NAME
mount_umap —
user and group ID
remapping file system layer
SYNOPSIS
mount_umap |
[-o
options] -g
gid-mapfile -u
uid-mapfile target
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, a
local file system on removable media brought from some foreign location that
uses a different user/group database, or could be a local file system for
another operating system which does not support Unix-style user/group IDs, or
which uses a different numbering scheme.
Both
target and
mount-point are
converted to absolute paths before use.
The options are as follows:
-
-
- -g
gid-mapfile
- Use the group ID mapping specified in
gid-mapfile. This flag is required.
-
-
- -o
- Options are specified with a -o flag
followed by a comma separated string of options. See the
mount(8) man page for
possible options and their meanings.
-
-
- -u
uid-mapfile
- Use the user ID mapping specified in
uid-mapfile. This flag is required.
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
environment 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.
target should be the current location of the sub-tree in the
local system's name space.
mount-point should be a directory
where the mapped subtree is to be placed.
uid-mapfile and
gid-mapfile describe the mappings to be made between
identifiers.
The format of the user and group ID mapping files is very simple. The first line
of the file is the total number of mappings present in the file. The remaining
lines each consist of two numbers: the ID in the mapped subtree and the ID in
the original subtree.
For example, to map uid 1000 in the original subtree to uid 2000 in the mapped
subtree:
For user IDs in the original subtree for which no mapping exists, the user ID
will be mapped to the user “nobody”. For group IDs in the original
subtree for which no mapping exists, the group ID will be mapped to the group
“nobody”.
There is a limit of 64 user ID mappings and 16 group ID mappings.
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 sub-tree if the ownership
or permissions on these files are improper. It will also report an error if
the count of mappings in the first line of the map files is not correct.
SEE ALSO
mount(8),
mount_null(8)
HISTORY
The
mount_umap utility first appeared in
4.4BSD.
BUGS
The implementation is not very sophisticated.