nsswitch.conf(5) — Linux manual page


NSSWITCH.CONF(5)          Linux Programmer's 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 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

       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.

       initgroups  Supplementary group access list, used by getgrouplist(3)

       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

       protocols   Network protocols, used by getprotoent(3) and related

       publickey   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.

       services    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.

       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 spec‐

       *  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.  Li‐
       braries called /lib/libnss_SERVICE.so.X will provide the named SER‐
       VICE.  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 addi‐
       tionally specify "compat" (see Compatibility mode below).  The ver‐
       sion 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:

           success     No error occurred and the requested entry is re‐
                       turned.  The default action for this condition is

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

           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 ac‐
                       tion 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 addi‐
       tionally 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.

               -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

               -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 speci‐
       fying any NSS service except "compat" itself as the source for the
       pseudo-databases passwd_compat, group_compat, and shadow_compat.

FILES         top

       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         top

       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

SEE ALSO         top

       getent(1), nss(5)

COLOPHON         top

       This page is part of release 5.09 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

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

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