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NAME    [Toc]    [Back]

       nsswitch.conf  - System Databases and Name Service Switch configuration

DESCRIPTION    [Toc]    [Back]

       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 (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

	      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 file could be look like (This is also the
       default if /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 as you can guess from the table above.
       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
       * 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 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 overriden by specifying
       `nisplus' as source for the pseudo-databases passwd_compat,  group_com-
       pat and shadow_compat.  This pseudo-databases are only available in GNU
       C Library.

FILES    [Toc]    [Back]

       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    [Toc]    [Back]

       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)
[ Back ]
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