%%% title = "mutfs 5" area = "File Formats Manual" workgroup = "Mutfs Filesystem" %%%

mutfs

Name

mutfs - immutable file system, with write grace period

Synopsis

mutfs [OPTION]... olddir newdir

Description

Mutfs is used as an overlay file system to make it immutable, write actions are only allowed when creating a new file (or within a user specific grace period). Once things exists, they can't be changed or deleted. A use-case might be to protect an backed up archive from a ransomware attack. The attack will still happen, but at least it can't delete the old files (nor the encrypted ones once created).

Options are:

• -o opt,..., where opt can be:
  • debug: enable debug logging.
  • null: change null permissions to 0644 (files), 0755 (dirs).
  • allow_other: everyone can access the files.
  • ro: make fully read-only.
  • log: enable logging when a destructive action is tried.
  • grace=duration, given a Go syntax duration will allow write operations for duration.

Using mount -t mutfs ~ /tmp/mut -o debug,grace=5s will use mutfs (if the executable (mount.mutfs) can be found in the path) to mount ~ under /tmp. For up to 5 seconds after file/directory creation destructive actions are allowed.

Note the grace period works by getting the files creation time via the statx system call, which the underlying filesystem should support.

Or you can install the following systemd mount unit:

[Unit]
Description=Immutable Filesystem
After=network.target

[Mount]
What=<olddir>
Where=<newdir>
Type=mutfs
Options=debug,allow_other

[Install]
WantedBy=multi-user.target

Install

Copy mutfs and mount.mutfs to /usr/sbin. And potentially add a line to /etc/fstab;

/home/miek    /tmp/mut         mutfs     log,nouser,allow_other   0 0

And adjust as needed.

Re-exporting the filesystem only works if allow_other is specifed, otherwise the SMB/NFS daemon cannot access the filesystem because only the user mounting it has access. Note unless you edit /etc/fuse.conf only root can create mounts with this option specified.

Examples

For example mount your home directory on a directory in /tmp: /mutfs ~ /tmp/mut, then you can create the file a, but we can't update it after it's creation:

% cd /tmp/mut
% echo 1 > a
% cat a
1
% echo 2 > a
zsh: permission denied: a
% rm a
rm: cannot remove 'a': Permission denied

See Also

fuse(8) stat(2)

Author

Miek Gieben miek@miek.nl.