nsswitch.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,
netgroups, 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 occurs.
The following sources are implemented:
Source |
Description |
files |
Local files, such as /etc/hosts, and
/etc/passwd. |
dns |
Internet Domain Name System. “hosts” and
“networks” use IN class entries, all other
databases use HS class (Hesiod) entries. |
mdnsd |
Use mdnsd(8) for
“hosts” lookups, acting as both a system-wide cache for
normal unicast DNS as well as providing multicast DNS
(“zeroconf”) lookups. |
multicast_dns |
Use mdnsd(8) only for
multicast DNS “hosts” lookups. This would normally be used
in conjunction with “dns”, which would then provide unicast
DNS resolver functions. |
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. |
The following databases are used by the following C library functions:
The following status codes are available:
Status |
Description |
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:
Action |
Description |
continue |
Try the next source |
return |
Return with the current result |
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 tryagain=continue]
).
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
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 ‘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 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.
To ensure compatibility with previous and current implementations,
the “compat” source must appear alone for a given
database.
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:
Database |
Default source list |
group |
compat |
group_compat |
nis |
hosts |
files dns |
netgroup |
files [notfound=return] nis |
passwd |
compat |
passwd_compat |
nis |
- /etc/nsswitch.conf
- The file
nsswitch.conf
resides in
/etc.
To lookup 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 notfound in nis, don't try files." This treats nis
as the authoritative source of information, except when the server is
down.
The nsswitch.conf
file format first appeared in
NetBSD 1.4.
Luke Mewburn ⟨lukem@NetBSD.org⟩ wrote this
freely distributable name-service switch implementation, using ideas from the
ULTRIX svc.conf(5) and Solaris
nsswitch.conf(4) manual
pages.