On 03/09/2015 02:29 PM, Traiano Welcome wrote:
Hi Alexander

  Thanks for the response:

On Mon, Mar 9, 2015 at 8:04 PM, Alexander Bokovoy <aboko...@redhat.com> wrote:
On Mon, 09 Mar 2015, Traiano Welcome wrote:
Hi List

I have AD trusts configured and working between an IPA server and a
"master" primary domain controller (dc-1) in a forest in one data
center. This allows me to connect with SSH to linux servers in the
same data-center, authenticating with my AD credentials.

I'm trying to test a scenario where I have an IPA server set up in
another data center, and trust is established with an AD domain
controller (dc-2) in that data-center.
This domain controller takes dc-1 as it's authoritative source.
Ideally, the IPA server will interact with the domain controller
nearest to it (i.e dc-2), however, from tcpdump, I note the following:

- IPA server communicates with dc-2 first
- dc-2 returns a list of domain controllers in all the datacenters,
including dc-1
the IPA server then begins querying ldap and kerberos ports on dc-1,
the domain controller furthest from it.
- Authentication on clients fail

My question is:  Is it possible to get IPA  to query and interact only
with the domain controller it initially established trust with?
Let's first separate multiple issues.

1. Terminology
   Cross-forest trust is established between domain controllers in forest
   domains. In case of IPA we need that access only once, when trust is
   created. You don't need to establish trust again with dc-2 once you
   have established it with dc-1.

   When trust is established, AD DCs will look up IPA masters via SRV
   records in DNS (_ldap._tcp.<ipa-domain>). We don't provide
   site-specific SRV records as IPA does not handle sites in this area

Thanks for the clarification.

   However, this is not your problem.

2. User and group lookups are done by SSSD against global catalog
   service (_gc._tcp.<ad-domain>) and AD DS (_ldap._tcp.<ad-domain>),
   along with Kerberos KDC (AD DCs).

   These DCs are found via SRV records in DNS and SSSD since 1.9.5
   uses site-specific SRV records for AD domain controllers lookups in
   case of IPA-AD trust.

You are interested in (2) being site-aware. However, you didn't say what
is your distribution and software versions so it is quite hard to give
any recommendation.
My objective is basically distribution of IPA servers across multiple
data centers linked by a WAN:

- There is a central 'head office' with an AD domain-controller, this
is the primary source of identity for the domain.
- A number of other data-centers each have their own local AD domain
controller linked to the head-office domain controller
- The datacenters are in different timezones.
- An IPA server in each data-center with trust established with the
local domain controller
- Each IPA server is configured with it's own realm, but provides
access to the global AD domain via AD Trusts

The idea was to prevent IPA authentication related traffic going
across the WAN (latency, different time-zones etc) to a central ad
domain controller at head office. So this setup seemed reasonable.

I'm using IPA v3.3 distributed with the CentOS 7 DVD. For reference,
the working setup was based on this howto:


... which works fine if the IPA server has a trust relationship
directly with the primary domain controller at head office (which it
is collocated with).

I can live without site-awareness per se if I can achieve IPA
centralised authentication across datacenters in different timezones,
with AD as the primary source of identity. An alternative setup that
I've considered testing:

- IPA server at the head office with trust established to the primary AD DC.
- A replica of the primary in each DC (different timezones), on the same realm

However, I doubt replicas can work across timezones, and with high WAN

As Alexander said the trust on the fores level.
There is no pairing between IPA replicas and AD DCs to a specific data center.

What you need to concentrate is making sure that SSSD on a client picks AD in the local data center and IPA in the same data center (for the additional lookup operations). I do not know whether one can explicitly set this in sssd.conf. This is something to ask SSSD list. The alternative would be to make DNS return the right server. For AD DC the AD DNS will be returning the server to authenticate against and there should be a way to configure AD DNS this way. I do not think it is possible to force specific DNS entry out of IPA - it does not support sites yet. But may be there is a way to override it on SSSD.

In case of other (legacy Linux or other platforms) clients that talk to IPA you can configure them with the explicit fail over list. They do not talk to AD directly. You might also consider configuring SSSD on the IPA server to use AD in the same location as a primary server.


/ Alexander Bokovoy

Thank you,
Dmitri Pal

Sr. Engineering Manager IdM portfolio
Red Hat, Inc.

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