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

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

       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

       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  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 functions.

	      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  func-

	      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 miss-

       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/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  brack-
       ets.  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 mean:

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

	      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 `continue'.

	      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 overriden 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 libnss_SER-
       VICE.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  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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