Simplified question is - "why do we require domain (external trust) or forest (forest trust) functional level 2 when using selective authentication?" -
Let's begin with what selective authentication (SA) does ... when configured across a particular trust it tells the KDCs within the domain at the end of the trust to perform an additional validity check before issuing the session ticket (we normally rely solely on authorization ... SA prevents the ticket from even being issued thus it is known as the authentication firewall). The additional validity check uses the SPN (service principal name) within the ticket request and resolves it to a computer object within the domain NC (nothing new so far) and looks for an "Allow" for the extended right "Allowed to authenticate" assigned to any SID within the requesting user's PAC or access token (this is the new validity check). "Allowed to authenticate" should be assigned against the computer object that represents the physical computer housing the resource. It must be assigned to the user or group from the trusted domain that you wish to grant access to. If the right is "allowed", the ticket is issued. If the right is "denied" or not listed/not applicable to the requesting user, the ticket is not issued and access will not be granted since authorization cannot proceed. It is important to note that this process is only performed against TGS requests originating in a foreign realm/domain for which the trust relationship's TDO (trusted domain object) indicates SA as opposed to forest wide authentication. Before a session ticket can be issued a requesting client must possess a TGT issued by a KDC authoritative over the server holding the target service. Upon requesting initial auth., the KDC in the trusting domain decrypts the TGS referral, validates the authenticator and, if valid, constructs a new TGT containing a near bit for bit copy of the PAC from the original ticket (PAC = privileged attribute certificate). At this juncture, a new SID is injected into the PAC dependant upon the trust's authentication type; selective or forest-wide. * If forest wide, the SID is "This Organization" = Well-known group = S-1-5-15 * If selective, the SID is "Other Organization" = Well-known group = S-1-5-1000 So how do we know whether or not to invoke this new behavior and which SID should be injected during the TGT's construction? We do that by determining where the ticket request originated. If memory serves, each ticket contains an attribute known as the transited path attribute which maintains a list of the domains/realms through which the ticket has passed to get here thereby allowing us to determine behaviors relevant to the ticket's origin. The presence of the "Other Org" SID within a TGT dictates that the new behavior (the extra validity check) must be used before issuing a session ticket. Since this behavior is only known to a 2003+ KDC, the need for a functional level is imposed. SA is also supported for downlevel NTLM-only clients ... they use a mechanism known as "pass-through authentication" in order to dynamically inject additional domain relevant SIDs ... this allows the DCs to detect the presence of the "Other Org" SID and perform the new validity check before returning the newly formed token (or not). Note also that since "This" and "Other Org" are SIDs (and therefore security principals), they can be assigned access to resources allowing you to permit or deny access to a any resource based on whether the request originated within a domain that is considered as part of _our_ organization or not. I've found it useful to keep the following in mind; when creating a trust between 2 domains or forests, treat the authentication type as follows - * If selective auth. is used then we're saying that we have 2 separate organizations wishing solely to share resources when suitable * If forest/domain-wide auth. is used then we're saying that although we have two isolated domains they still represent one organization and additional validity checks are not necessary Hope this proves useful ... that's my post quota for '05 ;-) -- Dean Wells MSEtechnology * Email: [EMAIL PROTECTED] http://msetechnology.com -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Mulnick, Al Sent: Monday, January 10, 2005 8:59 AM To: ActiveDir@mail.activedir.org Subject: RE: [ActiveDir] Forest trusts vs trusts within forests Actually Dean, would like to hear that explanation as to why if it's not too much trouble. It often helps to make the idea stick :) As for the replication, Dave I understood the replication differences to be more for security reasons than performance etc. Something along the lines of not putting information where it wasn't absolutely needed anyway. Was I off on that? Much of the conversation has been around protecting assets should some event occur. I get the sense that there is an operational component to this and that you have a well defined process to handle events should they occur. Could just be me though. Al -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Grillenmeier, Guido Sent: Monday, January 10, 2005 5:16 AM To: ActiveDir@mail.activedir.org Subject: RE: [ActiveDir] Forest trusts vs trusts within forests that's also my understanding Dean and that's how I've tested it that it works - but I certainly wouldn't mind the lengthy version of the explanation... I do have to say, that the statement to require FFL2 to use SA for forests trusts is somewhat of a joke though: you'll have to have both forests running at FFL2 anyways to create a forest trust in the first place ;-) /Guido -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Dean Wells Sent: Saturday, January 08, 2005 12:20 AM To: Send - AD mailing list Subject: RE: [ActiveDir] Forest trusts vs trusts within forests For forest trust: must be forest functional level 2 For external trust: must be domain functional level 2 If an explanation as to "why" is desirable, please ask ... it's lengthy. -- Dean Wells MSEtechnology * Email: [EMAIL PROTECTED] http://msetechnology.com -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Fugleberg, David A Sent: Friday, January 07, 2005 5:33 PM To: ActiveDir@mail.activedir.org Subject: RE: [ActiveDir] Forest trusts vs trusts within forests Al - that was basically the first question, and I did get the confirmation I was looking for. The other part was regarding the 'functional level' requirements for SA. I had read conflicting things there - the one that troubled me was this: "To enable selective authentication on forest trusts, the