Understanding the Patterns for Identity Consolidation and SSO enabled by Centrify with AD on UNIX, Linux and Macs

Background

In the almost two years of Centrifying we have discussed Identity Consolidation with AD and Single Sign-on at length.  90% of organizations have Active Directory but sometimes over-complicate things when it comes to identity consolidation and SSO.

I had the chance to speak about this in a seminar and these two videos consolidate many entries that we've covered in this blog over the years.

Direct Integration

Name Service Switch, Pluggable Authentication Modules, GSSAPI, Kerberos and Proxies

• OpenSSH SSO over an outgoing external non-transitive AD one-way trust
• NSS and PAM using Oracle DB as an example (externally identified user)
• GSSAPI using MongoDB
• Kerberos using Hadoop (MapR example)
• LDAP Proxy to enable Couchbase console access.

Web-Java, SAP and DB2 Plugins

SPNEGO Plugins (Apache/Java);  ERP Plugin (SAP Netweaver, SAPgui), DB2 Plugin

• Apache SPNEGO
• JBoss SPNEGO
• DB2 Plugin
• Toolset:  Centrify-enhanced psftp;  addns, adcert

Conceptual Diagram

The idea is to eliminate complexity and promote reuse by committing to Active Directory, let the Centrify DirectControl agent do the heavy-lifting for Direct Integration and use the SPNEGO plugins when needed.


For the full briefing, including marketing slideware go here.

Basics: A Primer on Active Directory External One-Way Trusts with Centrify on UNIX, Linux and OS X

Background

Active Directory trusts are a broad topic and we typically have conversations with UNIX, Linux or Mac SMEs are excited about Centrify capabilities, but don't understand the purpose of trusts.

As a matter of fact, sometimes even the AD SMEs don’t understand the types of trusts in their own environment; this has the makings for interesting conversations.
The purpose of this blog post is to provide a primer on AD trusts (mostly by way of references) and to cover the considerations of Centrify deployments on the most popular type of trust: The external forest one-way trust.

I think that especially now that I see a lot of organizations extend their AD infrastructure to IaaS (like Amazon, Azure, Rackspace, etc) it's very important to know how these trusts work and what are the considerations to their Centrified systems.

What you need to know to understand this article
You must understand Active Directory basic concepts like forest, domain, sites, domain controllers, global catalog, DNS, and network connectivity.   A quick search should yield some interesting results;  you can also look at my personal blog.

My next advice will be to read about trusts and how authentication works with them, but this can be a very long and deep read. The best article about how trusts work is this one:
https://technet.microsoft.com/en-us/library/cc773178%28v=ws.10%29.aspx

Continue reading, and keep in mind, the main object of this article is to help UNIX, Linux and Mac OS X Centrify administrators get a basic understanding of external one-way trusts plus key-in on considerations for their Centrify deployments.

Needless to say, if you're supporting Centrify you need to know the basics like Zones, DirectControl, DirectAuthorize, Access Manager, Centrify PowerShell, etc.

Basics

Why Active Directory trusts exist?
There are several reasons, some of them are backward-compatibility, support for Kerberos realms, etc., however the most common reason is to support the account-resource model.

What is the account-resource model?
It's a security model in which user and group accounts exist in a centralized realm and resources (systems, apps, files, etc) exist on a different realm.

Notice the direction of trust, vs. the direction of access.  Believe it or not, if you understand the concept outlined in this illustration, you will get one-way trusts with ease.

Note:  The account-resource model is just one reason why AD trusts exist.  The key reason is the modern enterprise;  Global companies need ways to accommodate different access models especially when mergers and acquisitions happen too.  Since the modern enterprise is changing to expand the barriers of the datacenter, more capabilities will be created to support these scenarios.  AD one-way trusts, RODCs, etc are just some of the tools of the trade.

Other facts about Active Directory Trusts:

• There are different types of trusts: External, Real, Forest, Shortcut
• Trusts have transitivity: non-transitive, transitive
• Trusts have direction: one-way, two-way
• Trusts have scope: trusts may allow domain-wide or selective authentication

A reference model to understand External one-way trusts


The layers are:  TCP/IP (network communications), name resolution, active directory and Centrify layer.  The idea is that the model mimics the OSI model, in which if a lower layer fails, the upper layers won't work either.

