mount.fuse

fuse(8)                     System Manager's Manual                    fuse(8)



NAME
       fuse - configuration and mount options for FUSE file systems

DESCRIPTION
       FUSE (Filesystem in Userspace) is a simple interface for userspace
       programs to export a virtual filesystem to the Linux kernel. FUSE also
       aims to provide a secure method for non privileged users to create and
       mount their own filesystem implementations.

DEFINITIONS
       FUSE   The in-kernel filesystem that forwards requests to a user-space
              process.

       filesystem
              The user-space process that responds to requests received from
              the kernel.

       libfuse
              The shared library that most (user-space) filesystems use to
              communicate with FUSE (the kernel filesystem). libfuse also
              provides the fusermount3 (or fusermount if you have older
              version of libfuse) helper to allow non-privileged users to
              mount filesystems.

       filesystem owner
              The user that starts the filesystem and instructs the kernel to
              associate it with a particular mountpoint. The latter is
              typically done by the filesystem itself on start-up. When using
              libfuse, this is done by calling the fusermount3 utility.

       client Any process that interacts with the mountpoint.

CONFIGURATION
       Some options regarding mount policy can be set in the file
       /etc/fuse.conf. Currently these options are:

       mount_max = NNN
              Set the maximum number of FUSE mounts allowed to non-root users.
              The default is 1000.

       user_allow_other
              Allow non-root users to specify the allow_other or allow_root
              mount options (see below).

       These limits are enforced by the fusermount3 helper, so they can be
       avoided by filesystems that run as root.

OPTIONS
       Most of the generic mount options described in mount are supported (ro,
       rw, suid, nosuid, dev, nodev, exec, noexec, atime, noatime, sync,
       async, dirsync). Filesystems are mounted with nodev,nosuid by default,
       which can only be overridden by a privileged user.

   General mount options:
       These are FUSE specific mount options that can be specified for all
       filesystems:

       default_permissions
              This option instructs the kernel to perform its own permission
              check instead of deferring all permission checking to the
              filesystem. The check by the kernel is done in addition to any
              permission checks by the filesystem, and both have to succeed
              for an operation to be allowed. The kernel performs a standard
              UNIX permission check (based on mode bits and ownership of the
              directory entry, and uid/gid of the client).

              This mount option is activated implicitly if the filesystem
              enables ACL support during the initial feature negotiation when
              opening the device fd. In this case, the kernel performs both
              ACL and standard unix permission checking.

              Filesystems that do not implement any permission checking should
              generally add this option internally.

       allow_other
              This option overrides the security measure restricting file
              access to the filesystem owner, so that all users (including
              root) can access the files.

       rootmode=M
              Specifies the the file mode of the filesystem's root (in octal
              representation).

       blkdev Mount a filesystem backed by a block device.  This is a
              privileged option. The device must be specified with the
              fsname=NAME option.

       blksize=N
              Set the block size for the filesystem. This option is only valid
              for 'fuseblk' type mounts. The default is 512.

              In most cases, this option should not be specified by the
              filesystem owner but set internally by the filesystem.

       max_read=N
              With this option the maximum size of read operations can be set.
              The default is infinite, but typically the kernel enforces its
              own limit in addition to this one. A value of zero corresponds
              to no limit.

              This option should not be specified by the filesystem owner. The
              correct (or optimum) value depends on the filesystem
              implementation and should thus be set by the filesystem
              internally.

              This mount option is deprecated in favor of direct negotiation
              over the device fd (as done for e.g. the maximum size of write
              operations). For the time being, libfuse-using filesystems that
              want to limit the read size must therefore use this mount option
              and set the same value again in the init() handler.

       fd=N   The file descriptor to use for communication between the
              userspace filesystem and the kernel.  The file descriptor must
              have been obtained by opening the FUSE device (/dev/fuse).

              This option should not be specified by the filesystem owner. It
              is set by libfuse (or, if libfuse is not used, must be set by
              the filesystem itself).

       user_id=N
              group_id=N Specifies the numeric uid/gid of the mount owner.

              This option should not be specified by the filesystem owner. It
              is set by libfuse (or, if libfuse is not used, must be set by
              the filesystem itself).

       fsname=NAME
              Sets the filesystem source (first field in /etc/mtab). The
              default is the name of the filesystem process.

       subtype=TYPE
              Sets the filesystem type (third field in /etc/mtab). The default
              is the name of the filesystem process. If the kernel suppports
              it, /etc/mtab and /proc/mounts will show the filesystem type as
              fuse.TYPE

              If the kernel doesn't support subtypes, the source filed will be
              TYPE#NAME, or if fsname option is not specified, just TYPE.


   libfuse-specific mount options:
       These following options are not actually passed to the kernel but
       interpreted by libfuse. They can be specified for all filesystems that
       use libfuse:

       allow_root
              This option is similar to allow_other but file access is limited
              to the filesystem owner and root.  This option and allow_other
              are mutually exclusive.

       auto_unmount
              This option enables automatic release of the mountpoint if
              filesystem terminates for any reason. Normally the filesystem is
              responsible for releasing the mountpoint, which means that the
              mountpoint becomes inaccessible if the filesystem terminates
              without first unmounting.

