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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 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.

       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 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 | 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 "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.

           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 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 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 specifying 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 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.

SEE ALSO

       getent(1), nss(5)

COLOPHON

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