplied by the IPA server.
> >The link to the related AD object is preferably the SID but if it is not
> >available on both sides the name of the object must be used. AD will
> >keep the responsibility for authentication and provide the AD
> >group-memberships of the object.
> >
> >= Use Cases =
> >* Migration from the FreeIPA Sync solution to the FreeIPA Trust solution
> >** [https://fedorahosted.org/freeipa/ticket/3318 https://fedorahosted.org/freeipa/ticket/3318]
> >* Migration/Consolidation of domains currently managed by other solutions, e.g. NIS
> >** [https://fedorahosted.org/freeipa/ticket/3979 https://fedorahosted.org/freeipa/ticket/3979] (contains some ideas about a solution)
> As I mentioned in the ticket not only that. Based on conversations
> with different potential consumers of the trust functionality the
> ability to use existing POSIX attributes and manage them in IPA
> while user accounts come from AD is a crucial next step.

Thank you for your feedback, it was very helpful but I'm afraid it might
also caused some new questions.

> >
> >= Design =
> >In Ticket [https://fedorahosted.org/freeipa/ticket/3979 https://fedorahosted.org/freeipa/ticket/3979] two aspects of a design are already explained.
> ># Instead of just offering a single override option the introduction of
> >   views are suggested. A suitable client can ask for a specific view
> >   with override options different from the default override
> >   view.
> Yes.
> >Questions:
> >#* Will the default view always be applied? I think it makes sense to
> >    always apply it to get a consistent default behavior. If there is a
> >    reason for a client to get the unmodified data a special view called
> >    e.g. NO_OVERRIDE can be used. This is e.g. needed for the extdom plugin
> >    to be able to send the raw data to the IPA client so that SSSD can use
> >    different views on the client which might be needed for docker/container
> >    use cases.
> Sounds reasonable to have the default view and apply it always. If
> the view does not contain posix attributes for the specific user we
> should use dynamic mapping based on SIDs.

Quite some time ago we have decided to not mix algorithmic mapping and
manually managed POSIX IDs. E.g. think about the case where a view with
POSIX ID was added for an existing user which already has an algorithmic
ID assigned on some clients.

I think the admin has to decide what he wants. Below you mentioned a
migration use case where old users should keep their IDs but new users
will get algorithmically generated ones. I think this is bad practice
and technically is it next to impossible without additional admin
effort to decide if a given AD user is an old or a new user. The admin
either has to add special flags/attributes to the old AD user objects or
we have to keep an immutable list of old users in IPA. Please note that
this has to be done for groups as well. Imo it would be
easier and safer for the admin to either do a full migration to
algorithmically mapped IDs or manage all POSIX IDs manually on the IPA

Additionally I think that in your use case there might be even the need
to manually manage POSIX IDs even for new user. E.g. in the case where a
larger amount of new users is added to AD which where managed by a
completely independent system before.

> >#* Will views only be applied to users from other domains or to IPA
> >    users as well?
> Design goal will be to allow them to be applied to all users.

Why, what is the use case to override attribute of IPA users which
cannot be solved by adding other attributes with the needed values to
the IPA user object directly?

> Implementation goal will be to apply them to external users first.
> Or I should say we need to figure out the procedure:
> 1) Pre migration: AD with no POSIX, LDAP/NIS with POSIX for some/same users.
> 2) Past migration: AD with no POSIX, no duplicate accounts in IdM,
> POSIX attributes for AD users are migrated into a view.
> How we do it is the question but we need to have a path. May be it
> would require some kind of tool:
> ipa create-view --check
> This will go through all users in IPA and check them against
> available trusted domains and would report which users will be
> removed becuase they are found in a specific domain.

I think it should only report conflicts. The admin should decide how to
solve the conflict.

> ipa create-view --create --except=<group list>
> This will actually do the work but if there are special users that
> need to be processed in some special way admin might temporarily put
> them into a specific group and then exclude this group or groups
> from cleanup.
> Just a thought...
> >#* Do we want stackable views?
> We want client oriented views. I.e. views should be attached to host groups.
> For the legacy clients there should be a way to expose a view as a
> separate base=DN so that one set of legacy clients can be pointed to
> one DN and another to another.

I'm not sure if this will stretch the compat tree too much, I let
Alexander and Nalin decided.

> If the host based views and DN based views do not have data for a
> specific user the data should be fetched from a global default view.
> I think we can use CoS for that in conjunction with the compat
> plugin but I would leave this to experts to decide.
> Bottom line: one global fallback view and then specific views for
> host groups and for legacy clients.
> I know I might be asking a lot ;-)

How shall this 'global default view' work? E.g. if there is an AD user
without a POSIX ID in AD and algorithmic mapping is not enabled for this
user. If there is no override object for this user in the current view
or if the override object does not contains a POSIX ID attribute where
should the ID come from?

Please note that it is a valid use-case that a user does not has a POSIX
ID e.g. web applications. I think it is not necessary to forcefully try
to assign some POSIX ID to a user. If the admin decides that the user
does not need one, why shall we try to create one?

> >#* Do we want to override everything or shall there be immutable
> >    attributes like e.g. the SID or the user name?
> I do not know why would overwrite the SID but I think that
> overwriting the user name would give us an interesting aliasing
> capability as a feature as a side effect.

