nsswitch.conf(5) File Formats Manual nsswitch.conf(5)
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 identi-
fied 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 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 func-
tions.
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-
tions.
shadow Shadow user passwords, used by getspnam(3) and related func-
tions.
The GNU C Library ignores databases with unknown names. Some applica-
tions use this to implement special handling for their own databases.
For example, sudo(8) consults the sudoers database. Delegation of sub-
ordinate user/group IDs can be configured using the subid database. Re-
fer 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 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 li-
braries installed, you may have access to further services such as "hes-
iod", "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=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 "con-
tinue".
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 dupli-
cate 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 addition-
ally 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.
+user:::::: Include the specified user from the NIS passwd map,
but override with non-empty passwd fields.
+@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 specify-
ing 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 re-
spective 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
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, in-
troduced a cleaner solution to the problem.
SEE ALSO
getent(1), nss(5)
Linux man-pages 6.9.1 2024-05-02 nsswitch.conf(5)
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