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Setting up chrooted SFTP-only access on a Synology DiskStation

August 28th, 2013 Leave a comment Go to comments

This has been on my list to try to figure out for a long time. I wanted SFTP only access to specific accounts, and to be able to chroot them. It took me a while and various attempts, only to get wind up landing on the most basic solution, of course.

I originally tried scponly and scponlyc (which I've used in the past) and rssh, however none of them worked properly for me.

Sure enough, the openssh package from optware worked right out of the box.*

wget http://wizjos.endofinternet.net/synology/archief/syno-mvkw-bootstrap_1.2-7_arm-ds111.xsh
ipkg install openssh openssh-sftp-server

Then edit /opt/etc/openssh/sshd_config, and put in:

Match User username
        ChrootDirectory /some/directory
        ForceCommand internal-sftp

Also edit the user account in /etc/passwd, change the home dir to the /some/directory, and give it "/bin/sh" for a shell.

Viola... when sshd is restarted next time it will just work.

The guys at optware made a neat startup script that will start their sshd on boot. So nothing to do there.

Make sure to disable synology's built-in ssh (Control Panel > Terminal) or you'll probably be hitting the wrong one!

If you are concerned about privileges, the way that Synology runs its units isn't very UNIX permission friendly (most files are world writable on the filesystem, and the expectation is the daemons will properly control the access.) I wound up creating a little cron job that chmods and chowns files to keep the secondary account I've created to be a "read only" account to that directory.

* As always with my tips, YMMV - this worked fine on my Atom-based DS2411+ unit. but when I tried the same setup on a DS213, it didn't seem to work. No idea why, there aren't much diagnostics or logs provided to use. Sorry.

UPDATE: After running this on the "working" NAS unit for a bit, it stopped working. The culprit was the ChrootDirectory became owned by the user, not by root:root. Changing it back (chown root:root /some/directory) fixes that. So it looks like OpenSSH wants that in place for the chroot stuff to work. That could have been the issue mentioned in the previous paragraph (couldn't test it anymore)

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