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+# schsh: Restricted file access via SSH
+
+## Introduction
+
+Welcome to [schsh](https://www.ralfj.de/projects/schsh), a schroot-based shell.
+
+Its purpose is simple: I want to provide users with scp, sftp and rsync access
+to my server, such that they can only operate in a certain subdirectory.
+There are plenty of solutions for this problem out there, and all have one
+drawback in common:
+You need to manually set up a bunch of chroots, and copy the files needed for
+scp, sftp and rsync into them.
+
+I didn't like that, so here is my alternative solution: Use schroot for the
+chroots. This gets OpenSSH out of the loop when it comes to chroots, instead
+the relevant users get a special shell (schsh, the schroot shell). That shell
+essentially calls schroot and runs the desired command inside the chroot. It
+also provides some very basic command restriction (so that you can allow scp,
+sftp and rsync and nothing else).
+
+Unfortunately, this still needs a (s)chroot to be set up for each user, but at
+least no files have to be copied: Instead, schroot is configured to bind-mount
+the relevant system folders into the user-chroot. Hence no files are
+duplicated, and system updates to the relevant tools are applied inside the
+chroots automatically. For additional hardening, these bind-mounts are
+configured to be read-only and no-setuid, while the only user-writeable folder
+is no-exec.
+
+## Setup
+
+Before you start, make sure you have the dependencies installed:
+schsh needs [Python 3](http://www.python.org) (I tested it with
+version 3.2) and [schroot](https://wiki.debian.org/Schroot)
+(version 1.6 or newer).
+
+Installation is simple: Just run `make install`. That will copy some files
+to `/usr/local/bin`, and some configuration to `/etc/schroot/`.
+Before you create any users, make sure the directory `/var/lib/schsh` and a
+group called `schsh` exist.
+
+You should also set up SSH to disallow port forwarding for users controlled by
+schsh. See `sshd_config` in the source folder for an appropriate snippet
+of OpenSSH configuration.
+
+Before you can set up schsh for a user, you need to create it first:
+
+ adduser sandboxed --disabled-password
+
+Any existing user can be "sandboxed" by running:
+
+ makeschsh sandboxed
+
+This does the following:
+
+* Change the user's shell to `/usr/local/bin/schsh`
+* Create a chroot base in `/var/lib/schsh/sandboxed` with some empty
+ subfolders as well as `/etc/passwd` and `/etc/group` containing
+ only root, this user and the `schsh` group
+* Add the user to the `schsh` group
+* Set up a schroot called `schsh-sandboxed` for the given folder, and an
+ fstab file in `/etc/schroot/schsh` used by this schroot
+
+Now if the user logs in via SSH, `/usr/local/bin/schsh` will be executed,
+and it will lock the user into the schroot `schsh-sandboxed`. It will
+only see some system folders and a folder called `/data` mapped to
+`/home/sandboxed/data`. If you want to give the user access to more
+folders, or another folder, simply edit `/etc/schroot/schsh/sandboxed.fstab`.
+The only part of schsh writing any files is `makeschsh`, so you can change
+the users' schroot configurations at your will.
+
+## Configuration
+
+There is not much to configure at the moment. However, there are some
+global variables at the top of both `schsh` and `makeschsh` to
+change the base paths, and to tell which commands are allowed.
+
+## Source, License
+
+You can find the sources in the [git
+repository](http://www.ralfj.de/git/schsh.git) (also available [on
+GitHub](https://github.com/RalfJung/schsh)). They are provided under the
+[GPLv3](https://www.gnu.org/licenses/gpl.html). In addition, all files except
+for `schsh-rrsync` are provided under the
+[GPLv2](https://www.gnu.org/licenses/old-licenses/gpl-2.0.html) or (at your
+option) any later version of the GPL. See the file `LICENSE-GPL3` for more
+details.
+
+## Contact
+
+If you found a bug, or want to leave a comment, please
+[send me a mail](mailto:post-AT-ralfj-DOT-de).
projects / schsh.git / blobdiff