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.Dd October 25, 2009
.Dt NSSWITCH.CONF 5
.Os
.Sh NAME
.Nm nsswitch.conf
.Nd name-service switch configuration file
.Sh DESCRIPTION
The
.Nm
file specifies how the
.Xr nsdispatch 3
(name-service switch dispatcher) routines in the C library should operate.
.Pp
The configuration file controls how a process looks up various databases
containing information regarding hosts, users (passwords), groups,
netgroups, etc.
Each database comes from a source (such as local files, DNS, and
.Tn NIS ) ,
and the order to look up the sources is specified in
.Nm nsswitch.conf .
.Pp
Each entry in
.Nm
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 occurs.
.Ss Sources
The following sources are implemented:
.Bl -column "multicast_dns" -offset indent -compact
.It Sy Source   Description
.It files       Local files, such as
.Pa /etc/hosts ,
and
.Pa /etc/passwd .
.It dns Internet Domain Name System.
.Dq hosts
and
.Dq networks
use
.Sy IN
class entries, all other databases use
.Sy HS
class (Hesiod) entries.
.It mdnsd       Use
.Xr mdnsd 8
for
.Dq hosts
lookups, acting as both a system-wide cache for normal unicast DNS
as well as providing multicast DNS
.Dq ( zeroconf )
lookups.
.It multicast_dns       Use
.Xr mdnsd 8
only for multicast DNS
.Dq hosts
lookups.
This would normally be used in conjunction with
.Dq dns ,
which would then provide unicast DNS resolver functions.
.It nis NIS (formerly YP)
.It compat      support
.Sq +/-
in the
.Dq passwd
and
.Dq group
databases.
If this is present, it must be the only source for that entry.
.El
.Ss Databases
The following databases are used by the following C library functions:
.Bl -column "netgroup" -offset indent -compact
.It Sy Database Used by
.It group Ta Xr getgrent 3
.It hosts Ta Xr gethostbyname 3
.It netgroup Ta Xr getnetgrent 3
.It networks Ta Xr getnetbyname 3
.It passwd Ta Xr getpwent 3
.It shells Ta Xr getusershell 3
.El
.Ss Status codes
The following status codes are available:
.Bl -column "tryagain" -offset indent -compact
.It Sy Status   Description
.It success     The requested entry was found.
.It notfound    The entry is not present at this source.
.It tryagain    The source is busy, and may respond to retries.
.It unavail     The source is not responding, or entry is corrupt.
.El
.Ss Actions
For each of the status codes, one of two actions is possible:
.Bl -column "continue" -offset indent -compact
.It Sy Action   Description
.It continue    Try the next source
.It return      Return with the current result
.El
.Ss Format of file
A
.Tn BNF
description of the syntax of
.Nm
is:
.Bl -column "\*[Lt]criterion\*[Gt]" -offset indent
.It \*[Lt]entry\*[Gt]   ::=
\*[Lt]database\*[Gt] ":" [\*[Lt]source\*[Gt] [\*[Lt]criteria\*[Gt]]]*
.It \*[Lt]criteria\*[Gt]        ::=
"[" \*[Lt]criterion\*[Gt]+ "]"
.It \*[Lt]criterion\*[Gt]       ::=
\*[Lt]status\*[Gt] "=" \*[Lt]action\*[Gt]
.It \*[Lt]status\*[Gt]  ::=
"success" | "notfound" | "unavail" | "tryagain"
.It \*[Lt]action\*[Gt]  ::=
"return" | "continue"
.El
.Pp
Each entry starts on a new line in the file.
A
.Sq #
delimits a comment to end of line.
Blank lines are ignored.
A
.Sq \e
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.
.Pp
The default criteria is to return on
.Dq success ,
and continue on anything else (i.e,
.Li [success=return notfound=continue unavail=continue tryagain=continue]
).
.Ss Compat mode: +/- syntax
In historical multi-source implementations, the
.Sq +
and
.Sq -
characters are used to specify the importing of user password and
group information from
.Tn NIS .
Although
.Nm
provides alternative methods of accessing distributed sources such as
.Tn NIS ,
specifying a sole source of
.Dq compat
will provide the historical behaviour.
.Pp
An alternative source for the information accessed via
.Sq +/-
can be used by specifying
.Dq passwd_compat: source .
.Dq source
in this case can be
.Sq dns ,
.Sq nis ,
or
any other source except for
.Sq files
and
.Sq compat .
.Ss Notes
Historically, many of the databases had enumeration functions, often of
the form
.Fn 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 multiple sources that contain similar information are
specified.
.Pp
To ensure compatibility with previous and current implementations, the
.Dq compat
source must appear alone for a given database.
.Ss Default source lists
If, for any reason,
.Nm nsswitch.conf
doesn't exist, or it has missing or corrupt entries,
.Xr nsdispatch 3
will default to an entry of
.Dq files
for the requested database.
Exceptions are:
.Bl -column passwd_compat "files dns" -offset indent
.It Sy Database Default source list
.It group       compat
.It group_compat        nis
.It hosts       files dns
.It netgroup    files [notfound=return] nis
.It passwd      compat
.It passwd_compat       nis
.El
.Sh FILES
.Bl -tag -width /etc/nsswitch.conf -compact
.It Pa /etc/nsswitch.conf
The file
.Nm
resides in
.Pa /etc .
.El
.Sh EXAMPLES
To lookup hosts in
.Pa /etc/hosts
and then from the DNS, and lookup user information from
.Tn NIS
then files, use:
.Bl -column "passwd:" -offset indent
.It hosts:      files dns
.It passwd:     nis [notfound=return] files
.It group:      nis [notfound=return] files
.El
.Pp
The criteria
.Dq [notfound=return]
sets a policy of "if the user is notfound in nis, don't try files."
This treats nis as the authoritative source of information, except
when the server is down.
.Sh SEE ALSO
.Xr getent 1 ,
.Xr nsdispatch 3 ,
.Xr resolv.conf 5 ,
.Xr named 8 ,
.Xr ypbind 8
.Sh HISTORY
The
.Nm
file format first appeared in
.Nx 1.4 .
.Sh AUTHORS
.An Luke Mewburn
.Aq lukem@NetBSD.org
wrote this freely distributable name-service switch implementation,
using ideas from the
.Tn ULTRIX
.Xr svc.conf 5
and
.Tn Solaris
.Xr nsswitch.conf 4
manual pages.