NSSWITCH.CONF(5) | File Formats Manual | NSSWITCH.CONF(5)

# NAME

`nsswitch.conf` —
name-service switch configuration file

# DESCRIPTION

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.

## Sources

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.

## Databases

The following databases are used by the following C library functions:

Database | Used by
group | getgrent(3)
hosts | gethostbyname(3)
netgroup | getnetgrent(3)
networks | getnetbyname(3)
passwd | getpwent(3)
shells | getusershell(3)

## Status codes

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.

## Actions

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

## 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 tryagain=continue]` ).

## 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 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’.

## Notes

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.

## 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:

Database | Default source list
group | compat
group_compat | nis
hosts | files dns
netgroup | files [notfound=return] nis
passwd | compat
passwd_compat | nis

# FILES

/etc/nsswitch.conf
  The file `nsswitch.conf` resides in /etc.

# EXAMPLES

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.

# SEE ALSO

getent(1), nsdispatch(3), resolv.conf(5), named(8), ypbind(8)

# HISTORY

The `nsswitch.conf` file format first appeared in NetBSD 1.4.

# AUTHORS

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.

October 25, 2009 | NetBSD 9.4