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

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 sepa‐
	     rated 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 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.

     target should be the current location of the sub-tree 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.

     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:

	   1
	   2000 1000

     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 origi‐
     nal 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 map‐
     pings 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.

BSD				 March 6, 2001				   BSD

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