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Bazaar over chrooted sftp

How to set up bzr for chrooted sftp users

How to restrict a user’s access to sftp://.../var/bzr

For the prototyping editor itself and for a lot of our clients’ projects we are heavy Bazaar users at pidoco° to manage our distributed workflow. When we started some years ago we just installed bzr on one of the test servers where all of the developers had ssh access anyway. We put the repositories in /var/bzr and used sftp to checkout/push/pull source changes. This was handy as a sftp server comes with openssh installed.

As the team grew over the years we got to a point where we wanted to give new developers access to the bzr repositories without giving them full ssh access. However we did not want to have to change all the urls for existing repositories. Luckily this can be achieved easily since Debian Lenny.

Per Server Settings

On our scm server we have a user group called bzr that grants read/write access to most of the repositories (except of some personal or release branches) to all users with bzr access. And now we added the group sftponly. All users in this group will be restricted to sftp access only instead of a full shell.

sudo addgroup sftponly
sudo addgroup bzr

You probably have to add ‘/usr/lib/sftp-server’ to /etc/shells to make it a valid shell, eg. like this:

root@host # echo '/usr/lib/sftp-server' >> /etc/shells

The following settings in /etc/ssh/sshd_config force the internal sftp server to be used by openssh and change the root directory for all users in the group sftponly to /var/chroot. Make sure to restart sshd afterwards.

Subsystem sftp internal-sftp
Match Group sftponly
    ChrootDirectory /var/chroot
    AllowTCPForwarding no
    X11Forwarding no
    ForceCommand internal-sftp

Up to now our repositories have been in /var/bzr. These need to be moved to /var/chroot/var/bzr to let the sftponly users access them. /var/chroot needs to have root:root as owner for openssh to work correctly. For the existing ssh users we add a symbolic link to keep the old paths working:

sudo mkdir /var/chroot
sudo chown root:root /var/chroot
sudo mkdir /var/chroot/var
sudo mv /var/bzr /var/chroot/var
sudo ln -s /var/chroot/var/bzr /var/bzr

Per User Settings

giving the user username sftp access, but nor ssh access:

USERNAME=username                                  # give the user a name
sudo adduser ${USERNAME}                           # add user and data to system
sudo usermod -s /usr/lib/sftp-server ${USERNAME}   # disallow ssh/bash, allow ssh/ftp (sftp)
sudo adduser ${USERNAME} bzr                       # allow group access to most bzr folders
sudo adduser ${USERNAME} sftponly                  # disallow access to /, allow access to /var/bzr

This changes user’s shell to sftp-server.

Result

As a result of these settings both normal ssh users as well as restricted users in the sftponly group can use the same url for their shared repositories
bzr checkout sftp://my.domain/var/bzr/my_repository. By using chroot however users in the group sftponly are restricted to using sftp and can only access the folders in the bzr subdirectory.

Sources

In the Debian Administration Weblog you can find information on how to setup an OpenSSH SFTP chroot() with ChrootDirectory and on how to restrict users to SFTP only instead of SSH.