.\"	$OpenBSD: mount_umap.8,v 1.16 2002/06/26 21:32:02 fgsch Exp $
.\"	$NetBSD: mount_umap.8,v 1.4 1996/03/05 02:36:42 thorpej Exp $
.\"
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.\" This code is derived from software donated to Berkeley by
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.\"	@(#)mount_umap.8	8.3 (Berkeley) 3/27/94
.\"
.Dd March 27, 1994
.Dt MOUNT_UMAP 8
.Os
.Sh NAME
.Nm mount_umap
.Nd sample file system layer
.Sh SYNOPSIS
.Nm mount_umap
.Op Fl o Ar options
-u
.Ar uid-mapfile
-g
.Ar gid-mapfile
.Ar target
.Ar mount_point
.Sh DESCRIPTION
The
.Nm
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 location that uses a different password file.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl o Ar options
Options are specified with a
.Fl o
flag followed by a comma separated string of options.
See the
.Xr mount 8
man page for possible options and their meanings.
.El
.Pp
The
.Nm
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.
For instance, user
smith might have UID 1000 in the original environment, while having
UID 2000 in the local environment.
The
.Nm
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.
.Pp
.Em target
should be the current location of the subtree in the
local system's name space.
.Ar mount_point
should be a directory
where the mapped subtree is to be placed.
.Em uid-mapfile
and
.Em gid-mapfile
describe the mappings 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 subsequent 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.
.Em uid-mapfile
should contain all UID
mappings, and
.Em gid-mapfile
should contain all GID mappings.
Any UIDs not mapped in
.Em uid-mapfile
will be treated as user NOBODY,
and any GIDs not mapped in
.Em 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.
.Pp
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.
.Nm
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.
.Pp
The layer created by the
.Nm
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.
.Sh SEE ALSO
.Xr mount 2 ,
.Xr mount 8 ,
.Xr mount_null 8
.Sh HISTORY
The
.Nm
utility first appeared in
.Bx 4.4 .