nsswitch.conf(5) - Manual pages

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, 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 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.
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.

Databases

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), setgrent(3), endgrent(3)
hosts: getaddrinfo(3), gethostbyaddr(3), gethostbyaddr_r(3), gethostbyname(3), gethostbyname2(3), gethostbyname_r(3), getipnodebyaddr(3), getipnodebyname(3)
networks: getnetbyaddr(3), getnetbyaddr_r(3), getnetbyname(3), getnetbyname_r(3)
passwd: getpwent(3), getpwent_r(3), getpwnam_r(3), getpwuid_r(3), setpwent(3), endpwent(3)
shells: getusershell(3)
services: getservent(3)
rpc: getrpcbyname(3), getrpcbynumber(3), getrpcent(3)
proto: getprotobyname(3), getprotobynumber(3), getprotoent(3)
netgroup: getnetgrent(3), getnetgrent_r(3), setnetgrent(3), endnetgrent(3), innetgr(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]).

Cache

You can enable caching for the particular database by specifying “cache” as the first source in the nsswitch.conf 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 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 do not 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 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
group: compat
group_compat: nis
hosts: files dns
passwd: compat
passwd_compat: nis
services: compat
services_compat: nis

FILES

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

EXAMPLES

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 notfound in nis, do not try files." This treats nis as the authoritative source of information, except when the server is down.

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