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. Tradition-
ally, this was done by using files (e.g., `/etc/passwd'),
but other nameservices (like the Network Information Ser-
vice (NIS) and the Domain Name Service (DNS)) became popu-
lar, 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)
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
network
Network names and numbers, used by getnetent(3)
functions.
passwd User passwords, used by getpwent(3) functions.
protocols
Network protocols, used by getprotoent(3) func-
tions.
publickey
Public and secret keys for secure_rpc used by NIS+
and NFS.
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 file could be look like
(This is also the default if /etc/nsswitch.conf is miss-
ing):
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 con-
tain 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 GNU C Library, you must have a file called /lib/lib-
nss_SERVICE.so.1 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'. This Ser-
vices will not be used by libc5 with NYS.
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 val-
ues 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 has 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.
FILES
A service named SERVICE is implemented by a shared object
library named libnss_SERVICE.so.1 that resides in /lib.
/etc/nsswitch.conf configuration file
/lib/libnss_compat.so.1 implements `compat' source for
GNU C Library 2.x
/lib/libnss_db.so.1 implements `db' source for GNU C
Library 2.x
/lib/libnss_dns.so.1 implements `dns' source for GNU C
Library 2.x
/lib/libnss_files.so.1 implements `files' source for GNU
C Library 2.x
/lib/libnss_hesoid.so.1 implements `hesoid' source for
GNU C Library 2.x
/lib/libnss_nis.so.1 implements `nis' source for GNU C
Library 2.x
/lib/libnss_nisplus.so.1 implements `nisplus' source for
GNU C Library 2.x
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, is isn't possible to link programs using the
NSS Service statically. With Linux, this is no problem.