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NSSWITCH.CONF(5)	   Linux Programmers Manual	     NSSWITCH.CONF(5)

       nsswitch.conf  - System Databases and Name Service Switch configuration

       Various functions in the C Library need to be configured to  work  cor
       rectly 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 (libc.so.6)
       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 fol
       low 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/nss
       witch.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  dif
       ferent 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 ser
       vice, for passwd and group compat, but not for shadow.

       For  glibc,  you  must  have a file called /lib/libnss_SERVICE.so.X 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).

       You can override certain passwd fields for a particular user  from  the
       NIS   passwd   map  by  using  the  extended  form  of  +user::::::  in
       /etc/passwd.  Non-empty fields override information in the  NIS	passwd

       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 doesnt 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.  These pseudo-databases are  only  available  in	GNU  C

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

       /etc/nsswitch.conf	configuration file
       /lib/libnss_compat.so.X	implements compat source for glibc2
       /lib/libnss_db.so.X	implements db source for glibc2
       /lib/libnss_dns.so.X	implements dns source for glibc2
       /lib/libnss_files.so.X	implements files source for glibc2
       /lib/libnss_hesiod.so.X	implements hesiod source for glibc2
       /lib/libnss_nis.so.X	implements nis source for glibc2
       /lib/libnss_nisplus.so.2 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 isnt 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

       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  1999-01-17		      NSSWITCH.CONF(5)

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