TCP/IP Layer

There are different communications requirements for things to work as expected, but we need to concentrate first in what kind of traffic we will be dealing with.

The considerations for communications are very important especially in DMZ scenarios;  the key here is that most likely there’s conversations between 3 different subject-matter-experts:  the UNIX, Linux, Mac SME, the AD SME and the Network/Firewall SME.

• Traffic between forests to establish the trust relationship in the first place  (not in scope for this article, but read the Network Ports used by Trusts section of this link.
• Traffic between the Centrify AD client and its local domain controllers
• Traffic between the Centrify AD client and remote domain controllers (from the trusted domain) for referrals (see below in the AD layer)
• Traffic between systems with AD tools and Centrify tools for referrals and the object picker (see below in the AD layer)
•  If DirectAudit is in use:  Traffic between the audit agent and the collector and between the collector and the database and management server.  <= this will be discussed later
• If DirectSecure is in use, GPO traffic to obtain PKI certificate auto-enrollment and IPSec rules implies connectivity between domain controllers (to get GPOs) and CAs or SubCAs to get the certificates themselves.   <= this will be discussed later

The ports required for AD communication can be determined very quickly with DirectControl, (just run the “adinfo –T” command)  but here’s a summary:

• TCP 53 (DNS) -  without name resolution there’s no communication.  Period.  This means that A and SRV records must be resolvable by all systems on each side of the trust relationship.
•  TCP 3268  (Global Catalog):    In AD, a domain controller with the Global Catalog role contains a partial replica of all objects in AD.  Global Catalog placement has performance implications.
• TCP and UDP 389 (LDAP):  This should be self-explanatory.
• TCP 445 (modern Server Message Block):  This is optional, but required for GPOs used for config management.
• TCP 88 (Kerberos KDC):  Self-explanatory;  this the port for Kerberos communications.
• TCP 464 (Kerberos password change):  This is the port for password change operations 
• TCP 123 (SNTP):  This is optional, to sync time with the domain controller’s Windows Time Service.
• Ephemeral ports(*):  These are often overlooked, but these are the high-ports used for socket communications.

(*)Note: Microsoft has changed the ephemeral port range over the years and the best way to see the ones that apply to your version of Windows is to look at the output of the netsh command with some options:

C:\Windows\system32>netsh int ipv4 show dynamicport tcp

Protocol tcp Dynamic Port Range

---------------------------------

Start Port      : 49152

Number of Ports : 16384

Bottom-line:  Expect to allow communications from Centrified systems and management stations to at least two domain controllers from your trusted forest. 

These can be read-only DCs  (RODCs) and depending on forest complexity, these may have to be Global Catalog.

This is not an issue on flat networks, but definitely more complex high-security scenarios like DMZs.

In the illustration above, this is a post-trust establishment scenario with one RODC in the trusting side and a firewall only allowing traffic for the RODC to be updated by the trusted domain DCs.

Both referral traffic from Centrified systems and management stations are serviced by the RODC.
Note:  This is just one way of going about this scenario, there are several ways with pros and cons.

Name Resolution Layer

Name resolution by way of DNS is often overlooked by many administrators, the key to understand here is that Active Directory clients use Kerberos for authentication and in a "Kerberized" world there’s no functionality without being able to resolve names;  DNS symmetry is key;  fortunately most AD deployments use domain controllers as DNS servers, but in scenarios in which other DNS servers are used, DNS Zones holding AD SRV records have to be accessible. 

In my demo environment, I used the “conditional forwarding” capabilities of Windows DNS.

Note that I can get away with this because my network is flat.  If I was dealing with a DMZ scenario I would either host a replica and protect it with firewall rules or IPSec encryption for DNS traffic.

For legacy apps, careful thought must be given to short names, especially watch out for duplicity or short names exceeding 15 characters.  Read the best practices for naming in Active Directory.

In addition there's the issue of Kerberos realms vs DNS Names:  with active directory these are often the same, but it does not have to be like that.

