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       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 ( 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

              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

       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/ 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 mean:

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

              The  lookup  process  succeeded,  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

       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
       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.


       A   service   named   SERVICE   is   implemented   by   a   shared  object  library  named that resides in /lib.

       /etc/nsswitch.conf       configuration file
       /lib/  implements `compat' source for glibc2
       /lib/      implements `db' source for glibc2
       /lib/     implements `dns' source for glibc2
       /lib/   implements `files' source for glibc2
       /lib/  implements `hesiod' source for glibc2
       /lib/     implements `nis' source for glibc2
       /lib/ 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.

       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.


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