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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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