But is there a use case for such a feature which is not solved better by
creating a new user with the given name? Please note the side effects,
e.g. with respect to the home-directory path. Typically the user name is
part of the of path. What if the original object has a home-directory
path shall we override it implicitly as well or keep it and who shall
decided? Or what about Kerberos principals, who shall user 'abc' know
that his name was overridden and his Kerberos principal is
'xyz at AD.REALM'?

> >#* Shall we allow different UIDs/GIDs in different views?
> Yes.

I hope the admin knows what he does in this case. I think it's similar
like with the user name, is there really a user-case for this with
cannot be solved better by creating a new user with the given UID? Think
about what happens if a host is moved to a new host group e.g. to change
the HBAC rules but by chance has now a different view with different

>From the OS point of view a user with a different UID is a different
user, from the peoples point of view a user with a different name is a
different user. I think what we achieve with allowing different values
for user names and/or UIDs is to allow different users share the same
password, Kerberos credentials etc and I'm not sure we really want this.

> >#* I think overriding UIDs/GIDs should only be allowed for
> >    ipa-ad-trust-posix idranges, mixing override with algorithmic mapping
> >    does not make sense imo.
> I think it does at least for the migration time. But it might not be
> achievable.
> The idea is that you really should be required to manage UID/GID for
> the users manually via views if it is an old user.
> If it is a new user that never was on the Linux side before the
> algorithmic mapping might be preferred.

see above

> But I also think that this should be controlled by a policy and
> admin would have to decide whether the IPA should generate UIDs or
> he would prefer to set the manually explicitly.

yes, this can already be controlled by the idrange type. But you have to
choose either algorithmic or manual mapping you cannot have both in a
given domain. What you can do is to create a domain in the AD forest for
the old users and one for the new users. Now you can use manual mapping
for the old-users-domain and algorithmic mapping for the

> ># The views will be stored in containers below
> >   cn=views,cn=accounts,$SUFFIX with containers for users and groups. The
> >   objectclasses will look similar to posixAccount and posixGroup
> >   objectclasses but with only optional (MAY) attributes. Questions:
> >#* Do we want to consider to allow to add arbitrary attributes to a view
> >    to cover requests like "We have this beautiful application which can get
> >    all user data via the SSSD D-BUS responder but our AD admin will not
> >    extend the AD LDAP schema to add the required attributes. Can IPA add
> >    them for users from trusted domains?"
> Yes but probably not as phase 1. So it is a separate enhancement.
> >#* It was suggested to use a UUID to reference the original objects. For
> >    AD users and groups the SID would be a good choice because it is unique
> >    and already contains a reference to the original domain. I wonder if we
> >    should add  a type prefix like 'SID:' to be able to add other types like
> >    'IPAUUID:domref:ef2b7400-a3c4-11e3-82e7-525400de2951' where domref will
> >    be a reference to the other IPA domain. On the other hand a type can be
> >    added later and if the type is missing a SID is assumed.
> >
> >On the SSSD side the views can be stored below the corresponding user or
> >group object in the cache and the SYSDB API has to be enhanced to return
> >merged results. The merging only happens in the responders (NSS, D-BUS)
> >before sending data to the clients.
> Why SSSD should know about different views?
> It should know raw and not raw but it always sees one merged view
> from the server.
> What am I missing?

You miss the docker/container use case where a central SSSD instance is
used by instances with different host names which might belong to
different host groups and hence require different views. So SSSD does
not only need to know all views but the host groups (we already have
them for HBAC) and their corresponding views as well.

Although this use case is work-in-progress I think we should include it
from the start because the extra work to read all views quite small and
might be a larger effort when added later.


> >
> >To manage the views a new set CLI tools/Web UI pages should be added.
> >Depending if we would like to allow to override IPA users as well or
> >only users from trusted domains the tools might get their own name
> >space, ipa override-*, or can be added below the trust name space, ipa
> >trust-override-*.  It has to be noted that changes of a view will only
> >be visible on the client after the related cached object is expired.
> OK.
> >
> >= Implementation =
> See comments above. I hope they give enough hints for
> implementation. At least for the first pass.
> >=Feature Management =
> >== UI ==
> >== CLI ==
> >
> >= Major configuration options and enablement =
> >Any configuration options? Any commands to enable/disable the feature or
> >turn on/off its parts?
> >
> >= Replication =
> >Any impact on replication?
> >
> >= Updates and Upgrades =
> >Any impact on updates and upgrades?
> >
> >= Dependencies =
> >Any new package and library dependencies.
> >
> >= External Impact =
> >Impact on other development teams and components
> >
> >= Backup and Restore =
> >Any files or configuration that needs to be taken care of in backup or
> >restore procedure.
> >
> >= Test Plan =
> >Test scenarios that will be transformed to test cases for FreeIPA
> >Continuous Integration during implementation or review phase.
> >
> >= RFE Author =
> >[[User:Sbose|Sumit Bose]]
> >
> >_______________________________________________
> >Freeipa-devel mailing list
> >Freeipa-devel at redhat.com
> >https://www.redhat.com/mailman/listinfo/freeipa-devel
> -- 
> Thank you,
> Dmitri Pal
> Sr. Engineering Manager IdM portfolio
> Red Hat, Inc.
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