Manpage of NSSWITCH.CONF

NSSWITCH.CONF

Section: Linux Programmer's Manual (5)
Updated: 2016-03-15
Index
 

NAME

nsswitch.conf - Name Service Switch configuration file  

DESCRIPTION

The Name Service Switch (NSS) configuration file, /etc/nsswitch.conf, is used by the GNU C Library to determine the sources from which to obtain name-service information in a range of categories, and in what order. Each category of information is identified by a database name.

The file is plain ASCII text, with columns separated by spaces or tab characters. The first column specifies the database name. The remaining columns describe the order of sources to query and a limited set of actions that can be performed by lookup result.

The following databases are understood by the GNU C Library:

aliases
Mail aliases, used by getaliasent(3) and related functions.
ethers
Ethernet numbers.
group
Groups of users, used by getgrent(3) and related functions.
hosts
Host names and numbers, used by gethostbyname(3) and related functions.
initgroups
Supplementary group access list, used by getgrouplist(3) function.
netgroup
Network-wide list of hosts and users, used for access rules. C libraries before glibc 2.1 supported netgroups only over NIS.
networks
Network names and numbers, used by getnetent(3) and related functions.
passwd
User passwords, used by getpwent(3) and related functions.
protocols
Network protocols, used by getprotoent(3) and related 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 related functions.
services
Network services, used by getservent(3) and related functions.
shadow
Shadow user passwords, used by getspnam(3) and related functions.

Here is an example /etc/nsswitch.conffile:

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 name. The remaining columns specify:

*
One or more service specifications, for example, "files", "db", or "nis". The order of the services on the line determines the order in which those services will be queried, in turn, until a result is found.
*
Optional actions to perform if a particular result is obtained from the preceding service, for example, "[NOTFOUND=return]".

The service specifications supported on your system depend on the presence of shared libraries, and are therefore extensible. Libraries called /lib/libnss_SERVICE.so.Xwill provide the named SERVICE. On a standard installation, you can use "files", "db", "nis", and "nisplus". For the hostsdatabase, you can additionally specify "dns". For the passwd, group, and shadowdatabases, you can additionally specify "compat" (see Compatibility modebelow). The version number Xmay be 1 for glibc 2.0, or 2 for glibc 2.1 and later. On systems with additional libraries installed, you may have access to further services such as "hesiod", "ldap", "winbind" and "wins".

An action may also be specified following a service specification. The action modifies the behavior following a result obtained from the preceding data source. Action items take the general form:

[STATUS=ACTION]
[!STATUS=ACTION]

where

STATUS=> success| notfound| unavail| tryagain
ACTION=> return| continue| merge

The ! negates the test, matching all possible results except the one specified. The case of the keywords is not significant.

The STATUSvalue is matched against the result of the lookup function called by the preceding service specification, and can be one of:

success
No error occurred and the requested entry is returned. The default action for this condition is "return".
notfound
The lookup succeeded, but the requested entry was not found. The default action for this condition is "continue".
unavail
The service is permanently unavailable. This can mean either that the required file cannot be read, or, for network services, that the server is not available or does not allow queries. The default action for this condition 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 for this condition is "continue".

The ACTIONvalue can be one of:

return
Return a result now. Do not call any further lookup functions. However, for compatibility reasons, if this is the selected action for the groupdatabase and the notfoundstatus, and the configuration file does not contain the initgroupsline, the next lookup function is always called, without affecting the search result.
continue
Call the next lookup function.
merge
[SUCCESS=merge]is used between two database entries. When a group is located in the first of the two group entries, processing will continue on to the next one. If the group is also found in the next entry (and the group name and GID are an exact match), the member list of the second entry will be added to the group object to be returned. Available since glibc 2.24.
 

Compatibility mode (compat)

The NSS "compat" service is similar to "files" except that it additionally permits special entries in corresponding files for granting users or members of netgroups access to the system. The following entries are valid in this mode:

For passwdand shadowdatabases:

+user
Include the specified userfrom the NIS passwd/shadow map.
+@netgroup
Include all users in the given netgroup.
-user
Exclude the specified userfrom the NIS passwd/shadow map.
-@netgroup
Exclude all users in the given netgroup.
+
Include every user, except previously excluded ones, from the NIS passwd/shadow map.

For groupdatabase:

+group
Include the specified groupfrom the NIS group map.
-group
Exclude the specified groupfrom the NIS group map.
+
Include every group, except previously excluded ones, from the NIS group map.

By default, the source is "nis", but this may be overridden by specifying any NSS service except "compat" itself as the source for the pseudo-databases passwd_compat, group_compat, and shadow_compat.  

FILES

A service named SERVICEis implemented by a shared object library named libnss_SERVICE.so.Xthat resides in /lib.
/etc/nsswitch.confNSS configuration file.
/lib/libnss_compat.so.X
implements "compat" source.
/lib/libnss_db.so.X
implements "db" source.
/lib/libnss_dns.so.X
implements "dns" source.
/lib/libnss_files.so.X
implements "files" source.
/lib/libnss_hesiod.so.X
implements "hesiod" source.
/lib/libnss_nis.so.X
implements "nis" source.
/lib/libnss_nisplus.so.X
implements "nisplus" source.

The following files are read when "files" source is specified for respective databases:

aliases/etc/aliases
ethers
/etc/ethers
group
/etc/group
hosts
/etc/hosts
initgroups
/etc/group
netgroup
/etc/netgroup
networks
/etc/networks
passwd
/etc/passwd
protocols
/etc/protocols
publickey
/etc/publickey
rpc
/etc/rpc
services
/etc/services
shadow
/etc/shadow
 

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.

Traditionally, there was only a single source for service information, often in the form of a single configuration file (e.g., /etc/passwd). However, as other name services, such as the Network Information Service (NIS) and the Domain Name Service (DNS), became popular, a method was needed that would be more flexible than fixed search orders coded into the C library. The Name Service Switch mechanism, which was based on the mechanism used by Sun Microsystems in the Solaris 2 C library, introduced a cleaner solution to the problem.  

SEE ALSO

getent(1), nss(5)


 

Index

NAME
DESCRIPTION
Compatibility mode (compat)
FILES
NOTES
SEE ALSO

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