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1=============2NFS ID Mapper3=============4 5Id mapper is used by NFS to translate user and group ids into names, and to6translate user and group names into ids. Part of this translation involves7performing an upcall to userspace to request the information. There are two8ways NFS could obtain this information: placing a call to /sbin/request-key9or by placing a call to the rpc.idmap daemon.10 11NFS will attempt to call /sbin/request-key first. If this succeeds, the12result will be cached using the generic request-key cache. This call should13only fail if /etc/request-key.conf is not configured for the id_resolver key14type, see the "Configuring" section below if you wish to use the request-key15method.16 17If the call to /sbin/request-key fails (if /etc/request-key.conf is not18configured with the id_resolver key type), then the idmapper will ask the19legacy rpc.idmap daemon for the id mapping. This result will be stored20in a custom NFS idmap cache.21 22 23Configuring24===========25 26The file /etc/request-key.conf will need to be modified so /sbin/request-key can27direct 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 will36expire. This parameter is optional for /usr/sbin/nfs.idmap. When the timeout37is 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 user42 gid: Find the GID for the given group43 user: Find the user name for the given UID44 group: Find the group name for the given GID45 46You can handle any of these individually, rather than using the generic upcall47program. If you would like to use your own program for a uid lookup then you48would 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. In58this case, /some/other/program will handle all uid lookups and59/usr/sbin/nfs.idmap will handle gid, user, and group lookups.60 61See Documentation/security/keys/request-key.rst for more information62about the request-key function.63 64 65nfs.idmap66=========67 68nfs.idmap is designed to be called by request-key, and should not be run "by69hand". This program takes two arguments, a serialized key and a key70description. The serialized key is first converted into a key_serial_t, and71then 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.idmap74determines the correct function to call by looking at the first part of the75description string. For example, a uid lookup description will appear as76"uid:user@domain".77 78nfs.idmap will return 0 if the key was instantiated, and non-zero otherwise.79