One of LDAP's chief advantages is its ability to consolidate multiple directory services into one. This chapter examines the pros and cons of using LDAP as a replacement for Sun's Network Information Service (NIS). NIS is used primarily by Unix clients to centralize management of user information and passwords, hostnames and IP addresses, automount maps (files that control the mounting of remote file systems), and other administrative information. NIS clients for other operating systems, such as Windows NT 4.0, exist, though they aren't particularly common.
 NIS was superseded by NIS+, which was never widely adopted. Describing how to replace NIS+ is beyond the scope of this book.
While the focus of this chapter is using an LDAP directory as a replacement for NIS domains, many other tools are used to distribute management information on Unix systems; for example, many sites use rsync(1) to push administrative files, such as /etc/passwd, to client machines. While this chapter assumes that you are replacing NIS with an LDAP directory, adapting these techniques I present to other schemes for sharing the data in /etc/passwd, /etc/hosts, and other key files should be straightforward:
Get the information you want to share into the directory.
Get your clients to use the directory.
Disable your old information-sharing mechanism.
There are two fundamental strategies for replacing NIS with an LDAP directory. The first solution, illustrated in Figure 6-1, involves setting up an NIS/LDAP gateway: i.e., an NIS server that accepts NIS queries, but answers the queries by retrieving information from an LDAP directory. This strategy doesn't require any client modifications, and therefore works with all NIS clients; it may be the easiest means of transitioning to a new LDAP-based information service. Sun Microsystems Directory Server 4.x supports this approach. Sadly, newer releases (5.x) of Sun's directory services product do not. An alternative solution is the NIS/LDAP gateway provided by a company named PADL Software (http://www.padl.com/). This gateway product is available for servers running Solaris, Linux, FreeBSD, or AIX, and will be discussed later in this chapter.
The second solution involves making a complete transition to LDAP. If you are willing to disable NIS lookups on all of your clients and install the necessary LDAP libraries and modules, you may prefer this approach. Clients access information directly from an LDAP directory, eliminating the gateway. Many modern operating systems support pluggable information retrieval modules; for example, Unix and Unix-like systems such as Solaris and Linux can use the LDAP Pluggable Authentication Modules (PAM) and Name Server Switch (NSS) modules that have been released by PADL Software under the GNU Lesser General Public License (LGPL). If you are unfamiliar with PAM and NSS, read the brief overview found in Appendix A.
 More information on the LGPL license can be found at http://www.fsf.org/licenses/licenses.html#LGPL.
To implement either solution?an NIS/LDAP gateway server or LDAP-enabled client lookups?we must define the attribute types and object classes needed to move the information served by NIS (or saved in static system files) into an LDAP directory.