Using AutoFS to mount EncFS over CIFS

Background

My hosting provider offers backup space accessible using FTP, SSH/SFTP, and CIFS. I want to make use of this space, but I don't want to put potentially sensitive data on there unencrypted, so I went looking for a solution.

I considered using LUKS, but it's not ideal because in this case I'd have to create a disk image on the remote storage which would be remounted via a loop device locally. In addition it would be complicated to resize the remote image.

I found EncFS to be a suitable alternative. It provides an encrypted filesystem in user-space, and works on file level instead of on a block level. This way there's no need to determine your block device size in advance.

I won't go into how to set up an EncFS mount as there's plenty of documentation out there for this.

First round: EncFS over CurlFTPfs

Initially I thought the hosting provider only gave FTP access to the backup space, so I went about stacking EncFS on top of CurlFTPfs - a most inelegant solution. It appeared to work, however I found that eventually doing an rsync through all the layers was causing problems. I tried limiting the rsync bandwidth with the --bwlimit option, but it only delayed the problem. I guessed this was CurlFTPfs so I did some more digging.

Second round: EncFS over CIFS/Samba

I found there was also CIFS (Samba) access to the backup space, so I tried out the same EncFS on top of the CIFS mount and it worked great. No more problems. So I wanted to automate this as much as possible, removing the extra logic to make sure the CIFS mount is in place before mounting the EncFS space.

AutoFS

Unfortunately this is where the bad news comes (although there is also good news later). I tried various approaches but was unable to get EncFS to mount via AutoFS, although it does work through fstab. This appears to be due to how EncFS and AutoFS manage creation of the mount point.

So the current setup is to use AutoFS to mount the CIFS share, and to mount the EncFS share with it's own idle handling logic.

The relevant configs are:
/etc/auto.master

/net  /etc/auto.hetzner  --timeout=150

/etc/auto.hetzner

ht-backup-raw  -fstype=cifs,credentials=/etc/backup/samba.auth  ://unique_id.your-backup.de/backup

EncFS

Creating an automated way to ount the EncFS space also required some hackery. There is an option to use an external password program, however this can't be referenced in /etc/fstab, so an additional FUSE wrapper script needs to be created. This script receives 4 arguments. This script I've written is not perfect, but enough for the task:

/usr/local/sbin/encfs-extpass

#!/bin/bash

SOURCE="$1"
MOUNTPOINT="$2"
ARG1="$3"
OPTIONS="$4"

if echo $4 | grep -q 'autofs'; then
       MOUNTPOINT_PATH=/$(basename $2)
       OPTIONS=${4/autofs,/}
       encfs --extpass="/etc/encfs${MOUNTPOINT_PATH}.sh" "$1" "${MOUNTPOINT_PATH}" -o "$OPTIONS"
else
       encfs --ondemand --idle=1 --extpass="/etc/encfs${2}.sh" "$1" "$2" -o "$4"
fi

You can see my feeble attempt in there to debug and mount via autofs, but it didn't work. I leave it there for others to try if they wish.

The final bit of the magic is:

/etc/fstab

encfs-extpass#/net/ht-backup-raw  /ht-backup  fuse  defaults  0 0

Here I have a line in /etc/fstab that uses FUSE to call my custom script, which in turn mounts my encfs space, but without an interactive prompt. This is because I have a script at /etc/encfs/ht-backup.sh that echo's out the password, which is all encfs needs. I make sure the password script is chmod 400 and owned by root and my password is relatively safe.

Conclusion

Now when the system comes up the EncFS space is mounted automatically. I haven't tested the idle unmounting thoroughly yet, but in theory it should work.