Active Directory Layer

The AD layer deals with all the Active Directory-related capabilities needed for the trust to work as expected and these include trusts, referrals, object picking, authorization and group scoping

Trusts

Because this topic focuses on the external non-transitive one-way trust, it’s important to know it may be selective; but to keep things simple, let’s assume it’s a domain-wide trust.

Going back to the basic principle with an example illustrated in the figure below.

If Fabrikam (HQ) trusts Contoso (CORP), that means that users from the corp.contoso.com can access resources from hq.fabrikam.com; 

Trusts are set up with a dual authorization (just like launching a rocket).  If you look at Active Directory Domains and Trusts for both sides you’ll see this:


The non-transitivity of the trust is what provides the assurance that accounts from the trusting domain won’t be able to access resources from the trusted domain.

Authorization

We have talked about authentication in most of this entry, but we haven’t addressed authorization.  

Going back to the basic principle of the example:

"If hq.fabrikam.com trusts corp.contoso.com, that means that users from the corp.contoso.com can access resources from hq.fabrikam.com"  

HOWEVER, this does not mean that users from corp.contoso.com have any privileges on hq.fabrikam.com;  this will only happen if it’s explicitly granted. 

The common practice in a centralized account-resource model is to add the Global Domain Administrators group from the trusted domain into the Builtin Local Administrators group from the trusting domain, this will allow for Centralized administration, and since there’s really no accounts used by users in the resource domains (other than contingency or service accounts), the unique credential principle is maintained.

This has implications to the Centrify best practices and we’ll discuss this on the Centrify layer.

Group Scoping

A key concept in the access model is group scoping.  In a nutshell it means that security groups (the groups used for authorization) have different scopes:  Universal, Global and Domain Local. 

The key here is that Domain Local groups can contain principals (users/global groups) from the local domain and any trusted domain.  Global groups can only contain principals from the same domain.

For the sake of simplicity I will ignore Universal groups.  Please see this article for an in depth comparison.

The best practice is to Assign Rights (or entitlements) to Domain Local Groups and add users or Global Groups to the Domain Local group.  From that point on, the add/moves/changes of rights and entitlements are managed via group memberships.

See the example above:   The Domain Local Administrators group from hq.fabrikam.com contains the Global groups from corp.contoso.com and hq.fabrikam.com and the local Administrator domain account.

Referrals

I’ve mentioned referrals before in this topic. 

A referral happens when a system from the trusting domain has to authenticate a user from the trusted domain.  Let’s look at an oversimplified example:

•  If the system centos66 in hq.fabrikam.com needs to authenticate user joe@hq.fabrikam.com, the authentication will happen locally against the fab-dc1.hq.fabrikam.com domain controller.
•  If the system centos66 in hq.fabrikam.com needs to authenticate user jerry@corp.contoso.com, provided that the user has a UNIX identity and a role that allows login, the authentication will be referred to dc1.corp.contoso.com to satisfy the request.

The same thing happens with Windows systems or tools like ADUC, Access Manager, PowerShell, etc.

For more information, please read the “Kerberos-Based Processing of Authentication Requests Over Forest Trusts”  section of this link.

Object Picker

When using tools like Active Directory users and computers or Access Manager, in order to pick foreign principals you use the object picker. 

Here’s an example below:

This screenshot is from Access Manager running from the trusting forest (hq.fabrikam.com)  and corp.contoso.com is the trusted external forest.

This means that in this case, the Access Manager console must have at least 2 layers (communication and  name resolution) in order to get to this point;
Note:  The fact that we can get there does not mean that we can browse the trusted forest, we must have a valid AD account from that forest.  Remember, we are going against the direction of access.

Contrast with this:

This screenshot is from Access Manager running from the trusted domain (corp.contoso.com) notice that you can’t even pick from the trusting domain (hq.fabrikam.com) because of the fundamental way trusts.  Principals from the trusting forest can’t access resources in the trusted forest.

This sounds like a broken record, but believe it or not, this is the main source of confusion for the IT leads we work with.

Centrify Layer

This layer covers the behavior of Centrified systems when an external one-way trust is established, management tools, and how the best practices change in the account-resource model.

