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 resolution 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 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.

If System Security Services Daemon (SSSD) is installed on your system, you can use this service with the `sss' keyword. SSSD supports the following databases: passwd, group, services and netgroup.

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'. However, if the selected action for the `group' database is `return', the next lookup function is always called, without affecting the search result.

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'.

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 3.22 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/.