1=============
2NFS ID Mapper
3=============
4
5Id mapper is used by NFS to translate user and group ids into names, and to
6translate user and group names into ids.  Part of this translation involves
7performing an upcall to userspace to request the information.  There are two
8ways NFS could obtain this information: placing a call to /sbin/request-key
9or by placing a call to the rpc.idmap daemon.
10
11NFS will attempt to call /sbin/request-key first.  If this succeeds, the
12result will be cached using the generic request-key cache.  This call should
13only fail if /etc/request-key.conf is not configured for the id_resolver key
14type, see the "Configuring" section below if you wish to use the request-key
15method.
16
17If the call to /sbin/request-key fails (if /etc/request-key.conf is not
18configured with the id_resolver key type), then the idmapper will ask the
19legacy rpc.idmap daemon for the id mapping.  This result will be stored
20in a custom NFS idmap cache.
21
22
23Configuring
24===========
25
26The file /etc/request-key.conf will need to be modified so /sbin/request-key can
27direct the upcall.  The following line should be added:
28
29``#OP	TYPE	DESCRIPTION	CALLOUT INFO	PROGRAM ARG1 ARG2 ARG3 ...``
30``#======	=======	===============	===============	===============================``
31``create	id_resolver	*	*		/usr/sbin/nfs.idmap %k %d 600``
32
33
34This will direct all id_resolver requests to the program /usr/sbin/nfs.idmap.
35The last parameter, 600, defines how many seconds into the future the key will
36expire.  This parameter is optional for /usr/sbin/nfs.idmap.  When the timeout
37is not specified, nfs.idmap will default to 600 seconds.
38
39id mapper uses for key descriptions::
40
41	  uid:  Find the UID for the given user
42	  gid:  Find the GID for the given group
43	 user:  Find the user  name for the given UID
44	group:  Find the group name for the given GID
45
46You can handle any of these individually, rather than using the generic upcall
47program.  If you would like to use your own program for a uid lookup then you
48would edit your request-key.conf so it look similar to this:
49
50``#OP	TYPE	DESCRIPTION	CALLOUT INFO	PROGRAM ARG1 ARG2 ARG3 ...``
51``#======	=======	===============	===============	===============================``
52``create	id_resolver	uid:*	*		/some/other/program %k %d 600``
53``create	id_resolver	*	*		/usr/sbin/nfs.idmap %k %d 600``
54
55
56Notice that the new line was added above the line for the generic program.
57request-key will find the first matching line and corresponding program.  In
58this case, /some/other/program will handle all uid lookups and
59/usr/sbin/nfs.idmap will handle gid, user, and group lookups.
60
61See Documentation/security/keys/request-key.rst for more information
62about the request-key function.
63
64
65nfs.idmap
66=========
67
68nfs.idmap is designed to be called by request-key, and should not be run "by
69hand".  This program takes two arguments, a serialized key and a key
70description.  The serialized key is first converted into a key_serial_t, and
71then passed as an argument to keyctl_instantiate (both are part of keyutils.h).
72
73The actual lookups are performed by functions found in nfsidmap.h.  nfs.idmap
74determines the correct function to call by looking at the first part of the
75description string.  For example, a uid lookup description will appear as
76"uid:user@domain".
77
78nfs.idmap will return 0 if the key was instantiated, and non-zero otherwise.
79