              At the moment, this option implies that the filesystem will also
              be mounted with nodev and nosuid (even when mounted by root).
              This restriction may be lifted in the future.


   High-level mount options:
       These following options are not actually passed to the kernel but
       interpreted by libfuse. They can only be specified for filesystems that
       use the high-level libfuse API:

       kernel_cache
              This option disables flushing the cache of the file contents on
              every open(2).  This should only be enabled on filesystems,
              where the file data is never changed externally (not through the
              mounted FUSE filesystem).  Thus it is not suitable for network
              filesystems and other "intermediate" filesystems.

              NOTE: if this option is not specified (and neither direct_io)
              data is still cached after the open(2), so a read(2) system call
              will not always initiate a read operation.

       auto_cache
              This option is an alternative to kernel_cache. Instead of
              unconditionally keeping cached data, the cached data is
              invalidated on open(2) if the modification time or the size of
              the file has changed since it was last opened.

       umask=M
              Override the permission bits in st_mode set by the filesystem.
              The resulting permission bits are the ones missing from the
              given umask value.  The value is given in octal representation.

       uid=N  Override the st_uid field set by the filesystem (N is numeric).

       gid=N  Override the st_gid field set by the filesystem (N is numeric).

       entry_timeout=T
              The timeout in seconds for which name lookups will be cached.
              The default is 1.0 second. For all the timeout options, it is
              possible to give fractions of a second as well (e.g.
              entry_timeout=2.8)

       negative_timeout=T
              The timeout in seconds for which a negative lookup will be
              cached. This means, that if file did not exist (lookup retuned
              ENOENT), the lookup will only be redone after the timeout, and
              the file/directory will be assumed to not exist until then.  The
              default is 0.0 second, meaning that caching negative lookups are
              disabled.

       attr_timeout=T
              The timeout in seconds for which file/directory attributes are
              cached.  The default is 1.0 second.

       ac_attr_timeout=T
              The timeout in seconds for which file attributes are cached for
              the purpose of checking if auto_cache should flush the file data
              on  open. The default is the value of attr_timeout

       noforget

       remember=T
              Normally, libfuse assigns inodes to paths only for as long as
              the kernel is aware of them. With this option inodes are instead
              assigned for at least T seconds (or, in the case of noforget,
              the life-time of the filesystem). This will require more memory,
              but may be necessary when using applications that make use of
              inode numbers.

       modules=M1[:M2...]
              Add modules to the filesystem stack.  Modules are pushed in the
              order they are specified, with the original filesystem being on
              the bottom of the stack.


   mount.fuse3 options:
       These options are interpreted by mount.fuse3 and are thus only
       available when mounting a file system via mount.fuse3 (such as when
       mounting via the generic mount(1) command or /etc/fstab). Supported
       options are:

       setuid=USER
              Switch to USER and its primary group before launching the FUSE
              file system process. mount.fuse3 must be run as root or with
              CAP_SETUID and CAP_SETGID for this to work.

       drop_privileges
              Perform setup of the FUSE file descriptor and mounting the file
              system before launching the FUSE file system process.
              mount.fuse3 requires privilege to do so, i.e. must be run as
              root or at least with CAP_SYS_ADMIN and CAP_SETPCAP. It will
              launch the file system process fully unprivileged, i.e. without
              capabilities(7) and prctl(2) flags set up such that privileges
              can't be reacquired (e.g. via setuid or fscaps binaries). This
              reduces risk in the event of the FUSE file system process
              getting compromised by malicious file system data.


FUSE MODULES (STACKING)
       Modules are filesystem stacking support to high level API. Filesystem
       modules can be built into libfuse or loaded from shared object

   iconv
       Perform file name character set conversion.  Options are:

       from_code=CHARSET
              Character set to convert from (see iconv -l for a list of
              possible values). Default is UTF-8.

       to_code=CHARSET
              Character set to convert to.  Default is determined by the
              current locale.

   subdir
       Prepend a given directory to each path. Options are:

       subdir=DIR
              Directory to prepend to all paths.  This option is mandatory.

       rellinks
              Transform absolute symlinks into relative

       norellinks
              Do not transform absolute symlinks into relative.  This is the
              default.

SECURITY
       The fusermount3 program is installed set-user-gid to fuse. This is done
       to allow users from fuse group to mount their own filesystem
       implementations.  There must however be some limitations, in order to
       prevent Bad User from doing nasty things.  Currently those limitations
       are:

       1.     The user can only mount on a mountpoint, for which it has write
              permission

       2.     The mountpoint is not a sticky directory which isn't owned by
              the user (like /tmp usually is)

       3.     No other user (including root) can access the contents of the
              mounted filesystem.

NOTE
       FUSE filesystems are unmounted using the fusermount3(1) command
       (fusermount3 -u mountpoint).

AUTHORS
       FUSE is currently maintained by Nikolaus Rath <Nikolaus@rath.org>

       The original author of FUSE is Miklos Szeredi <mszeredi@inf.bme.hu>.

       This man page was originally written by Bastien Roucaries
       <roucaries.bastien+debian@gmail.com> for the Debian GNU/Linux
       distribution.

SEE ALSO
       fusermount3(1) fusermount(1) mount(8)



                                                                       fuse(8)