linux/Documentation/filesystems/nfs/idmapper.txt

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ID Mapper
=========
Id mapper is used by NFS to translate user and group ids into names, and to
translate user and group names into ids.  Part of this translation involves
performing an upcall to userspace to request the information.  There are two
ways NFS could obtain this information: placing a call to /sbin/request-key
or by placing a call to the rpc.idmap daemon.

NFS will attempt to call /sbin/request-key first.  If this succeeds, the
result will be cached using the generic request-key cache.  This call should
only fail if /etc/request-key.conf is not configured for the id_resolver key
type, see the "Configuring" section below if you wish to use the request-key
method.

If the call to /sbin/request-key fails (if /etc/request-key.conf is not
configured with the id_resolver key type), then the idmapper will ask the
legacy rpc.idmap daemon for the id mapping.  This result will be stored
in a custom NFS idmap cache.


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Configuring
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The file /etc/request-key.conf will need to be modified so /sbin/request-key can
direct the upcall.  The following line should be added:

#OP	TYPE	DESCRIPTION	CALLOUT INFO	PROGRAM ARG1 ARG2 ARG3 ...
#======	=======	===============	===============	===============================
create	id_resolver	*	*		/usr/sbin/nfs.idmap %k %d 600

This will direct all id_resolver requests to the program /usr/sbin/nfs.idmap.
The last parameter, 600, defines how many seconds into the future the key will
expire.  This parameter is optional for /usr/sbin/nfs.idmap.  When the timeout
is not specified, nfs.idmap will default to 600 seconds.

id mapper uses for key descriptions:
	  uid:  Find the UID for the given user
	  gid:  Find the GID for the given group
	 user:  Find the user  name for the given UID
	group:  Find the group name for the given GID

You can handle any of these individually, rather than using the generic upcall
program.  If you would like to use your own program for a uid lookup then you
would edit your request-key.conf so it look similar to this:

#OP	TYPE	DESCRIPTION	CALLOUT INFO	PROGRAM ARG1 ARG2 ARG3 ...
#======	=======	===============	===============	===============================
create	id_resolver	uid:*	*		/some/other/program %k %d 600
create	id_resolver	*	*		/usr/sbin/nfs.idmap %k %d 600

Notice that the new line was added above the line for the generic program.
request-key will find the first matching line and corresponding program.  In
this case, /some/other/program will handle all uid lookups and
/usr/sbin/nfs.idmap will handle gid, user, and group lookups.

See <file:Documentation/security/keys-request-key.txt> for more information
about the request-key function.


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nfs.idmap
=========
nfs.idmap is designed to be called by request-key, and should not be run "by
hand".  This program takes two arguments, a serialized key and a key
description.  The serialized key is first converted into a key_serial_t, and
then passed as an argument to keyctl_instantiate (both are part of keyutils.h).

The actual lookups are performed by functions found in nfsidmap.h.  nfs.idmap
determines the correct function to call by looking at the first part of the
description string.  For example, a uid lookup description will appear as
"uid:user@domain".

nfs.idmap will return 0 if the key was instantiated, and non-zero otherwise.