Linux Network Administrators Guide
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Chapter 13. The Network Information System
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13.7. 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 to use NIS maps for the host and password lookup functions. The former is especially useful if you do not have the BIND name service. The password lookup lets all users log into their accounts from any system in the NIS domain; this usually goes along with sharing a central /home directory among all hosts via NFS. The password map is explained detail in the next section.
Other maps, like services.byname , don't provide such dramatic gains, but do save you some editing work. The services.byname map is valuable if you install any network applications that use a service name not in the standard services file.
Generally, you want to have some choice of when a lookup function uses the local files, when it queries the NIS server, and when it uses other servers such as DNS. GNU libc 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 GNU libc, the file contains a line naming the services to use.
The right order of services depends on the type of data each service is offering. It is unlikely that the services.byname map will contain entries differing from those in the local services file; it will only contain additional entries. So it appears reasonable to query the local files first and check NIS only if the service name isn't found. Hostname information, on the other hand, may change very frequently, so 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. For hostnames, therefore, you normally want to check the local file last.
The following example shows how to force gethostbyname and gethostbyaddr to look in NIS and DNS before the hosts file and how to have the getservbyname function look in the local files before querying NIS. These resolver functions will try each of the listed services in turn; if a lookup succeeds, the result is returned; otherwise, they will try the next service in the list. The file setting for these priorities is:
# small sample /etc/nsswitch.conf
#
hosts: nis dns files
services: files nis
The following is a complete list of services and locations that may be used with an entry in the nsswitch.conf file. The actual maps, files, servers, and objects queried depend on the entry name. The following can appear to the right of a colon:
- nis
- nisplus or nis+
- dns
- files
- compat
- db
Use the current domain NIS server. The location of the server queried is configured in the yp.conf file, as shown in the previous section. For the hosts entry, the hosts.byname and hosts.byaddr maps are queried.
Use the NIS+ server for this domain. The location of the server is obtained from the /etc/nis.conf file.
Use the DNS name server. This service type is useful only with the hosts entry. The name servers queried are still determined by the standard resolv.conf file.
Use the local file, such as the /etc/hosts file for the hosts entry.
Be compatible with older file formats. This option can be used when either NYS or glibc 2.x is used for NIS or NIS+ lookups. While these versions normally can't interpret older NIS entries in passwd and group files, compat option allows them to work with those formats.
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