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The Name Service Switch (NSS) configuration file, /etc/nsswitch.conf,
is used by the GNU C Library to determine the sources from which to
obtain name-service information in a range of categories, and in what
order. Each category of information is identified by a database name.
The file is plain ASCII text, with columns separated by spaces or tab
characters. The first column specifies the database name. The remain-
ing columns describe the order of sources to query and a limited set of
actions that can be performed by lookup result.
The following databases are understood by the GNU C Library:
aliases Mail aliases, used by getaliasent(3) and related functions.
ethers Ethernet numbers.
group Groups of users, used by getgrent(3) and related functions.
hosts Host names and numbers, used by gethostbyname(3) and
initgroups Supplementary group access list, used by getgrouplist(3)
netgroup Network-wide list of hosts and users, used for access
rules. C libraries before glibc 2.1 supported netgroups
only over NIS.
networks Network names and numbers, used by getnetent(3) and related
passwd User passwords, used by getpwent(3) and related functions.
protocols Network protocols, used by getprotoent(3) and related func-
publickey Public and secret keys for Secure_RPC used by NFS and NIS+.
rpc Remote procedure call names and numbers, used by getrpcby-
name(3) and related functions.
services Network services, used by getservent(3) and related func-
shadow Shadow user passwords, used by getspnam(3) and related
Here is an example /etc/nsswitch.conf file:
* Optional actions to perform if a particular result is obtained from
the preceding service, for example, "[NOTFOUND=return]".
The service specifications supported on your system depend on the pres-
ence of shared libraries, and are therefore extensible. Libraries
called /lib/libnss_SERVICE.so.X will provide the named SERVICE. On a
standard installation, you can use "files", "db", "nis", and "nisplus".
For the hosts database, you can additionally specify "dns". For the
passwd, group, and shadow databases, you can additionally specify "com-
pat" (see Compatibility mode below). The version number X may be 1 for
glibc 2.0, or 2 for glibc 2.1 and later. On systems with additional
libraries installed, you may have access to further services such as
"hesiod", "ldap", "winbind" and "wins".
An action may also be specified following a service specification. The
action modifies the behavior following a result obtained from the pre-
ceding data source. Action items take the general form:
STATUS => success | notfound | unavail | tryagain
ACTION => return | continue
The ! negates the test, matching all possible results except the one
specified. The case of the keywords is not significant.
The STATUS value is matched against the result of the lookup function
called by the preceding service specification, and can be one of:
success No error occurred and the requested entry is returned.
The default action for this condition is "return".
notfound The lookup succeeded, but the requested entry was not
found. The default action for this condition is "con-
unavail The service is permanently unavailable. This can mean
either that the required file cannot be read, or, for
network services, that the server is not available or
does not allow queries. The default action for this
condition 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 for this
condition is "continue".
bers of netgroups access to the system. The following entries are
valid in this mode:
+user Include the specified user from the NIS passwd map.
+@netgroup Include all users in the given netgroup.
-user Exclude the specified user from the NIS passwd map.
-@netgroup Exclude all users in the given netgroup.
+ Include every user, except previously excluded ones,
from the NIS passwd map.
By default the source is "nis", but this may be overridden by specify-
ing "nisplus" as the source for the pseudo-databases passwd_compat,
group_compat, and shadow_compat.
A service named SERVICE is implemented by a shared object library named
libnss_SERVICE.so.X that resides in /lib.
/etc/nsswitch.conf NSS configuration file.
/lib/libnss_compat.so.X implements "compat" source.
/lib/libnss_db.so.X implements "db" source.
/lib/libnss_dns.so.X implements "dns" source.
/lib/libnss_files.so.X implements "files" source.
/lib/libnss_hesiod.so.X implements "hesiod" source.
/lib/libnss_nis.so.X implements "nis" source.
/lib/libnss_nisplus.so.X implements "nisplus" source.
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.
Traditionally, there was only a single source for service information,
often in the form of a single configuration file (e.g., /etc/passwd).
However, as other name services, such as the Network Information Ser-
vice (NIS) and the Domain Name Service (DNS), became popular, a method
was needed that would be more flexible than fixed search orders coded
into the C library. The Name Service Switch mechanism, which was based
on the mechanism used by Sun Microsystems in the Solaris 2 C library,
introduced a cleaner solution to the problem.
Linux 2013-02-12 NSSWITCH.CONF(5)