NSSWITCH.CONF(5) Linux Programmer's Manual NSSWITCH.CONF(5)
nsswitch.conf - Name Service Switch configuration file
The Name Service Switch (NSS) configuration file, /etc/nsswitch.conf,
is used by the GNU C Library and certain other applications 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
remaining 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
related functions.
initgroups Supplementary group access list, used by getgrouplist(3)
function.
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 functions.
passwd User passwords, used by getpwent(3) and related
functions.
protocols Network protocols, used by getprotoent(3) and related
functions.
publickey Public and secret keys for Secure_RPC used by NFS and
NIS+.
rpc Remote procedure call names and numbers, used by
getrpcbyname(3) and related functions.
services Network services, used by getservent(3) and related
functions.
shadow Shadow user passwords, used by getspnam(3) and related
functions.
The GNU C Library ignores databases with unknown names. Some
applications use this to implement special handling for their own
databases. For example, sudo(8) consults the sudoers database.
Here is an example /etc/nsswitch.conf file:
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 name. The remaining columns
specify:
* One or more service specifications, for example, "files", "db", or
"nis". The order of the services on the line determines the order
in which those services will be queried, in turn, until a result
is found.
* 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
presence 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 "compat" (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
preceding data source. Action items take the general form:
[STATUS=ACTION]
[!STATUS=ACTION]
where
STATUS => success | notfound | unavail | tryagain
ACTION => return | continue | merge
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
"continue".
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".
The ACTION value can be one of:
return Return a result now. Do not call any further lookup
functions. However, for compatibility reasons, if
this is the selected action for the group database
and the notfound status, and the configuration file
does not contain the initgroups line, the next lookup
function is always called, without affecting the
search result.
continue Call the next lookup function.
merge [SUCCESS=merge] is used between two database entries.
When a group is located in the first of the two group
entries, processing will continue on to the next one.
If the group is also found in the next entry (and the
group name and GID are an exact match), the member
list of the second entry will be added to the group
object to be returned. Available since glibc 2.24.
Note that merging will not be done for getgrent(3)
nor will duplicate members be pruned when they occur
in both entries being merged.
Compatibility mode (compat)
The NSS "compat" service is similar to "files" except that it
additionally permits special entries in corresponding files for
granting users or members of netgroups access to the system. The
following entries are valid in this mode:
For passwd and shadow databases:
+user Include the specified user from the NIS
passwd/shadow map.
+@netgroup Include all users in the given netgroup.
-user Exclude the specified user from the NIS
passwd/shadow map.
-@netgroup Exclude all users in the given netgroup.
+ Include every user, except previously excluded
ones, from the NIS passwd/shadow map.
For group database:
+group Include the specified group from the NIS group
map.
-group Exclude the specified group from the NIS group
map.
+ Include every group, except previously excluded
ones, from the NIS group map.
By default, the source is "nis", but this may be overridden by
specifying any NSS service except "compat" itself 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.
The following files are read when "files" source is specified for
respective databases:
aliases /etc/aliases
ethers /etc/ethers
group /etc/group
hosts /etc/hosts
initgroups /etc/group
netgroup /etc/netgroup
networks /etc/networks
passwd /etc/passwd
protocols /etc/protocols
publickey /etc/publickey
rpc /etc/rpc
services /etc/services
shadow /etc/shadow
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 Service (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.
getent(1), nss(5)
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latest version of this page, can be found at
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Linux 2017-05-03 NSSWITCH.CONF(5)
Pages that refer to this page: getent(1), gethostbyname(3), host.conf(5), resolv.conf(5), nscd(8), nss-myhostname(8), nss-mymachines(8), nss-resolve(8), nss-systemd(8)