Verifying TCP/IP Connectivity using Centrify CLI Tools

Like I explained before, the adinfo –T command will be very handy.  Let’s look at the output of this command from two perspectives:

corp.contoso.com (trusted)

hq.fabrikam.com (trusting)

[george@cen1 ~]$ adinfo -T && adinfo -T hq.fabrikam.com Domain Diagnostics:   Domain: corp.contoso.com     DNS query for: _ldap._tcp.corp.contoso.com     DNS query for: _gc._tcp.corp.contoso.com   Testing Active Directory connectivity:     Global Catalog: dc1.corp.contoso.com       gc:       3268/tcp - good     Domain Controller: dc1.corp.contoso.com       ldap:      389/tcp - good       ldap:      389/udp - good       smb:       445/tcp - good       kdc:        88/tcp - good       kpasswd:   464/tcp - good       ntp:       123/udp - good Domain Diagnostics:   Domain: hq.fabrikam.com     DNS query for: _ldap._tcp.hq.fabrikam.com     DNS query for: _gc._tcp.hq.fabrikam.com   Testing Active Directory connectivity:     Global Catalog: fab-dc1.hq.fabrikam.com       gc:       3268/tcp - good     Domain Controller: fab-dc1.hq.fabrikam.com       ldap:      389/tcp - good       ldap:      389/udp - good       smb:       445/tcp - good       kdc:        88/tcp - good       kpasswd:   464/tcp - good       ntp:       123/udp - good

[rpimentel@centos66 ~]$ adinfo -T && adinfo -T corp.contoso.com Domain Diagnostics:   Domain: hq.fabrikam.com     DNS query for: _ldap._tcp.hq.fabrikam.com     DNS query for: _gc._tcp.hq.fabrikam.com   Testing Active Directory connectivity:     Global Catalog: fab-dc1.hq.fabrikam.com       gc:       3268/tcp - good     Domain Controller: fab-dc1.hq.fabrikam.com       ldap:      389/tcp - good       ldap:      389/udp - good       smb:       445/tcp - good       kdc:        88/tcp - good       kpasswd:   464/tcp - good       ntp:       123/udp - good Domain Diagnostics:   Domain: corp.contoso.com     DNS query for: _ldap._tcp.corp.contoso.com     DNS query for: _gc._tcp.corp.contoso.com   Testing Active Directory connectivity:     Global Catalog: dc1.corp.contoso.com       gc:       3268/tcp - good     Domain Controller: dc1.corp.contoso.com       ldap:      389/tcp - good       ldap:      389/udp - good       smb:       445/tcp - good       kdc:        88/tcp - good       kpasswd:   464/tcp - good       ntp:       123/udp - good

This confirms that Centrified systems in Fabrikam will have no issues with queries or referral traffic.

Centrify Agents and Trusts

On startup, join, forced cache expiration or when the 8-hour domain discovery threshold is met, the Centrify agent interrogates domain controllers and if required, it will update its Domain Map.

A key command to review the domain map is the “adinfo --sysinfo domain”; let’s look at the output from both sides               

corp.contoso.com (trusted)

hq.fabrikam.com (trusting)

$ dzdo adinfo --sysinfo domain AD Password: System Diagnostic ========Domain info map======== DC=corp,DC=contoso,DC=com     CN              = CORP.CONTOSO.COM     SID             = S-1-5-21-2180375406-786980114-1643973036     TRUST_ATTRS     = 0x20     TRUST_DIRECTION = 3     TRUST_TYPE      = 2     NTLM NAME       = CORP     LOCAL FOREST    = YES CN=hq.fabrikam.com,CN=System,DC=corp,DC=contoso,DC=com     CN              = HQ.FABRIKAM.COM     SID             = S-1-5-21-2165579280-668341325-710928106     TRUST_ATTRS     = 0x0     TRUST_DIRECTION = 1     TRUST_TYPE      = 2     NTLM NAME       = HQ     LOCAL FOREST    = NO

