MOUNT_UMAP(8)           OpenBSD System Manager's Manual          MOUNT_UMAP(8)

NAME
     mount_umap - sample file system layer

SYNOPSIS
     mount_umap [-o options] -u uid-mapfile -g gid-mapfile target mount_point

DESCRIPTION
     The mount_umap command is used to mount a subtree of an existing file
     system that uses a different set of UIDs and GIDs than the local system.
     Such a file system could be mounted from a remote site via NFS or it
     could be a file system on removable media brought from some foreign loca-
     tion that uses a different password file.

     The options are as follows:

     -o options
             Options are specified with a -o flag followed by a comma separat-
             ed string of options.  See the mount(8) man page for possible op-
             tions and their meanings.

     The mount_umap command uses a set of files provided by the user to make
     correspondences between UIDs and GIDs in the subtree's original environ-
     ment and some other set of IDs in the local environment.  For instance,
     user smith might have UID 1000 in the original environment, while having
     UID 2000 in the local environment.  The mount_umap command allows the
     subtree from smith's original environment to be mapped in such a way that
     all files with owner UID 1000 look like they are actually owned by UID
     2000.

     target should be the current location of the subtree in the local sys-
     tem's name space.  mount_point should be a directory where the mapped
     subtree is to be placed.  uid-mapfile and gid-mapfile describe the map-
     pings to be made between identifiers.  Briefly, the format of these files
     is a count of the number of mappings on the first line, with each subse-
     quent line containing a single mapping.  Each of these mappings consists
     of an ID from the original environment and the corresponding ID in the
     local environment, separated by whitespace.  uid-mapfile should contain
     all UID mappings, and gid-mapfile should contain all GID mappings.  Any
     UIDs not mapped in uid-mapfile will be treated as user NOBODY, and any
     GIDs not mapped in gid-mapfile will be treated as group NULLGROUP.  At
     most 64 UIDs can be mapped for a given subtree, and at most 16 groups can
     be mapped by a given subtree.

     The mapfiles can be located anywhere in the file hierarchy, but they must
     be owned by root, and they must be writable only by root.  mount_umap
     will refuse to map the subtree if the ownership or permissions on these
     files are improper.  It will also balk if the count of mappings in the
     first line of the map files is not correct.

     The layer created by the mount_umap command is meant to serve as a simple
     example of file system layering.  It is not meant for production use.
     The implementation is not very sophisticated.

SEE ALSO
     mount(8), mount_null(8)

HISTORY
     The mount_umap utility first appeared in 4.4BSD.

OpenBSD 2.9                     March 27, 1994                               1
