nsswitch.conf(5)nsswitch.conf(5)NAMEnsswitch.conf - name-service switch configuration file
The nsswitch.conf file specifies how the nsdispatch(3) (name-service
switch dispatcher) routines in the C library should operate.
The configuration file controls how a process looks up various data‐
bases containing information regarding hosts, users (passwords),
groups, net-groups, etc. Each database comes from a source (such as
local files, DNS, and NIS), and the order to look up the sources is
specified in nsswitch.conf.
Each entry in nsswitch.conf consists of a database name, and a space
separated list of sources. Each source can have an optional trailing
criterion that determines whether the next listed source is used, or
the search terminates at the current source. Each criterion consists
of one or more status codes, and actions to take if that status code
The following sources may be used: Description Local files, such as
/etc/hosts and /etc/passwd. Internet Domain Name System. ``hosts''
and `networks' use IN class entries, all other databases use HS class
(Hesiod) entries. NIS (formerly YP) support `+/-' in the ``passwd''
and ``group'' databases. If this is present, it must be the only source
for that entry.
The following databases use the switch: Used by getesdfent() getesd‐
The following status codes are available: Codes The requested entry was
found. The entry was not present at this source. The source is busy
and may respond to retires. The source is not responding, or entry is
For each of the status codes, one if two actions is possible: Descrip‐
tion Try the next source. Return with the current result.
Format of the File
A BNF description of the syntax of nsswitch.conf is:
<entry> ::= <database> ":" [<source> [<criteria>]]*
<criteria> ::= "[" <criterion>+ "]"
<criterion> ::= <status> "=" <action>
<status> ::= "success" | "notfound" | "unavail" | "tryagain"
<action> ::= "return" | "continue"
Each entry starts on a new line in the file. A `#' delimits a comment
to end of line. Blank lines are ignored. A `\' at the end of a line
escapes the newline, and causes the next line to be a continuation of
the current line. All entries are case-insensitive.
The default criteria is to return on ``success'', and continue on any‐
thing else (i.e, [success=return notfound=continue unavail=continue
Compat Mode: +/- syntax
In historical multi-source implementations, the `+' and `-' characters
are used to specify the importing of user password and group informa‐
tion from NIS. Although nsswitch.conf provides alternative methods of
accessing distributed sources such as NIS, specifying a sole source of
``compat'' will provide the historical behaviour.
An alternative source for the information accessed via `+/-' can be
used by specifying ``passwd_compat: source''. ``source'' in this case
can be `dns', `nis', or any other source except for `files' and `com‐
Historically, many of the databases had enumeration functions, often of
the form getXXXent(). These made sense when the databases were in
local files, but don't make sense or have lesser relevance when there
are possibly multiple sources, each of an unknown size. The interfaces
are still provided for compatibility, but the source may not be able to
provide complete entries, or duplicate entries may be retrieved if mul‐
tiple sources that contain similar information are specified.
To ensure compatibility with previous and current implementations, the
``compat'' source must appear alone for a given database.
Default Source Lists
If, for any reason, nsswitch.conf doesn't exist, or it has missing or
corrupt entries, nsdispatch(3) will default to an entry of ``files''
for the requested database. Exceptions are: Default Source List compat
nis files dns files [notfound=return] nis compat nis
To look up hosts in /etc/hosts and then from the DNS, and lookup user
information from NIS then files, use:
hosts: files dns
passwd: nis [notfound=return] files
group: nis [notfound=return] files
The criteria ``[notfound=return]'' sets a policy of "if the user is not
found in nis, don't try files." This treats nis as the authoritative
source of information, except when the server is down.
SEE ALSOgetesdfent(3), getesdvent(3), getesfient(3), getespwent(3), getest‐
cent(3), getgrent()getprent(3), gethostbyname(3), getnetgrent(3), get‐
netbyname(3), getpwent(3), getprotobyname(3), getnameinfo(3), nsdis‐