Of course, you didn't buy this book to read about the Domain Name System. And it's not likely that you were looking for a general discussion of directory services. This book is about a particular kind of directory service?namely, a service for directories that implement the Lightweight Directory Access Protocol (LDAP). LDAP has become somewhat of a buzzword in contemporary IT shops. If you are like me, sometimes you just have to ask, "Why all the fuss?" The fuss is not so much about LDAP itself, but about the potential of LDAP to consolidate existing services into a single directory that can be accessed by LDAP clients from various vendors. These clients can be web browsers, email clients, mail servers, or any one of a myriad of other applications.
By consolidating information into a single directory, you are not simply pouring the contents of your multitude of smaller pots into a larger pot. By organizing your information well and thinking carefully about the common information needed by client applications, you can reduce data redundancy in your directories and therefore reduce the administrative overhead needed to maintain that data. Think about all the directory services that run on your network and consider how much information is duplicated. Perhaps hosts on your network use a DHCP server. This server has a certain amount of information about IP addresses, Ethernet addresses, hostnames, network topology, and so forth in its configuration files. Which other applications use the same or similar information and could share it if it were stored in a directory server? DNS comes immediately to mind, as does NIS. If you have networked printers as well, think about the amount of information that's replicated on each client of the printing system (for example, /etc/printcap files).
Now consider the applications that use your user account information. The first ones that probably come to mind are authentication services: users need to type usernames and passwords to log in. Your mail server probably uses the same username information for mail routing, as well as for services such as mailing lists. There may also be online phone books that keep track of names, addresses, and phone numbers, as well as personnel systems that keep track of job classifications and pay scales.
Imagine the administrative savings that would result if all the redundant data on your network could be consolidated in a single location. What would it take to delete a user account? We all know what that takes now: you delete the user from /etc/passwd, remove him by hand from any mailing lists, remove him from the company phone list, and so on. If you're clever, you've probably written a script or two to automate the process, but you're still manipulating the same information that's stored in several different places. What if there was a single directory that was the repository for all this information, and deleting a user was simply a matter of removing some records from this directory? Life would become much simpler. Likewise, what would it take to track host-related information? What would it be worth to you if you could minimize the possibility that machines and users use out-of-date information?
This sounds like a network administrator's utopia. However, I believe that as more and more client applications use LDAP directories, making an investment in setting up an LDAP server will have a huge payoff long-term. Realistically, we're not headed for a utopia. We're going to be responsible for more servers and more services, running on more platforms. The dividends of our LDAP investment come when we significantly reduce the number of directory technologies that we have to understand and administer. That is our goal.