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NSSWITCH.CONF(5) Linux Programmer's 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 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 data‐
base 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.

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 ser‐
vice, 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 Compati‐
bility 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

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 pre‐
ceding 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 con‐
dition 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 con‐
tain the initgroups line, the next lookup function is always called, with‐
out affecting the search result.

continue Call the next lookup function.

Compatibility mode (compat)
The NSS "compat" service is similar to "files" except that it additionally permits special
entries in /etc/passwd for granting users or members of netgroups access to the system.
The following entries are valid in this mode:

+user Include the specified user from the NIS passwd 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 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 specifying "nisplus" 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_SER‐
VICE.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.

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 popu‐
lar, 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 prob‐
lem.