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 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:

       aliases
	      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.

       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/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 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 )+ `]'

       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).

       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 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 suffi-
       cient, 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 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.

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

COLOPHON

       This page is part of release 3.27 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)