NSSWITCH.CONF(5)            Linux Programmer's 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

       initgroups  Supplementary group access list, used by getgrouplist(3)

       netgroup    Network-wide list of hosts and users, used for access rules.
                   C libraries before glibc 2.1 supported netgroups only over

       networks    Network names and numbers, used by getnetent(3) and related

       passwd      User passwords, used by getpwent(3) and related functions.

       protocols   Network protocols, used by getprotoent(3) and related

       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

       shadow      Shadow user passwords, used by getspnam(3) and related

       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

       *  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 => 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

           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

       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

   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

               +@netgroup  Include all users in the given netgroup.

               -user       Exclude the specified user from the NIS passwd/shadow

               -@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

       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.

       getent(1), nss(5)

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       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                              2017-05-03                   NSSWITCH.CONF(5)