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
       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 "nis-
       plus".  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 ser-
                       vices, 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  connec-
                       tions.  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, pro-
                       cessing 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 mecha-
       nism, 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 4.15 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)