Mount drivers

A driver turns a mount spec into a mounted folder. Every driver goes through your system ssh, so your keys, agent, known_hosts and ~/.ssh/config apply as they are.

Driver	Linux	macOS
sshfs	sshfs 3 + fuse3 (fusermount3)	sshfs with macFUSE or FUSE-T
rclone	rclone mount + fuse3	rclone mount with macFUSE or FUSE-T
rclone-nfs	unavailable (“macOS only”)	rclone nfsmount (needs /sbin/mount_nfs, no FUSE)

bifrost drivers shows what each probe found and which driver auto picks. It probes locally when the daemon is down.

$ bifrost drivers
✓ sshfs       /usr/bin/sshfs   SSHFS version 3.7.3, fusermount3
✓ rclone      /usr/bin/rclone  rclone v1.75.1, fusermount3
✗ rclone-nfs                   macOS only
default (auto): sshfs

Choosing a driver

Set driver on a static mount, in a provider’s [discovery.mount], or as mount.default_driver. The default everywhere is auto, which takes the first available driver in mount.auto_order:

OS	Default auto_order
Linux	sshfs, rclone
macOS	rclone-nfs, rclone, sshfs

[mount]
default_driver = "auto"            # auto | sshfs | rclone | rclone-nfs
auto_order = ["sshfs", "rclone"]

• A named driver is never substituted. If it is unavailable, the mount waits and says why.
• auto is sticky: a mount keeps its driver until its spec changes, so a driver that comes and goes never remounts anything.

SSHFS

The default on Linux. Bifröst runs sshfs 3 with reconnect, so a short network drop is handled inside the mount instead of by remounting. On macOS it adds volname=<id> (and noappledouble with macFUSE).

rclone

rclone always goes through OpenSSH with --sftp-ssh; its built-in SSH library, which skips host-key checks, is never used. It runs with --config=/dev/null and a per-mount VFS cache in <state>/rclone/<id>.

[mount]
vfs_cache_mode = "writes"          # off | minimal | writes | full

rclone logs “No host key validation is being performed”. That refers to the unused internal library, not to your mounts.

Shared SSH settings

Bifröst passes these options to ssh, to sshfs and to rclone’s --sftp-ssh:

BatchMode=yes ConnectTimeout=10 ServerAliveInterval=15 ServerAliveCountMax=3 ControlMaster=no ControlPath=none

None of them touches host-key checking, and no config key can. To use a separate ssh config for Bifröst, set mount.ssh_config; it is passed as -F. Before every mount, an ssh -s <host> sftp preflight runs, and its error becomes the mount’s last_error.

Unmounting

• Graceful by default. A busy mount (files open) stays mounted, reports unmount blocked: busy (files open), and is retried later.
• Forced with bifrost unmount --force, and automatically for dead mounts and mounts hung past offline_grace_period: a lazy detach (fusermount3 -uz, or diskutil unmount force on macOS). Open files keep working until they are closed.
• Nothing is killed. Bifröst never signals a process that holds your files. The only process it ever signals is its own mount child that never finished mounting.
• Stopping the daemon unmounts nothing. Mounts keep serving and the next bifrostd adopts them.

macOS

Caution

macOS support is built but not field-tested: the release binaries are built on GitHub’s macOS runners and CI builds and unit-tests the workspace there, but mounting on a real Mac (macFUSE, FUSE-T, rclone nfsmount) has not been exercised yet.

• FUSE detection: macFUSE at /Library/Filesystems/macfuse.fs; FUSE-T at /Library/Application Support/fuse-t or /usr/local/lib/libfuse-t.dylib.
• rclone-nfs comes first in the default order because it needs no kernel extension. It serves the mount on an unauthenticated 127.0.0.1 NFS port that other local users could reach, so on a shared Mac put rclone or sshfs first. nfsmount may also need root. With vfs_cache_mode below writes it would be read-only, so Bifröst raises it to writes.
• Mount table via getmntinfo; unmount with /sbin/umount; force with diskutil unmount force.
• Permissions: macFUSE needs its system extension approved. When a mount’s log mentions kernel extension, System Extension or not permitted, the error gets the hint allow the macFUSE system extension in System Settings → Privacy & Security, and bifrost doctor repeats it. Approve it there (and reboot if asked), or use FUSE-T or rclone-nfs, which need no kernel extension.