$ adinfo --sysinfo domain System Diagnostic ========Domain info map======== DC=hq,DC=fabrikam,DC=com     CN              = HQ.FABRIKAM.COM     SID             = S-1-5-21-2165579280-668341325-710928106     TRUST_ATTRS     = 0x20     TRUST_DIRECTION = 3     TRUST_TYPE      = 2     NTLM NAME       = HQ     LOCAL FOREST    = YES CN=corp.contoso.com,CN=System,DC=hq,DC=fabrikam,DC=com     CN              = CORP.CONTOSO.COM     SID             = S-1-5-21-2180375406-786980114-1643973036     TRUST_ATTRS     = 0x4     TRUST_DIRECTION = 2     TRUST_TYPE      = 2     NTLM NAME       = CORP     LOCAL FOREST    = NO

The key fields here are the trust type and trust direction.  Direction 3 means two-way (always with the local domain),  Direction 2 means one-way;

Workstation Mode vs. Zone Mode

In my test environment in the trusting (resource) domain (hq.fabrikam.com) I have two systems:

• A CentOS 7 system joined in workstation mode called cen2
• A CentOS 6.6 system joined in zone mode called centos66

Workstation mode (Auto zone) won't work in a 1-way trust scenario.   The reason has to do with the way AZ generates UNIX identities.  The UID/GID parts of the UNIX identity are generated on the fly based on the user or group's Active Directory SID (unique identifier), however since the system is in a resource forest, based on the security model it can't even read the object's SID.  The alternative is to use Centrify Zones. 



Note:  AutoZone will work over a transitive 2-way trust.
 

In Zone mode, only authorized users from the local and trusted forests that that have been UNIX-enabled will have access to the system. When you use Access Manager and use the object picker to select an object from the account domain, you must do so with a authenticated user from the trusted domain, at that point our software can see the user/group information and will create the proper identity information in the zone.

Note the red arrow next to Jerry’s name.  This indicates a foreign principal.

Now let’s look at the output of "adquery user -P" (to show user principal names only) from our zoned computer (centos66):

$ adquery user -P

felderi.cibao@hq.fabrikam.com

jerry.seinfeld@corp.contoso.com

rpimentel@hq.fabrikam.com

And the UNIX identity is defined as per the zone settings:

$ adquery user -A jerry

unixname:jerry

uid:1149240405

gid:1149240405

gecos:

home:/home/jerry

shell:/bin/bash

auditLevel:AuditIfPossible

isAlwaysPermitLogin:false

dn:CN=b8cdb1f6fdf447d5b2c43f77e6609ab2,CN=Foreign User,CN=One Way Trust,DC=corp,DC=contoso,DC=com

samAccountName:jerry.seinfeld

sid:S-1-5-21-2180375406-786980114-1643973036-1109

userPrincipalName:jerry.seinfeld@corp.contoso.com

canonicalName:corp.contoso.com/One Way Trust/Foreign User/b8cdb1f6fdf447d5b2c43f77e6609ab2

passwordHash:x

guid:f6b1cdb8-f4fd-d547-b2c4-3f77e6609ab2

zoneEnabled:true

Express Mode

In Express (limited freeware) mode, one-way trusts are not supported because it uses Auto zone mode.

Access Manager and the Delegation Model

In order to enforce Separation of Duties, Centrify best practices promote the separation of Governance activities vs. Operations activities.  However, the group scope and where accounts reside determine many of the practices.

For example, if you used Access Manager to generate the recommended deployment structure, most likely you’ll have these groups:

Notice the scope “Security Group – Domain Local” -  this means that if you’re in a pure account/resource model, you will have to add the Global Security groups from the trusted forest into the corresponding group.

E.g. if you want the Global group Centrify Administrators from the corp.contoso.com account domain  to manage Access/Privileges, I would add them to the cfyA_Global_* Domain Local groups.

Zone Provisioning Agent

The Centrify Zone Provisioning agent (ZPA) is a key tool for Identity and Access provisioning.   Some other customers may be using the Centrify DirectControl SDK, Centrify PowerShell or the TCL-based adedit scripting language to automate these actions.

When creating the Active Directory Security groups for Automatic UNIX identity provisioning for users and groups you have think about the access and trust direction before you decide your strategy. 

