mount_umap - sample file system layer
mount_umap [-o options] -u uid-mapfile -g gid-mapfile target
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
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
that uses a different password file.
The options are as follows:
Options are specified with a -o flag followed by a
string of options. See the mount(8) man page for
and their meanings.
Use the group ID mapping specified in gid-mapfile.
This flag is
Use the user ID mapping specified in uid-mapfile.
This flag is
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 environment
and some other set of IDs in the local environment.
user smith might have UID 1000 in the original environment,
UID 2000 in the local environment. The mount_umap command
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
target should be the current location of the subtree in the
name space. mount_point should be a directory where
subtree is to be placed. uid-mapfile and gid-mapfile describe the mappings
to be made between identifiers.
The format of these files is very simple. The first line is
a count of
the number of mappings. Each subsequent line contains two
first is the ID in the mapped subtree, and the second is the
ID in the
original subtree. IDs in the original subtree for which no
will be mapped to ``nobody''. 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.
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.
mount(2), mount(8), mount_null(8)
The mount_umap utility first appeared in 4.4BSD.
It is not meant for production use. The implementation is
not very sophisticated.
OpenBSD 3.6 March 27, 1994
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