NSSWITCH.CONF(5) BSD File Formats Manual 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 databases
containing information regarding hosts, users (passwords), groups, etc.
Each database comes from a source (such as local files, DNS, NIS, and
cache), and the order to look up the sources is specified in
Each entry in nsswitch.conf consists of a database name, and a space sep‐
arated list of sources. Each source can have an optional trailing crite‐
rion 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 occurs.
The following sources are implemented:
files Local files, such as /etc/hosts, and /etc/passwd.
db Local database.
dns Internet Domain Name System. “hosts” and ‘networks’ use IN class
entries, all other databases use HS class (Hesiod) entries.
nis NIS (formerly YP)
compat support ‘+/-’ in the “passwd” and “group” databases. If this is
present, it must be the only source for that entry.
cache makes use of the nscd(8) daemon.
The following databases are used by the following C library functions:
Database Used by
group getgrent(3), getgrent_r(3), getgrgid_r(3), getgrnam_r(3),
hosts getaddrinfo(3), gethostbyaddr(3), gethostbyaddr_r(3),
gethostbyname(3), gethostbyname2(3), gethostbyname_r(3),
networks getnetbyaddr(3), getnetbyaddr_r(3), getnetbyname(3),
passwd getpwent(3), getpwent_r(3), getpwnam_r(3), getpwuid_r(3),
rpc getrpcbyname(3), getrpcbynumber(3), getrpcent(3)
proto getprotobyname(3), getprotobynumber(3), getprotoent(3)
netgroup getnetgrent(3), setnetgrent(3), innetgr(3)
The following status codes are available:
success The requested entry was found.
notfound The entry is not present at this source.
tryagain The source is busy, and may respond to retries.
unavail The source is not responding, or entry is corrupt.
For each of the status codes, one of two actions is possible:
continue Try the next source
return Return with the current result
Format of 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 anything
else (i.e, [success=return notfound=continue unavail=continue
You can enable caching for the particular database by specifying “cache”
as the first source in the nsswitch.conf(5) file. You should also enable
caching for this database in nscd.conf(5). If for the particular query
“cache” source returns success, no further sources are queried. On the
other hand, if there are no previously cached data, the query result will
be placed into the cache right after all other sources are processed.
Note, that “cache” requires nscd(8) daemon to be running.
Compat mode: +/- syntax
In historical multi-source implementations, the ‘+’ and ‘-’ characters
are used to specify the importing of user password and group information
from NIS. Although nsswitch.conf provides alternative methods of access‐
ing 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 ‘compat’.
Historically, many of the databases had enumeration functions, often of
the form getXXXent(). These made sense when the databases were in local
files, but do not make sense or have lesser relevance when there are pos‐
sibly multiple sources, each of an unknown size. The interfaces are
still provided for compatibility, but the source may not be able to pro‐
vide complete entries, or duplicate entries may be retrieved if multiple
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 does not exist, or it has missing or
corrupt entries, nsdispatch(3) will default to an entry of “files” for
the requested database. Exceptions are:
Database Default source list
hosts files dns
/etc/nsswitch.conf The file nsswitch.conf resides in /etc.
To lookup hosts in cache, then in /etc/hosts and then from the DNS, and
lookup user information from NIS then files, use:
hosts: cache 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, do not try files." This treats nis as the authoritative
source of information, except when the server is down.
If system got compiled with WITHOUT_NIS you have to remove ‘nis’ entries.
FreeBSD's Standard C Library (libc, -lc) provides stubs for compatibility
with NSS modules written for the GNU C Library nsswitch interface. How‐
ever, these stubs only support the use of the “passwd” and “group” data‐
SEE ALSOnsdispatch(3), nscd.conf(5), resolv.conf(5), nscd(8), named(8), ypbind(8)HISTORY
The nsswitch.conf file format first appeared in FreeBSD 5.0. It was
imported from the NetBSD Project, where it appeared first in NetBSD 1.4.
Luke Mewburn ⟨email@example.com⟩ wrote this freely distributable name-ser‐
vice switch implementation, using ideas from the ULTRIX svc.conf(5) and
Solaris nsswitch.conf(4) manual pages.
BSD April 4, 2010 BSD