Authorization is the process of deciding if entity X is allowed to have access to resource Y. Determining the identity of X is the job of the authentication process. One task of authorization in computer networks is to define and determine which user has access to which computers in the network. On Linux, the tendency exists to create a local account for each single user who should be allowed to logon to a computer. This is typically the case because a user not only needs login privileges to a computer but also additional resources like a home directory to actually do some work. Creating a local account on every computer takes care of all this. The problem with this approach is that these local accounts can be inconsistent with each other. The same user name could have a different user ID and/or group ID on different computers. Even more problematic is when two different accounts share the same user ID and group ID on different computers: User joe on computer1 could have user ID 1234 and group ID 56 and user jane on computer2 could have the same user ID 1234 and group ID 56. This is a big security risk in case shared resources like NFS are used. These two different accounts are the same for an NFS server so that these users can wipe out each other's files. The solution to this inconsistency problem is to have only one central, authoritative data source for this kind of information and a means of providing all your computers with access to this central source. This is what a ''Directory Service'' is. The two directory services most widely used for centralizing authorization data are the Network Information Service (NIS, formerly known as Yellow Pages or YP) and Lightweight Directory Access Protocol (LDAP).
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