nsswitch.conf - System Databases and Name Service Switch configuration file
DESCRIPTION
Various functions in the C Library need to be configured to work
correctly in the local environment.
Traditionally, this was done by
using files (e.g., /etc/passwd), but other nameservices (like the
Network Information Service (NIS) and the Domain Name Service (DNS))
became popular, and were hacked into the C library, usually with a fixed
search order.
The Linux libc5 with NYS support and the GNU C Library 2.x (libc.so.6)
contain a cleaner solution of this problem.
It is designed after a method
used by Sun Microsystems in the C library of Solaris 2.
We follow their
name and call this scheme "Name Service Switch" (NSS).
The sources for
the "databases" and their lookup order are specified in the
/etc/nsswitch.conf
file.
The following databases are available in the NSS:
aliases
Mail aliases, used by
sendmail(8).
Presently ignored.
An example
/etc/nsswitch.conf
(namely, the default used when
/etc/nsswitch.conf
is missing):
passwd:
compat
group:
compat
shadow:
compat
hosts:
dns [!UNAVAIL=return] files
networks:
nis [NOTFOUND=return] files
ethers:
nis [NOTFOUND=return] files
protocols:
nis [NOTFOUND=return] files
rpc:
nis [NOTFOUND=return] files
services:
nis [NOTFOUND=return] files
The first column is the database.
The rest of the line specifies how the lookup process works.
You can specify the way it works for each database individually.
The configuration specification for each database can contain two
different items:
* The service specification like `files', `db', or `nis'.
* The reaction on lookup result like `[NOTFOUND=return]'.
For libc5 with NYS, the allowed service specifications are `files', `nis',
and `nisplus'.
For hosts, you could specify `dns' as extra service, for
passwd and group `compat', but not for shadow.
For glibc, you must have a file called
/lib/libnss_SERVICE.so.X
for every SERVICE you are using.
On a standard installation, you could use
`files', `db', `nis', and `nisplus'.
For hosts, you could specify `dns' as
extra service, for passwd, group, and shadow `compat'.
These services will not
be used by libc5 with NYS.
The version number
X
is 1 for glibc 2.0 and 2 for glibc 2.1.
The second item in the specification gives the user much finer
control on the lookup process.
Action items are placed between two
service names and are written within brackets.
The general form is
`[' ( `!'? STATUS `=' ACTION )+ `]'
where
STATUS => success | notfound | unavail | tryagain
ACTION => return | continue
The case of the keywords is insignificant.
The STATUS values are
the results of a call to a lookup function of a specific service.
They mean:
success
No error occurred and the wanted entry is returned.
The default
action for this is `return'.
notfound
The lookup process works ok but the needed value was not found.
The default action is `continue'.
unavail
The service is permanently unavailable.
This can either mean the
needed file is not available, or, for DNS, the server is not
available or does not allow queries.
The default action is
`continue'.
tryagain
The service is temporarily unavailable.
This could mean a file is
locked or a server currently cannot accept more connections.
The default action is `continue'.
Interaction with +/- syntax (compat mode)
Linux libc5 without NYS does not have the name service switch but does
allow the user some policy control.
In
/etc/passwd
you could have entries of the form +user or +@netgroup
(include the specified user from the NIS passwd map),
-user or -@netgroup (exclude the specified user),
and + (include every user, except the excluded ones, from the NIS
passwd map).
Since most people only put a + at the end of
/etc/passwd
to include everything from NIS, the switch provides a faster
alternative for this case (`passwd: files nis') which doesn't
require the single + entry in
/etc/passwd,
/etc/group,
and
/etc/shadow.
If this is not sufficient, the NSS `compat' service provides full
+/- semantics.
By default, the source is `nis', but this may be
overridden by specifying `nisplus' as source for the pseudo-databases
passwd_compat,
group_compat
and
shadow_compat.
These pseudo-databases are only available in GNU C Library.
FILES
A service named SERVICE is implemented by a shared object library named
libnss_SERVICE.so.X
that resides in
/lib.
/etc/nsswitch.conf
configuration file
/lib/libnss_compat.so.X
implements `compat' source for glibc2
/lib/libnss_db.so.X
implements `db' source for glibc2
/lib/libnss_dns.so.X
implements `dns' source for glibc2
/lib/libnss_files.so.X
implements `files' source for glibc2
/lib/libnss_hesiod.so.X
implements `hesiod' source for glibc2
/lib/libnss_nis.so.X
implements `nis' source for glibc2
/lib/libnss_nisplus.so.2
implements `nisplus' source for glibc 2.1
NOTES
Within each process that uses
nsswitch.conf,
the entire file is read only once; if the file is later changed, the
process will continue using the old configuration.
With Solaris, it isn't possible to link programs using the NSS Service
statically.
With Linux, this is no problem.
COLOPHON
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
