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, provides a system-wide mechanism to redirect mail for local recipients. Used by mail transfer agents
              such  as  Postfix or sendmail(8).  Note: On Linux, not like on other Unices, Sendmail uses its own aliases resolu-
              tion system independent on `/etc/nsswitch.conf'.

       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 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 installa-
       tion, 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 spe-
       cific 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 pro-
       vides 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_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.

COLOPHON
       This page is part of release 3.25 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)