Ubuntu Manpage: nsswitch.conf - Name Service Switch configuration file

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 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.

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

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.

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_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.

COLOPHON

       This page is part of release 4.04 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
       http://www.kernel.org/doc/man-pages/.

Linux                                              2015-12-05                                   NSSWITCH.CONF(5)

NAME

DESCRIPTION

FILES

NOTES

SEE ALSO

COLOPHON