On Tue, 2012-04-03 at 18:45 +0200, Ondrej Hamada wrote:
> On 03/13/2012 01:13 AM, Dmitri Pal wrote:
> > On 03/12/2012 06:10 PM, Simo Sorce wrote:
> >> On Mon, 2012-03-12 at 17:40 -0400, Dmitri Pal wrote:
> >>> On 03/12/2012 04:16 PM, Simo Sorce wrote:
> >>>> On Mon, 2012-03-12 at 20:38 +0100, Ondrej Hamada wrote:
> >>>>> USER'S operations when connection is OK:
> >>>>> -------------------------------------------------------
> >>>>> read data ->  local
> >>>>> write data ->  forwarding to master
> >>>>> authentication:
> >>>>> -credentials cached -- authenticate against credentials in local cache
> >>>>>                           -on failure: log failure locally, update
> >>>>> data
> >>>>> about failures only on lock-down of account
> >>>>> -credentials not cached -- forward request to master, on success
> >>>>> cache
> >>>>> the credentials
> >>>>>
> >>>> This scheme doesn't work with Kerberos.
> >>>> Either you have a copy of the user's keys locally or you don't, there is
> >>>> nothing you can really cache if you don't.
> >>>>
> >>>> Simo.
> >>>>
> >>> Yes this is what we are talking about here - the cache would have to
> >>> contain user Kerberos key but there should be some expiration on the
> >>> cache so that fetched and stored keys periodically cleaned following the
> >>> policy an admin has defined.
> >> We would need a mechanism to transfer Kerberos keys, but that would not
> >> be sufficient, you'd have to give read-only servers also the realm
> >> krbtgt in order to be able to do anything with those keys.
> >>
> >> The way MS solves hits (I think) is by giving a special RODC krbtgt to
> >> each RODC, and then replicating all RODC krbtgt's with full domain
> >> controllers. Full domain controllers have logic to use RODC's krbtgt
> >> keys instead of the normal krbtgt to perform operations when user's
> >> krbtgt are presented to a different server. This is a lot of work and
> >> changes in the KDC, not something we can implement easily.
> >>
> >> As a first implementation I would restrict read-only replicas to not do
> >> Kerberos at all, only LDAP for all the lookup stuff necessary. to add a
> >> RO KDC we will need to plan a lot of changes in the KDC.
> >>
> >> We will also need intelligent partial replication where the rules about
> >> which object (and which attributes in the object) need/can be replicated
> >> are established based on some grouping+filter mechanism. This also is a
> >> pretty important change to 389ds.
> >>
> >> Simo.
> >>
> > I agree. I am just trying to structure the discussion a bit so that all
> > what you are saying can be captured in the design document and then we
> > can pick a subset of what Ondrej will actually implement. So let us
> > capture all the complexity and then do a POC for just LDAP part.
> >
> Sorry for inactivity, I was struggling with a lot of school stuff.
> 
> I've summed up the main goals, do you agree on them or should I 
> add/remove any?
> 
> 
> GOALS
> ===========================================
> Create Hub and Consumer types of replica with following features:
> 
> * Hub is read-only
> 
> * Hub interconnects Masters with Consumers or Masters with Hubs
>    or Hubs with other Hubs
> 
> * Hub is hidden in the network topology
> 
> * Consumer is read-only
> 
> * Consumer interconnects Masters/Hubs with clients
> 
> * Write operations should be forwarded to Master
> 
> * Consumer should be able to log users into system without
>    communication with master

We need to define how this can be done, it will almost certainly mean
part of the consumer is writable, plus it also means you need additional
access control and policies, on what the Consumer should be allowed to
see.

> * Consumer should cache user's credentials

Ok what credentials ? As I explained earlier Kerberos creds cannot
really be cached. Either they are transferred with replication or the
KDC needs to be change to do chaining. Neither I consider as 'caching'.
A password obtained through an LDAP bind could be cached, but I am not
sure it is worth it.

> * Caching of credentials should be configurable

See above.

> * CA server should not be allowed on Hubs and Consumers


Missing points:
- Masters should not transfer KRB keys to HUBs/Consumers by default.

- We need selective replication if you want to allow distributing a
partial set of Kerberos credentials to consumers. With Hubs it becomes
complicated to decide what to replicate about credentials.

Simo.

-- 
Simo Sorce * Red Hat, Inc * New York

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