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dockersh

A user shell for isolated, containerized environments.

What is this?

dockersh is designed to be used as a login shell on machines with multiple interactive users.

When a user invokes dockersh, it will bring up a Docker container (if not already running), and
then spawn a new interactive shell in the container's namespace.

dockersh can be used as a shell in /etc/passwd or as an ssh ForceCommand.

This allows you to have a single ssh process on the normal ssh port which places user
sessions into their own individual docker containers in a secure and locked down manner.

Why do I want this?

You want to allow multiple users to ssh onto a single box, but you'd like some isolation
between those users. With dockersh each user enters their
own individual docker container (acting like a lightweight virtual machine), with their home directory mounted from the host
system (so that user data is persistent between container restarts), but with it's own kernel namespaces for
processes and networking.

This means that the user is isolated from the rest of the system, and they can only see their own processes,
and have their own network stack. This gives better privacy between users, and can also be used for more easily
separating each user's processes from the rest of the system with per user constraints.

Normally to give users individual containers you have to run an ssh daemon in each
container, and either have have a different port for each user to ssh to or some nasty
Forcecommand hacks (which only work with agent forwarding from the client).

Dockersh eliminates the need for any of these techiques by acting like a regular
shell which can be used in /etc/passwd or as an ssh
ForceCommand.
This allows you to have a single ssh process, on the normal ssh port, and gives
a secure way to connect users into their own individual docker
containers.

SECURITY WARNING

dockersh tries hard to drop all privileges as soon as possible, including disabling
the suid, sgid, raw sockets and mknod capabilities of the target process (and all children),
however this doesn't mean that it is safe enough to allow public access to dockersh containers!

WARNING: Whilst this project tries to make users inside containers have lowered privileges
and drops capabilities to limit users ability to escalate their privilege level, it is not certain
to be completely secure. Notably when Docker adds user namespace support, this can be used
to further lock down privileges.

SECOND WARNING: The dockersh binary needs the suid bit set so that it can make the syscalls to adjust
kernel namespaces, so any security issues in this code are likely to be exploitable to root.

Requirements

Linux >= 3.8

Docker >= 1.2.0

If you want to build it locally (rather than in a docker container), Go >= 1.2

Installation

With docker

(This is the recommended method).

Build the Dockerfile in the local directory into an image, and run it like this:

$ docker build .
# Progress, takes a while the first time..
....
Successfully built 3006a08eef2e 
$ docker run -v /usr/local/bin:/target 3006a08eef2e

Without docker

You need to install golang (tested on 1.2 and 1.3), then you should just be able to run:

go get
make

and a 'dockersh' binary will be generated in your $GOPATH (or your current
working directory if $GOPATH isn't set). N.B. This binary needs to be moved to where
you would like to install it (recommended /usr/local/bin), and owned by root + u+s
(suid). This is done automatically if you use the Docker based installed, but
you need to do it manually if you're compiling the binary yourself.

Invoking dockersh

There are two main methods of invoking dockersh. Either:

  1. Put the path to dockersh into /etc/shells, and then change the users shell
    in /etc/passwd (e.g. chsh myuser -s /usr/local/bin/dockersh)
  2. Set dockersh as the ssh ForceCommand in the users $HOME/.ssh/config, or
    globally in /etc/ssh/ssh_config

Note: The dockersh binary needs the suid bit set to operate!

Configuration

We use gcfg to read configs in an ini style format.

The global config file, /etc/dockershrc has a [dockersh] block in it, and zero or more [user "foo"] blocks.

This can be used to set settings globally or per user, and also to enable the setting
of settings in the (optional) per user configuration file (~/.dockersh), if enabled.

