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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 and certain other applications  to  deter-
       mine  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 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  re-
                   lated 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
                   functions.

       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.

       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
       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 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  ac-
                       cept  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  du-
                       plicate  members  be pruned when they occur in both en-
                       tries 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
       Within each process that uses nsswitch.conf, the entire  file  is  read
       only once.  If the file is later changed, the process will continue us-
       ing 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  Ser-
       vice  (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.

SEE ALSO
       getent(1), nss(5)

COLOPHON
       This  page  is  part of release 5.05 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
       https://www.kernel.org/doc/man-pages/.

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

NAME | DESCRIPTION | FILES | NOTES | SEE ALSO | COLOPHON