The main reason is that principals from the trusting forest can’t see the group memberships of the trusted forest.

If you make the mistake of

1. Create a Domain Local group for Identity Provisioning
2.  Nest a Global Group from the trusted forest

What will happen is that ZPA won’t be able to see any add/moves or changes to the group and provisioning won’t happen as expected.
Let’s look at this from the agent’s perspective using CLI tools.

I have a system called CEN1 that belongs to a zone in corp.contoso.com.  I will attempt to use the adquery group command to view the members of a well-known AD group:

[george@cen1 ~]$ adquery group -A "Cert Publishers@corp.contoso.com" | grep members

members:corp.contoso.com/Domain Controllers/DC1

Let’s now try to see if we can see the same group memberships from the trusting forest Fabrikam.

[george@cen1 ~]$ adquery group -A "Cert Publishers@hq.fabrikam.com" | grep members

members:hq.fabrikam.com/Domain Controllers/FAB-DC1

As you can see, this is consistent with the direction of access principle of AD trusts, and since group memberships are public to authenticated users from trusted forests, George has no issue seeing this information.

Let’s repeat the same process from cen2 in Fabrikam.

[jerry.seinfeld@corp.contoso.com@cen2 ~]$ adquery group -A "Cert Publishers@hq.fabrikam.com" | grep members

members:hq.fabrikam.com/Domain Controllers/FAB-DC1

Let’s now try to see if we can see the group memberships from the same group from Contoso:

[jerry.seinfeld@corp.contoso.com@cen2 ~]$ adquery group -A "Cert Publishers@corp.contoso.com" | grep members

Error: No such group Cert Publishers@corp.contoso.com

This is where working with Centrify Professional Services yields the best results.  They have tools to enable these kinds of scenarios.

The Missing Layer:  Automation, Orchestration and Interoperability

It’s very easy to focus on a single product or infrastructure, it’s also quite common for niche solutions to be closed or require specialized connectors (or extra investment to facilitate automation, orchestration and interoperability), however, with Centrify that’s not the case.

• This can be overlooked however those are the building blocks that make technology and business processes work well.  Here are some examples:
• Contoso may be using ServiceNOW to provide workflow, approvals and service management, there are SN considerations as well as the 4 layers.
  For example:  When a user requests access (or is removed) to/from a server or group of servers the resulting action is an add/removal from an Active Directory group.  Most likely this will be a Domain Local Group in Fabrikam.
• If Contoso is using ARCSight for log aggregation, the communication path for agents (syslog, event log) has to be clear, as well as for any additional infrastructure.
• Contoso can also be using  Centrify Privilege Service (CPS) to enable Password Management and Session Brokering, perhaps a Cloud Connector may be needed.

Conclusion

In summary, Active Directory Trusts are very powerful, but unfortunately not well understood.  Designs may vary, but a trust design that has been well researched can safely extend the boundaries of organizations.

The key concept in an external, non-transitive one-way trust is that the direction of access is opposite the direction of trust, effectively protecting the trusted domain;

In addition, network connectivity, name resolution, Active Directory considerations and Centrify design and tools have to be cohesive for things to work.

At Centrify, we have this basic test when it comes to one-way trusts:   Show me a Windows client work as expected.  If it does, a Centrified AD client should work as well.

Business Problems: Using Centrify on Mixed Kerberos Environments (UNIX, Linux, OS X)

Background

Organizations typically use Centrify's Server Suite to simplify the implementation of Kerberos by using Active Directory and the DirectControl (adclient).  However, in special situations, there may be a need to support independent MIT Kerberos realms along with Active Directory participation.

