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
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.
- nisplus or
nis+
Use the NIS+ server for this domain. The location of the server is obtained
from the /etc/nis.conf file.
- dns
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.
- files
Use the local file, such as the /etc/hosts file for the
hosts entry.
- compat
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.
- db
Look up the information from DBM files located in the
/var/db directory. The corresponding NIS map name is used for that file.
Currently, the NIS support in GNU libc caters to the following
nsswitch.conf databases:
aliases,
ethers.group,
hosts,
netgroup,
network,
passwd,
protocols,
publickey,
rpc,
services, and
shadow.
More entries are likely to be added.
Example 13-2 shows a more complete example that
introduces another feature of nsswitch.conf. The
[NOTFOUND=return] keyword in the
hosts entry tells the NIS client to
return if the desired item couldn't be found in the NIS or DNS database. That
is, the NIS client will continue searching the local files
only if calls to the NIS and DNS servers fail for some
other reason. The local files will then be used only at boot time and as a
backup when the NIS server is down.
Example 13-2. Sample nsswitch.conf File
# /etc/nsswitch.conf
#
hosts: nis dns [NOTFOUND=return] files
networks: nis [NOTFOUND=return] files
services: files nis
protocols: files nis
rpc: files nis |
GNU libc provides some other actions that are described in the
nsswitch manpage.
