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nsswitch.conf(5) File Formats 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.
Delegation of subordinate user/group IDs can be configured using
the subid database. Refer to subuid(5) and subgid(5) for more
details.
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
Starting with glibc 2.33, nsswitch.conf is automatically reloaded
if the file is changed. In earlier versions, the entire file was
read only once within each process. If the file was later
changed, the process would 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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
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Linux man-pages 6.15 2025-05-17 nsswitch.conf(5)
Pages that refer to this page: getent(1), gethostbyname(3), host.conf(5), nss(5), resolv.conf(5), hostname(7), pcap-filter(7), nscd(8), nss-myhostname(8), nss-mymachines(8), nss-resolve(8), nss-systemd(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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