Now that we've covered what Microsoft is doing with their directory products, let's review some of the issues around integrating a mixed directory environment. As we mentioned earlier, supporting multiple directories within a large organization is a necessary practice. You may already have several directories deployed, some of which are not Microsoft-based. A common question in this scenario is how to get your directories to work together.
Perhaps the most common use of a directory is to access employee, customer, or student information. One of the problems of supporting multiple directories is that for each directory to be useful, it needs to store similar data. It would be very helpful if there were a standard RFC that defined a replication scheme for LDAP directories, but unfortunately there is not. As a result, each directory vendor has implemented their own way to replicate data between servers. This is where metadirectories come into play. The primary purpose of a metadirectory is to facilitate data flow and provisioning across systems. If you have several directories, and writing your own scripts to replicate data is not a possibility, implementing a metadirectory is a valid option.
One of the biggest reasons for not wanting to implement multiple directories is that they have to be managed differently. Fortunately, both Active Directory and AD/AM are based on LDAP, so any of the standard LDAP SDK tools such as ldapsearch and ldapadd will work. Also, the Microsoft LDP tool, a graphical user interface for querying and managing content in Active Directory, has become very popular. LDP is an LDAP-based tool and works against any LDAP directory. The same cannot be said for tools such as ADSI Edit and the Active Directory administrative snap-ins, which works only with Active Directory.
One popular approach for managing content in SunONE and OpenLDAP directories is to use the LDAP Data Interchange Format (LDIF). LDIF has a strict format that is both human- and machine-readable, but it is easy to work with. Microsoft provides the LDIFDE program on the Windows Server platforms, which allows for importing and exporting LDIF files. You can also use an LDIF-based tool on a non-Windows platform to manage content in Active Directory.
The story for porting scripts is much the same as the one for using similar tools for managing different directories. Most directories today are LDAP-based, so if your scripts are using an LDAP API, they should work regardless of what directory is being used. That said, there are some fairly significant differences with how Active Directory was implemented that may cause problems in your scripts. Most LDAP directories, including AD/AM, have a flat namespace. That means you can make a single query to a server and retrieve all objects the server knows about. With Active Directory, it is a little different in multidomain environments. When you implement multiple domains, you are essentially segregating your LDAP namespace. A domain controller knows about only the objects in its domain. For this reason, Microsoft designed the Global Catalog so that you can perform a single query to search against all objects in a forest, but the GC contains only a subset of information for all objects. The impact to scripts may be less than obvious, but to perform a query such as retrieving all attributes for any user in the forest that has a department equal to "Sales", you first must query the GC. To then retrieve all defined attributes for each user, you have to run separate queries against the domains the users are in. The other option is to skip the GC and query the domains individually, but regardless this simple task can require several queries.
If you foresee supporting multiple directories, you might have the notion of trying to unify the namespace used by each. So perhaps your Active Directory root is dc=mycorp,dc=com and you have an OpenLDAP server that has a root at dc=apps,dc=mycorp,dc=com. You can create referral objects using the crossRef objectclass so that a query for dc=apps,dc=mycorp,dc=com against an Active Directory domain controller will refer the client to an OpenLDAP server. The LDIF representation of the referral object looks like the following, where nCName is the name of the partition and dNSRoot is the hostname to refer clients too:
dn: cn=OpenLDAP,cn=Partitions,cn=Configuration,dc=mycorp,dc=com objectclass: crossRef nCName: dc=apps,dc=mycorp,dc=com dNSRoot: openldap.mycorp.com
An issue with using referrals to access data in different directories is that clients from each directory typically can't authenticate in each. Unless you are synchronizing user accounts and passwords between directories or you allow anonymous binds, when the referral passes to the OpenLDAP server, the query will fail due to a logon failure.