trusting forest in which shared resources are located must have the forest functional level set to Windows Server 2003. To enable selective authentication on external trusts, the trusting domain in which shared resources are located must have the domain functional level set to Windows 2000 native." (From http://www.microsoft.com/resources/documentation/WindowsServ/2003/all/te chref/en-us/Default.asp?url=/Resources/Documentation/windowsserv/2003/al l/techref/en-us/w2k3tr_trust_security.asp) The second sentence sounds as though the trusting domain can be at Win2K Native and still use SA on an external trust. The info I see other places (including a post from John) sounds like the trusting domain must be at least Win2K3 Domain Functional Level. I'm not still not sure which is true, as I haven't tried it in the lab yet :) My guess is that SA is not available til the trusting domain (which would have to stamp the Other Organization SID in the token) is at W2K3 DFL. Dave -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Mulnick, Al Sent: Friday, January 07, 2005 4:09 PM To: ActiveDir@mail.activedir.org Subject: RE: [ActiveDir] Forest trusts vs trusts within forests Out of curiosity, did you get your question answered? The original that I read was that you wanted to know if you had two separate forests with trusts, would that create the same risks as if they were in the same forest. I *think* I read that correctly. I think John had a lot of great information in there, but I got to the thread too late which makes it harder to read and tell what was said etc. Just curious mostly. Al -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Fugleberg, David A Sent: Friday, January 07, 2005 3:50 PM To: ActiveDir@mail.activedir.org Subject: RE: [ActiveDir] Forest trusts vs trusts within forests Thanks John. To answer your questions: 1) the topology is hub/spoke. I would put a couple DCs for the new forest in the hub location. 2) Regarding replication, most of these sites have few to no Exchange users - those that do use OWA. So, I'm not worried losing the common GC that a single forest provides. I'll need to work with the Exchange team to see if/how any future plans impact this assessment, of course. Bandwidth is not the issue for wanting to compartmentalize replication. It's more about having a r/w copy of the internal directory at all of these sites that have no use for it. 3) The applications would by and large be at the central location. Some could live in the second forest (see #1). I'm certain that the business will want some of these users to access some apps in the internal forest, though- hence the need to trust the new forest. I'm also sure that our support people will want the new forest to trust the internal forest to make it easier to support. There's no illusion on my part that any configuration gives me a 100% security guarantee - if there was, someone would have found it an all of us in info security would have to find real jobs! Thanks again for the insights. I truly appreciate getting a sanity check. Around my company I'm the one people go to for AD expertise, so when I need to bounce things off of people it's often on this list. Happy Friday! Dave -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of John Reijnders Sent: Friday, January 07, 2005 10:36 AM To: 'ActiveDir@mail.activedir.org' Subject: RE: [ActiveDir] Forest trusts vs trusts within forests Hi David, Take 2 ;-). See inline comments for my ideas. 1) we have some remote sites with workstations that authenticate to the domain so they can be managed with GPOs and software distribution. They have no real need to access MS resources at the main site. In some cases, there are enough of these workstations to warrant a local DC. We don't want DCs for the one and only existing domain in some of these locations, because we can't always control physical access to them. An isolated forest (no trusts) for these would protect the internal forest in the event the new forest was compromised, compartmentalizing the damage. I'm interested in the physical structure of your network. Are the 'evil' sites fully connected to all other sites (centrally and the other 'evil' sites), or is the network topology more like a hub-and-spoke model? Implementing a separate domain or forest for the 'evil' sites would require some sort of connectivity between them or the implementation of DC for this domain/forest in your centrally and trustworthy site. But you're right that an isolated forest would take care of this. 2) there's no need to replicate the thousands of internal user and computer accounts to the locations mentioned above - a new domain, whether it's in a new forest or not, would eliminate this unwanted replication. There's no need to replicate the usr and cptr accounts, but there might be a need to replicate things like GC info for an Exchange address book? Replication has become very efficient in W2003 and I wouldn't be surprised if replication traffic wouldn't pose a problem. It really depends on the bandwith you have, but I havn't seen many implementations in which replication traffic forced me to implements multiple forest/domains. 3) some applications require access by vendors, suppliers, etc. There is some desire to keep such accounts physically seperate from the internal directory. Part of this was because many intranet resources are granted to 'authenticated users', and people have a hard time realizing that some clerk at one of our suppliers is just as much an 'authenticated user' as an internal employee[1]. If such accounts were in a completely isolated forest (no trusts), they would not be authenticated users in our internal domain. Yep! This calls for a federated forest construction. But are these applications located at the 'evil' sites or is this a totally different geographical spreading that might require an additional forest in the centrally managed site? What I'm trying to figure out is whether a seperate forest with trusts in both directions (with SA and SID Filtering) gets me closer to the objective than a new domain in the existing forest. It seems to me that a new domain in the existing forest would take care of #2, but not the other issues, which brings up the new forest idea. I just don't want to introduce a new forest only to find that the required trusts put me right back in the same situation as if I had just added a child domain to the existing forest. Comments ? The most obvious way to ensure 1 and 3 (I don't consider 2 to be a 'real' issue, but just one of those arguments that