On 04/04/2012 06:16 PM, Ondrej Hamada wrote:
On 04/04/2012 03:02 PM, Simo Sorce wrote:
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
-credentials cached -- authenticate against credentials in local cache -on failure: log failure locally, update
about failures only on lock-down of account
-credentials not cached -- forward request to master, on success
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.


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.


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?

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
Right, in such case the Consumers and Hubs will have to be masters (from 389-DS's point of view).

* 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.
Add point:
- storing of the Krb creds must be configurable and disabled 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.


Rich mentioned that they are planning support for LDAP filters in fractional replication in the future, but currently it is not supported.

Ad distribution of user's Krb creds:
When the user logs on any Consumer for a first time, he has to authenticate against master. If succeeds, he will be added to a specific user group. Each consumer will have one of these groups. These groups will be used by LDAP filters in fractional replication to distribute the Krb creds to the chosen Consumers only.

This will be more complicated because of the HUBs (as Simo already said). The easiest way will be to let the Hubs contain all the creds - but this solution might result in massive network traffic when the users start logging in. Other solution might be creating similar user group for each Hub. Because every replica will be Master(from 389-DS point of view) - it will hold record about everyone of its suppliers and consumers and thus it will be possible to decide who belongs to which user group.


Ondrej Hamada
FreeIPA team
IRC: ohamada

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