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man:nsswitch

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
                 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
                 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:         com-
         pat
         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
                     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
                     duplicate  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.
             +@netgroup  Include all users in the given netgroup.
  1. user Exclude the specified user from the NIS

passwd/shadow map.

  1. @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.
  1. 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
     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 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

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

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