sftp chroot jail in Ubuntu

(Update 16 Mar 2011: Since writing this post, I’ve learned of an easier way to create this chroot jail. Newer versions of OpenSSH enable the “ChrootDirectory” configuration directive. I recommend that you take a look at George Ornbo’s tutorial on chrooting sftp users in Intrepid for the details.)

(Updated 08 Feb 2011 to reflect xplicit’s experience on Ubuntu 10.04.)

I wanted to give a buddy access to a website hosted on my box. So I tried scponly, since I only wanted to provide SFTP access to that particular directory, using a chroot jail. The steps are as follows.

1. Install the scponly package using Ubuntu’s APT package management system.
2. Use the script provided to set up your first jail and your user’s home directory. For the location of the user’s jail, give the path of the directory you want to share.
3. Provide a password for the new user.
4. Ensure that the new user has permissions to read and write all the necessary directories in your Web site.


$ sudo apt-get install scponly
$ gzip -dc /usr/share/doc/scponly/setup_chroot/setup_chroot.sh.gz > /tmp/setup_chroot.sh
$ cp /usr/share/doc/scponly/setup_chroot/config.h /tmp

The previous step copies the “config.h” file to help things go more smoothly, as Luke found.

$ chmod +x /tmp/setup_chroot.sh
$ cd /tmp
$ sudo ./setup_chroot.sh


Next we need to set the home directory for this scponly user.
please note that the user's home directory MUST NOT be writeable
by the scponly user. this is important so that the scponly user
cannot subvert the .ssh configuration parameters.
For this reason, a writeable subdirectory will be created that
the scponly user can write into.

Note that I removed the /incoming subdirectory created by this script. There was no need for a separate directory for my buddy to upload files. He could have permissions over the whole site tree.


-en Username to install [scponly]
bob
-en home directory you wish to set for this user [/home/bob]
/var/www/sites/bobsite/htdocs
-en name of the writeable subdirectory [incoming]


-e
creating /var/www/sites/bobsite/htdocs/incoming directory for uploading files


Your platform (Linux) does not have a platform specific setup script.
This install script will attempt a best guess.
If you perform customizations, please consider sending me your changes.
Look to the templates in build_extras/arch.
- joe at sublimation dot org


please set the password for bob:
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
if you experience a warning with winscp regarding groups, please install
the provided hacked out fake groups program into your chroot, like so:
cp groups /var/www/sites/bobsite/htdocs/bin/groups


This script added certain directories to the site root (/var/www/sites/bobsite/htdocs). Every other directory needed to be writable by Bob. So let’s add Bob to a special group, and allow that group write access on all the website’s files.


$ sudo adduser bob www-data


We can ignore /bin, /etc, /lib and other directories added to the chroot jail (the website filesystem):


$ sudo find . \! -user root -exec chgrp www-data \{\} \;
$ sudo find . \! -user root -exec chmod g+w \{\} \;


Good to go!