Choosing the Right Maps

Having made sure you can reach the NIS server, you have to decide which configuration files to replace or augment with NIS maps. Commonly, you will want use NIS maps for the host and password lookup functions. The former is especially useful if you do not run BIND. The latter permits all users to log into their account from any system in the NIS domain; this usually requires sharing a central /home directory between all hosts via NFS. It is explained detail in section- below. Other maps, like services.byname, aren't such a dramatic gain, but save you some editing work if you install any network applications that use a service name that's not in the standard services file.

Generally, you want to have some freedom of choice when a lookup function uses the local files, and when it queries the NIS server. NYS allows you to configure the order in which a function accesses these services. This is controlled through the /etc/nsswitch.conf file, which stands for Name Service Switch but of course isn't limited to the name service. For any of the data lookup functions supported by NYS, it contains a line naming the services to use.

The right order of services depends on the type of data. It is unlikely that the services.byname map will contain entries differing from those in the local services file; it may only contain more. So a good choice may be to query the local files first, and check NIS only if the service name wasn't found. Hostname information, on the other hand, may change very frequently, so that DNS or the NIS server should always have the most accurate account, while the local hosts file is only kept as a backup if DNS and NIS should fail. In this case, you would want to check the local file last.

The example below shows how to configure gethostbyname(2), gethostbyaddr(2), and getservbyname(2) functions as described above. They will try the listed services in turn; if a lookup succeeds, the result is returned, otherwise the next service is tried.

The complete list of services that may be used with an entry in the nsswitch.conf file is shown below. The actual maps, files, servers and objects being queried depend on the entry name.

Currently, NYS supports the following nsswitch.conf entries: hosts, networks, passwd, group, shadow, gshadow, services, protocols, rpc, and ethers. More entries are likely to be added.

Figure- shows a more complete example which introduces another feature of nsswitch.conf: the [NOTFOUND=return] keyword in the hosts entry tells NYS to return if the desired item couldn't be found in the NIS or DNS database. That is, NYS will continue and search the local files only if calls to the NIS and DNS servers failed for some other reason. The local files will then only be used at boot time and as a backup when the NIS server is down.

Figure: Sample nsswitch.conf file.