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Documentation/filesystems/nfs/idmapper.txt (2.6.37)

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.  Id mapper will
user request-key to perform this upcall and cache the result.  The program
/usr/sbin/nfs.upcall should be called by request-key, and will perform the
translation and initialize a key with the resulting information.

 NFS_USE_NEW_IDMAPPER must be selected when configuring the kernel to use this

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:

#======	=======	===============	===============	===============================
create	id_resolver	*	*		/usr/sbin/nfs.upcall %k %d 600

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

#======	=======	===============	===============	===============================
create	id_resolver	uid:*	*		/some/other/program  %k %d 600
create	id_resolver	*	*		/usr/sbin/nfs.upcall %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.upcall will handle gid, user, and group lookups.

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

nfs.upcall 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.upcall
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

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

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