Active Directory and Centrify:  Plug-n-play Kerberos for UNIX, Linux and Mac

Aside from establishing a secure communications channel with AD, provide identity information and privilege management, in UNIX, Linux or OS X systems, Centrify takes care of the Kerberos environment.  At a high-level, here's what happens:

Kerberos Configuration

• Configuration: The /etc/krb5.conf file is modified to include information about Active Directory's Kerberos realm, this includes encryption levels, realms (domains), KDCs (domain controllers) and trusted realms (using Microsoft's Kerberos extensions)
  This capability is very convenient, because when AD administrators add or decommission domain controllers or establish trusts, there's no need to go back and update the krb5.conf file.
  In failover scenarios, authentication also "just works" provided there's communication with the target DCs.
• System Key Table: The system keytab (typically /etc/krb5.keytab) is updated with entries for the Service Principal Names (SPNs) and information required for it to work in Active Directory.
• Kerberos Tools:   Centrify's MIT Kerberos tools are placed in the /usr/share/centrifydc/kerberos/bin folder.  These tools are optimized to work with Microsoft's Kerberos extensions.  In addition, tools like adkeytab are available to administrators.  adkeytab provides much more robust capability than traditional tools like kutil.

Other Configuration:
Name Resolution:  Although the Centrify's adclient will respect the settings in /etc/resolv.conf, Centrified clients support advanced capabilities like Dynamic DNS updates and maintain an independent DNS cache.  Part of the telemetry calculations performed when making sure DC connectivity is optimal include DNS sweeps.
Time Synchronization:  Kerberos has a mechanism to deter replay attacks that relies on KDCs and Kerberos clients to keep with a skew of 5 minutes or less.  Upon system join, system time uses the AD domain controllers as time source.

Finally, there's the issue of Kerberos principal format.  With Centrify, regardless of the case sensitivity of the AD user or UPN (Kerberos expects user@REALM) , Centrify will always work Active Directory implementations.

 

What about support for Mixed Environments?

Centrify has very large organizations that may fall in the following categories:

• Government, commercial, educational or research organizations that have MIT Kerberos realms defined, but need to provide interoperability while they migrate completely to Active Directory.
  We have also seen this with commercial organizations that have implemented MIT Kerberos realms for a particular application (e.g. Hadoop), but are ready to eliminate the duplicity and complexity introduced.
• Organizations that have legitimate reasons to maintain MIT Kerberos implementations for critical apps or other situations.

There are 3 categories of implications:

• Name resolution implications:  In an ideal scenario, DNS is always symmetric and hierarchical; but we've seen situations in which systems have asymmetric configurations.
• Time source implications:  Although ideally, all systems sync up time with a hierarchical time source, sometimes IT organizations are quite fragmented.
• Kerberos configuration implications: 
• The Centrify client will only write information about local and trusted Active Directory realms and KDCs
• The system keytab is automatically set up or overwritten by adclient during system join.

With all those challenges, the idea is to eliminate additional complexity. 

 

Centrify Parameters to Support Mixed Scenarios

Here's a sample mixed scenario:

In a normal Centrify setup, James won't be able to log in or use tools in the EXAMPLE.COM realm, however everything will work fine for UNIX-enabled users in the corp.contoso.com.  Other key areas are the different time sources and potential asymmetry of DNS.

To overcome these issues, Centrify has 4 parameters (with corresponding Group Policy Objects):

adclient.krb5.autoedit:  when set to true (default), centrify's agent will maintain the system's Kerberos configuration (/etc/krb5.conf) file.  when set to false, the file will be left alone.

adclient.krb5.keytab:/path/to/file.keytab:  when set, this parameter will enable administrators to place the keytab used with AD in a different location than the default system keytab (/etc/krb5.keytab)

dns.dc.domain_name:  This configuration parameter allows administrators to specify domain controllers that service the target domain.  E.g.
dns.dc.corp.contoso.com: dc1.corp.contoso.com dc2.corp.contoso.com

adclient.sntp.enabled:  This parameter tells the agent if it will use the Windows Time Service running in the domain controller as the time source.


Tradeoffs
With this unorthodox configuration (basically multiple authentication sources) there tradeoffs;  part of the reason of any Centrify deployment is to promote simplicity.  Once the autoedit parameter is introduced, organizations lose the automatic maintenance of AD topology for Kerberos, however, this can be managed via group policy, or any config management tool (like Chef, Puppet, etc), but it requires constant communication between the UNIX/Linux or Mac administrators and Active Directory Administrators.
The benefit here is that Centrify has the maturity and flexibility to support these kinds of scenarios.