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NSSWITCH.CONF(5)	   Linux Programmer's Manual	      NSSWITCH.CONF(5)



NAME
       nsswitch.conf  - System Databases and Name Service Switch configuration
       file

DESCRIPTION
       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:

       aliases
	      Mail aliases, provide a system-wide mechanism to	redirect  mail
	      for local recipients. Used by mail transfer agents such as Post-
	      fix or sendmail(8).  Note: On Linux,  unlike  on	other  Unixes,
	      Sendmail	uses  its own aliases resolution system independent of
	      '/etc/nsswitch.conf' by default.

       ethers Ethernet numbers.

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

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

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

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

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

       protocols
	      Network protocols, used by getprotoent(3) 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 similar functions.

       services
	      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  ser-
       vices  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 )+ ']'

       where

       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
       mean:

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

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

       unavail
	      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
	      '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 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_com-
       pat,  group_compat  and	shadow_compat.	This pseudo-databases are only
       available in GNU C Library.

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



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