nsswitch.conf(5) — Linux manual page


nsswitch.conf(5)           File Formats Manual          nsswitch.conf(5)

NAME         top

       nsswitch.conf - Name Service Switch configuration file

DESCRIPTION         top

       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

       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:

              Mail aliases, used by getaliasent(3) and related

       ethers Ethernet numbers.

       group  Groups of users, used by getgrent(3) and related

       hosts  Host names and numbers, used by gethostbyname(3) and
              related functions.

              Supplementary group access list, used by getgrouplist(3)

              Network-wide list of hosts and users, used for access
              rules.  C libraries before glibc 2.1 supported netgroups
              only over NIS.

              Network names and numbers, used by getnetent(3) and
              related functions.

       passwd User passwords, used by getpwent(3) and related functions.

              Network protocols, used by getprotoent(3) and related

              Public and secret keys for Secure_RPC used by NFS and

       rpc    Remote procedure call names and numbers, used by
              getrpcbyname(3) and related functions.

              Network services, used by getservent(3) and related

       shadow Shadow user passwords, used by getspnam(3) and related

       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.
       Delegation of subordinate user/group IDs can be configured using
       the subid database.  Refer to subuid(5) and subgid(5) for more

       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

       •  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 => 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:

                  No error occurred and the requested entry is returned.
                  The default action for this condition is "return".

                  The lookup succeeded, but the requested entry was not
                  found.  The default action for this condition is

                  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".

                  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.

                  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.

                      Include all users in the given netgroup.

               -user  Exclude the specified user from the NIS
                      passwd/shadow map.

                      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

               -group Exclude the specified group from the NIS group

               +      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

FILES         top

       A service named SERVICE is implemented by a shared object library
       named libnss_SERVICE.so.X that resides in /lib.

                  NSS configuration file.
                  implements "compat" source.
                  implements "db" source.
                  implements "dns" source.
                  implements "files" source.
                  implements "hesiod" source.
                  implements "nis" source.
                  implements "nisplus" source.

       The following files are read when "files" source is specified for
       respective databases:

           ethers /etc/ethers
           group  /etc/group
           hosts  /etc/hosts
           passwd /etc/passwd
           rpc    /etc/rpc
           shadow /etc/shadow

NOTES         top

       Starting with glibc 2.33, nsswitch.conf is automatically reloaded
       if the file is changed.  In earlier versions, the entire file was
       read only once within each process.  If the file was later
       changed, the process would 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         top

       getent(1), nss(5)

Linux man-pages (unreleased)     (date)                 nsswitch.conf(5)

Pages that refer to this page: getent(1)gethostbyname(3)host.conf(5)resolv.conf(5)hostname(7)nscd(8)nss-myhostname(8)nss-mymachines(8)nss-resolve(8)nss-systemd(8)