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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 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.
- 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)
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 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'.
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_compat and
shadow_compat. This
pseudo-databases are only available in GNU C Library.
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
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.
