NSSWITCH.CONF
Section: Linux Programmer's Manual (5)
Updated: 2017-05-03
Page Index
NAME
nsswitch.conf - Name Service Switch configuration file
DESCRIPTION
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.
FILES
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
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.
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.
SEE ALSO
getent(1),
nss(5)
COLOPHON
This page is part of release 5.10 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.