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       nsswitch.conf  - System Databases and Name Service Switch configuration


       Various functions in the C  Library  need  to  be  configured  to  work
       correctly  in  the  local environment.  Traditionally, this was done by
       using files (e.g., ‘/etc/passwd’), but  other  nameservices  (like  the
       Network  Information  Service  (NIS) and the Domain Name Service (DNS))
       became popular, and were hacked into the  C  library,  usually  with  a
       fixed search order.

       The  Linux libc5 with NYS support and the GNU C Library 2.x (
       contain a cleaner solution of this problem.  It  is  designed  after  a
       method  used  by  Sun  Microsystems  in  the C library of Solaris 2. We
       follow their name and call this scheme "Name Service Switch" (NSS). The
       sources for the "databases" and their lookup order are specified in the
       /etc/nsswitch.conf file.

       The following databases are available in the NSS:

              Mail aliases, used by sendmail(8).  Presently ignored.

       ethers Ethernet numbers.

       group  Groups of users, used by getgrent(3) functions.

       hosts  Host names and numbers, used  by  gethostbyname(3)  and  similar

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

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

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

              Network protocols, used by getprotoent(3) functions.

              Public and secret keys for Secure_RPC used by NFS and NIS+.

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

              Network services, used by getservent(3) functions.

       shadow Shadow user passwords, used by getspnam(3).

       An   example   /etc/nsswitch.conf   (namely,   the  default  used  when
       /etc/nsswitch.conf is missing):

       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.  The rest of the line  specifies  how
       the  lookup  process  works.  You can specify the way it works for each
       database individually.

       The configuration specification  for  each  database  can  contain  two
       different items:
       * The service specification like ‘files’, ‘db’, or ‘nis’.
       * The reaction on lookup result like ‘[NOTFOUND=return]’.

       For  libc5  with  NYS,  the allowed service specifications are ‘files’,
       ‘nis’, and ‘nisplus’. For hosts,  you  could  specify  ‘dns’  as  extra
       service, for passwd and group ‘compat’, but not for shadow.

       For  glibc,  you  must  have a file called /lib/ for
       every SERVICE you are using. On a standard installation, you could  use
       ‘files’, ‘db’, ‘nis’, and ‘nisplus’. For hosts, you could specify ‘dns’
       as extra  service,  for  passwd,  group,  and  shadow  ‘compat’.  These
       services will not be used by libc5 with NYS.  The version number X is 1
       for glibc 2.0 and 2 for glibc 2.1.

       The second item in the specification gives the user much finer  control
       on  the  lookup  process.   Action items are placed between two service
       names and are written within brackets.  The general form is

       ‘[’ ( ‘!’? STATUS ‘=’ ACTION )+ ‘]’


       STATUS => success | notfound | unavail | tryagain
       ACTION => return | continue

       The case of the keywords is insignificant. The STATUS  values  are  the
       results  of  a  call  to a lookup function of a specific service.  They

              No error occurred and the wanted entry is returned. The  default
              action for this is ‘return’.

              The  lookup process works ok but the needed value was not found.
              The default action is ‘continue’.

              The service is permanently unavailable.  This  can  either  mean
              the needed file is not available, or, for DNS, the server is not
              available or does not allow  queries.   The  default  action  is

              The  service is temporarily unavailable.  This could mean a file
              is locked or a server currently cannot accept more  connections.
              The default action is ‘continue’.

   Interaction with +/- syntax (compat mode)
       Linux  libc5 without NYS does not have the name service switch but does
       allow the user some policy  control.  In  /etc/passwd  you  could  have
       entries  of  the  form  +user or +@netgroup (include the specified user
       from the NIS passwd map), -user or -@netgroup  (exclude  the  specified
       user),  and  +  (include every user, except the excluded ones, from the
       NIS passwd map). Since  most  people  only  put  a  +  at  the  end  of
       /etc/passwd  to  include  everything  from  NIS,  the switch provides a
       faster alternative for this case (‘passwd: files  nis’)  which  doesn’t
       require the single + entry in /etc/passwd, /etc/group, and /etc/shadow.
       If this is not sufficient, the NSS ‘compat’ service provides  full  +/-
       semantics.  By default, the source is ‘nis’, but this may be overridden
       by  specifying   ‘nisplus’   as   source   for   the   pseudo-databases
       passwd_compat,  group_compat  and shadow_compat.  This pseudo-databases
       are only available in GNU C Library.


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

       /etc/nsswitch.conf       configuration file
       /lib/  implements ‘compat’ source for glibc2
       /lib/      implements ‘db’ source for glibc2
       /lib/     implements ‘dns’ source for glibc2
       /lib/   implements ‘files’ source for glibc2
       /lib/  implements ‘hesiod’ source for glibc2
       /lib/     implements ‘nis’ source for glibc2
       /lib/ implements ‘nisplus’ source for glibc 2.1


       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.
       With  Solaris, it isn’t possible to link programs using the NSS Service
       statically. With Linux, this is no problem.

       On a Debian system other mail transport agents may or  may  not  ignore
       the  aliases  file.   For example, unlike sendmail Exim does not ignore