Config file values

Setting name Type Description Default value Example value
imagename String The name of the container image to launch for the user. The %u sequence will interpolate the username busybox ubuntu, or %u/mydockersh
containername String The name of the container (per user) which is launched. %u_dockersh %u-dsh
mounthome Bool If the users home directory should be mounted in the target container false true
mounttmp Bool If /tmp should be mounted into the target container (so that ssh agent forwarding works). N.B. Security risk false true
mounthometo String Where to map the user's home directory inside the container. %h /opt/home/myhomedir
mounthomefrom String Where to map the user's home directory from on the host. %h /opt/home/%u
usercwd String Where to chdir into the container when starting a shell. %h /
containerusername String Username which should be used inside the container. %u root
shell String The shell that should be started for the user inside the container. /bin/ash /bin/bash
mountdockersocket Bool If to mount the docker socket from the host. (DANGEROUS) false true
dockersocket String The location of the docker socket from the host. /var/run/docker.sock /opt/docker/var/run/docker.sock
entrypoint String The entrypoint for the persistent process to keep the container running internal /sbin/yoursupervisor
cmd Array of Strings Additional parameters to pass when launching the container as the command line -c'/echo foo'
dockeropt Array of Strings Additional options to pass to docker when launching the container. Can be used to mount additional volumes or limit memory etc. -v /some/place:/foovol
enableuserconfig Bool Set to true to enable reading of per user ~/.dockersh files false true
enableuserimagename Bool Set to true to enable reading of imagename parameter from ~/.dockersh files false true
enableusercontainername Bool Set to true to enable reading of containername parameter from ~/.dockersh files. (Dangerous!) false true
enableusermounthome Bool Set to true to enable reading of mounthome parameter from ~/.dockersh files false true
enableusermounttmp Bool Set to true to enable reading of mounttmp parameter from ~/.dockersh files false true
enableusermounthometo Bool Set to true to enable reading of mounthometo parameter from ~/.dockersh files false true
enableusermounthomefrom Bool Set to true to enable reading of mounthomefrom parameter from ~/.dockersh files false true
enableuserusercwd Bool Set to true to enable reading of usercwd parameter from ~/.dockersh files false true
enableusercontainerusername bool Set to true to enable reading of containerusername parameter from ~/.dockersh files false true
enableusershell Bool Set to true to enable reading of shell parameter from ~/.dockersh files false true
enableuserentrypoint Bool Set to true to enable users to set their own supervisor daemon / entry point to the container for PID 1 false true
enableusercmd Bool Set to true to enable users to set the additional command parameters to the entry point false true
enableuserdockeropt Bool Set to true to enable users to set additional options to the docker container that's started. (Dangerous!) false true

Notes:

  • Boolean settings are set by just putting the setting name in the config (see examples below).
  • You must set both enableuserconfig and the specific enableuserxxx setting that you want in /etc/dockersh to
    get any values parsed from ~/.dockersh
  • Array values are represented by having the same config key appear multiple times, once per value.

Config interpolations

The following sequences are interpolated if found in configuration variables:

Sequence Interpolation
%u The username of the user running dockersh
%h The homedirectory (from /etc/passwd) of the user running dockersh

Example configs

A very restricted environment, with only the busybox container, limited to 32M of memory, /etc/dockersh looks like this:

[dockersh]
imagename = busybox
shell = /bin/ash
usercwd = /

A fairly restricted shell environment, but with homedirectories and one admin user being allowed additional privs, set the following /etc/dockersh

[dockersh]
imagename = ubuntu:precise
shell = /bin/bash
mounthome

[user "someadminguy"]
mounttmp
mountdockersocket

In a less restrictive environment, you may allow users to choose their own container and shell, from a 'shell' container
they have uploaded to the registry, and have ssh agent forwarding working, with the following /etc/dockersh

[dockersh]
imagename = "%u/shell"
mounthome
mounttmp
enableuserconfig
enableusershell

[user "someadminguy"]
mountdockersocket

And an example user's ~/.dockersh

[dockersh]
shell = /bin/zsh

Or just allowing your users to run whatever container they want:

[dockersh]
mounthome
mounttmp
enableuserconfig
enableuserimagename

Caveats

  • User namespaces are not supported (yet) so if users escalate to root inside the container, they can probably escape
  • Tty/Pty handling is not great - whilst things appear to work, they don't go well in unusual circumstances (e.g. your process being killed due to OOM).
  • This code has not been audited by a 3rd party or a container expert, there are probably issues waiting to be found!

TODO

  • How do we deal with changed settings (i.e. when to recycle the container)
    • Document just kill 1 inside the container?
  • Fix up go panics when exiting the root container.
  • getpwnam so that we can interpolate the user's shell from /etc/shells (if used in ForceCommand mode!)
  • Decent test cases
  • Use libcontainer a lot more, in favour of our code:
  • Find a better way to make ssh agent sockets work than to bind /tmp

Contributing

Patches are very very welcome!

This is our first real Go project, so we apologise about the shoddy quality of the code.

Please make a branch and send us a pull request.

Please ensure that you use the supplied pre-commit hook to correctly format your code
with go fmt:

ln -s hooks/pre-commit .git/hooks/pre-commit

Copyright

Copyright (c) 2014 Yelp. Some rights are reserved (see the LICENSE file for more details).

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