comes in handy to add another one to the list of pro's to achieve your goal ;-), is a separate Forest. This does not put you right back in the same situation, because several extra steps are introduced that makes it tougher to do whatever you're not allowed to do on the other side. From a technical point of view, the FedFor construction with SA and Sidfiltering (be aware that this breaks SIDHistory!) is a very solid solution. This does not give you a 100% safety garanty. You will need to monitor your environment (non techical/social hacking can be far more dangerous!) for strange events. [1]Yeah, I know that I could put them in another OU, and the resources should really be ACLed so only intended groups have access instead of relying on 'authenticated users'. Maybe that's the path I should push for regarding #3 - your comments are welcome! Duh ... No further comments your honour! I rest my case ... Cheers! John Reijnders -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of John Reijnders Sent: Friday, January 07, 2005 1:42 AM To: 'ActiveDir@mail.activedir.org' Subject: RE: [ActiveDir] Forest trusts vs trusts within forests Happy New Year to you as well! In order to make a good decision for yourself whether or not you can and need to protect yourself against clever DomaAdmins, Service Admins and/or people with physical access to your DC's some extra info: Ways to bypass standard security: - Add the Enterprise Admin SID to your token (ex in you SidHistory). This can be done by using a 'improved' version of kerberos.dll, which will add the enterpr adm sid to every service ticket. - You can modify the system software or Directory db to bypass sec checks by: o Changing the default sec.descriptor for an objclass o Add a user to the enterprise adm Univ.Group on a GC o Execute a logon script in a site GPO - Or schedule an AT job which runs under local system credentials. (Partial) solutions to these problems are: * Delegation of control * Physical protection of ALL DCs * SID filtering (enabled by default) * Pro active Monitoring (!) * Multiple Forests (!!) Some benefits of W2K3 trusts: * Transitive (not really a sexy feature in you 2 single dom forest design) * You can use kerberos logon in stead of NTLM. * You can use both implicit and explicit UPN logon over the trust Selective Authentication (which is disabled by default and applies to external, realm and forest trusts): This option provides a method that you can use to achieve better granularity for authentication requests that come across a trust. When you enable it, all authentication is examined on the service DC. The service DC verifies that the user is explicitly allowed to authenticate to the resource before allowing the authentication request through. Because of this, you need to specify which users who come across the trust can authenticate to which resources in the domain when you enable the SA option across a trust. You can do this if you set up the "Allowed to Authenticate" control access right on an object for that particular user or group from the other forest or domain. When a user authenticates across a trust with the SA option enabled, a special "Other Organization" SID is added to the user's authorization data. The presence of this SID triggers a verification on the service domain to ensure that the user is allowed to authenticate to the particular service. After the user is authenticated, the server to which the user authenticates adds another SID, the "This Organization" SID. * You can disable the corresponding DomainInfo record for the domain or the TopLevelName record for the tree in the UI. This method is useful when only a small part (read domain) of the other forest is not trusted. Note that only authentication requests from users in that domain are disabled when you disable a DomainInfo record. When you disable a DomainInfo record, authentication requests are not disabled if those authentication requests are received from users who are in the local forest if those users want to gain access to resources that are in the disabled domain. This is not really applicable in your scenario. If you're going for the multiple forest scenario, consider the security benefits this will give you and compare them to the additional costs (extra hardware, no super GC is available by default unless you start using stuff like MIIS :-), extra management, etc.). Let us know what you end up with and ... why ;-) Cheers, John Reijnders -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Fugleberg, David A Sent: donderdag 6 januari 2005 21:32 To: activedir@mail.activedir.org Subject: [ActiveDir] Forest trusts vs trusts within forests Happy New Year ! I'm having a design discussion with myself about adding a forest vs adding a domain to an existing forest. I understand about the automatic transitive trust between domains in a forest, and how it's possible for a clever domain admin in a subdomain to compromise the entire forest. What I'm shaky on is this: If you had two single-domain forests, and established trusts in both directions between them, do you have the same issues ? I would think not, because the configuration and schema NCs are not shared between them, but I'm looking for some confirmation on that. Also, since we're talking about two single-domain forests, I'm guessing that the 'forest trusts' available in W2K3 FFL don't really come into play here, correct ? In other words, getting the first domain to W2K3 FFL doesn't buy anything with respect to this trust ? Thanks, Dave List info : http://www.activedir.org/mail_list.htm List FAQ : http://www.activedir.org/list_faq.htm List archive: http://www.mail-archive.com/activedir%40mail.activedir.org/ This e-mail and any attachment is for authorised use by the intended recipient(s) only. It may contain proprietary material, confidential information and/or be subject to legal privilege. It should not be copied, disclosed to, retained or used by, any other party. If you are not an intended recipient then please promptly delete this e-mail and any attachment and all copies and inform the sender. Thank you. This e-mail and any attachment is for authorised use by the intended recipient(s) only. It may contain proprietary material, confidential information and/or be subject to legal privilege. It should not be copied, disclosed to, retained or used by, any other party. If you are not an intended recipient then please promptly delete this e-mail and any attachment and all copies and inform the sender. Thank you. 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