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 pro-
grams 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 pro-
vides 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 typi-
cally done by the filesystem itself on start-up. When using lib-
fuse, 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 en-
ables 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 ac-
cess to the filesystem owner, so that all users (including root)
can access the files.
rootmode=M
Specifies the file mode of the filesystem's root (in octal rep-
resentation).
blkdev Mount a filesystem backed by a block device. This is a privi-
leged 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 implementa-
tion 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 de-
fault 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 supports
it, /etc/mtab and /proc/mounts will show the filesystem type as
fuse.TYPE
If the kernel doesn't support subtypes, the source field 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 in-
terpreted 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 in-
terpreted 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 uncon-
ditionally 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_time-
out=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 returned
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 avail-
able 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 get-
ting 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 possi-
ble values). Default is UTF-8.
to_code=CHARSET
Character set to convert to. Default is determined by the cur-
rent 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 implemen-
tations. 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 (fuser-
mount3 -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 <rou-
caries.bastien+debian@gmail.com> for the Debian GNU/Linux distribution.
SEE ALSO
fusermount3(1) fusermount(1) mount(8) fuse(4)
fuse(8)
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