Section: File Formats (5)
Updated: 1 October 2010
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nfsidmap - The NFS idmapper upcall program  


nfsidmap [-v] [-t timeout] key desc
nfsidmap [-v] [-c]
nfsidmap [-v] [-u|-g|-r user]  


The file /usr/sbin/nfsidmap is used by the NFS idmapper to translate user and group ids into names, and to translate user and group names into ids. Idmapper uses request-key to perform the upcall and cache the result. /usr/sbin/nfsidmap is called by /sbin/request-key, and will perform the translation and initialize a key with the resulting information.

nfsidmap can also used to clear the keyring of all the keys or revoke one particular key. This is useful when the id mappings have failed to due to a lookup error resulting in all the cached uids/gids to be set to the user id nobody.  


Clear the keyring of all the keys.
-g user
Revoke the gid key of the given user.
-r user
Revoke both the uid and gid key of the given user.
-t timeout
Set the expiration timer, in seconds, on the key. The default is 600 seconds (10 mins).
-u user
Revoke the uid key of the given user.
Increases the verbosity of the output to syslog (can be specified multiple times).


The file /etc/request-key.conf will need to be modified so /sbin/request-key can properly direct the upcall. The following line should be added before a call to keyctl negate:

create id_resolver     *       *       /usr/sbin/nfsidmap -t 600 %k %d

This will direct all id_resolver requests to the program /usr/sbin/nfsidmap. The -t 600 defines how many seconds into the future the key will expire. This is an optional parameter for /usr/sbin/nfsidmap and will default to 600 seconds when not specified.

The idmapper system uses four 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 choose to 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 looks similar to this:

create id_resolver     uid:*   *       /some/other/program %k %d

create id_resolver     *               *       /usr/sbin/nfsidmap %k %d

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


Bryan Schumaker, <>