MOUNT_UMAP(8) BSD System Manager's Manual MOUNT_UMAP(8)NAMEmount_umap - sample file system layer
SYNOPSISmount_umap [-o generic-options] [-g gidmap] [-u uidmap] source-directory
mount-point
DESCRIPTION
The mount_umap command is used to mount a sub-tree 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
Generic options, specified as a comma separated list. See the
mount(8) man page for other (generic) options and their meanings.
-g groupmap
File containing group-id mappings.
-u usermap
File containing user-id mappings.
The mount_umap command uses a set of files provided by the user to make
correspondences between uids and gids in the sub-tree'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 owning uid 1000 look like they are actually owned by uid
2000.
The source-directory argument should be the current location of the sub-
tree in the local system's name space. The mount-point argument should
be a directory where the mapped subtree is to be placed. The uid-mapfile
and gid-mapfile arguments describe the mappings to be made between iden-
tifiers. Briefly, the format of these files is a count of the number of
mappings on the first line, with each subsequent line containing a single
mapping. Each of these mappings consists of an id from the original en-
vironment and the corresponding id in the local environment, separated by
white space. The uid-mapfile file 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 giv-
en 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. The mount_umap
command will refuse to map the sub-tree 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.
EXAMPLE
# mount -t umap -o -u=uidmap,-g=gidmap /usr/src /mnt
SEE ALSOmount(8), mount_null(8)HISTORY
The mount_umap utility first appeared in 4.4BSD.
