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This is a text-only version of the following page on https://raymii.org:
---
Title : bdsh - Whitelist Restricted Shell
Author : Remy van Elst
Date : 29-06-2013
URL : https://raymii.org/s/software/bdsh.html
Format : Markdown/HTML
---

bdsh is a shell where you whitelist commands and only those commands can be
executed. Either via ssh, as an interactive shell or launched with commands.
Logs everything and escapes "dangerous" characters.

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bdsh stands for Boa Diminish SHell. You can probably guess why. It might have to
to with snakes and restricting users, no pun intended.

### What is the reason you wrote bdsh?

Sometimes sysadmins are forced to work with insecure systems or badly written
applications. Critical systems working with a push model instead of a pull
model, things that use ssh and break when stuff changes or users that need to be
audit trailed.

I couldn't find an easy way to set a shell for a user that both logs and is
configurable via a whitelist. I tried scripts in `authorized_keys` but then
things `scp` and `sftp` break. bdsh tries to solve that and is fairly successful
in that.

### Requirements

* Python 2.6+
* Python 3 is supported!

bdsh is tested on Ubuntu, CentOS, OpenSUSE and Debian all with Python 2.7 or
above.

### Download

Either from github:

git clone https://github.com/RaymiiOrg/boa-diminish-restricted-shell.git

Or from this website:

wget http://raymii.org/s/inc/software/bdsh.py

### Installation

* Clone the git repo &&&& cd in
* Install bdsh sytemwide
* `sudo cp bdsh.py /usr/bin/bdsh`
* `sudo chmod +x /usr/bin/bdsh`
* Edit and place whitelist
* `vi example_whitelist.conf &&&& sudo cp example_whitelist.conf /etc/bdsh_whitelist.conf`
* Edit `/etc/shells` and add `/usr/bin/bdsh`
* Set the shell for the user, either via:
* `sudo chsh -s /usr/bin/bdsh $USERNAME`
* or
* `vi /etc/passwd`

### Tips

#### Enable SFTP/SCP

Put these two lines in the whitelist file:

scp
/usr/lib/openssh/sftp-server

Note that you might have to change `scp` to `/usr/bin/scp`.

### Important

bdsh only checks if the command is whitelisted, not the arguments. So if you
allow `ls`, you also allow `ls -la`, and `ls -d` and such.

bdsh is not 100% safe, but it does provide a layer of security.

Read this article about restriced shells: http://pen-
testing.sans.org/blog/2012/06/06/escaping-restricted-linux-shells

Don't change the "Dangerous Characters" array, if you for example remove the
`&&` then you can do something like this: `ssh user@host "ls &&&& perl -e 'exec
"/bin/bash"'"`.

It does try its best to catch restriction-escaping:

Jun 29 17:41:07 localhost bdsh: [RESTRICTED SHELL]: user "testshell" executed vim
Jun 29 17:41:11 localhost bdsh: [RESTRICTED SHELL]: user "testshell" NOT allowed for /usr/bin/bdsh -c bash

### See Also

Another way to restrict ssh, written by me:
<https://github.com/RaymiiOrg/restrict_ssh> The github page:
<https://github.com/RaymiiOrg/boa-diminish-restricted-shell>

[1]: https://www.digitalocean.com/?refcode=7435ae6b8212

---

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