Authenticate SSH users keys with GitHub
Project description
GitHub-SSH-Auth
About
This project aims to provide a way for SSHd
to authenticate users on shell boxes using GitHub API v3 SSH keys of users in your organization.
How it works
SSH Authentication against GitHub API is done using a feature of OpenSSH
, namely AuthorizedKeysCommand
and AuthorizedKeysCommandUser
.
Everytime a user connects, the script will be called with the login as command line parameter.
So, everytime an user connects using SSH, the following happens :
-
sshd
deamon runsgithub-ssh-auth
under the user defined byAuthorizedKeysCommandUser
option. -
github-ssh-auth
reads its configuration file (by default/etc/github-ssh-auth/conf
) -
according to the configuration file, it looks up the username given by
sshd
and checks if that user is granted permission to connect to this host -
if yes, it tries to read the cache file (recommended but can be disabled) to find user's keys. If no cache file found or if disabled, then it queries GitHub API to get the keys and creates the cache (if enabled).
The rest is handled by sshd
itself, i.e. checking validity of that public key and the rest of the connection handling.
So basically there are three possible outcomes:
-
User shall be granted no access to this shell, the return (to stdout) will be empty, user will effectively be denied access
-
User shall be granted access but user has no key set, the return (to stdout) will be empty too, user will therefore be denied access
-
User shall be granted access and user has keys, they are given back to
sshd
to be processed further bysshd
itself :+1:
Updating keys and cache use
To avoid flooding GitHub API and consequently being temporarily banned from using them in case of massive connects, it is recommended to keep the cache enabled and update the keys only few times a day. The periodicity is yours and that is why there is a special update
command line parameter for that.
Consider the following scenario:
- cache is enabled
- cache file already exists
- a new user has joined the team OR an existing user replaced his/her keys
In such case, the cache file will NOT be updated when authentication happens, this is the behavior set by design to separate concerns and prevent connection to the outside world being in the critical path for authentication.
Instead, a locally defined cron
should either:
- call
github-ssh-update
to update cache - delete cache file (by default
/etc/github-ssh-auth/cache.json
) which will force recreation when next auth happens
Both will have the same outcome but the former is cleaner than the latter.
All in all, choice is yours :wink:
Installation
Since this Python module deals with SSH authentication, it should be installed globally, hence:
$ sudo pip install github-ssh-auth
This will install the following program and its shortcuts:
github-ssh
The real application, handling all options, but for convenience the shortcuts described after can be used.
Usage
Usage: github-ssh [OPTIONS] COMMAND [ARGS]...
Options:
--version Show the version and exit.
--help Show this message and exit.
Commands:
auth Authenticate user.
update Update GitHub SSH Auth cache file (users, teams, keys).
github-ssh-auth
Responsible for authentication itself, this one is to be called by sshd
itself.
Usage
Usage: github-ssh-auth [OPTIONS] LOGIN
Authenticate user.
Options:
-c, --config TEXT
--help Show this message and exit.
github-ssh-update
Responsible for updating cache file, it can be scheduled to run periodicaly to ensure synchronization with updated keys from Github.
Usage
Usage: github-ssh-update [OPTIONS]
Update GitHub SSH Auth cache file (users, teams, keys).
Options:
-c, --config TEXT
--help Show this message and exit.
Configuration
SSH configuration in /etc/ssh/sshd_config
These lines should be somewhere in your sshd
configuration file. Usually in /etc/ssh/sshd_config
:
AuthorizedKeysCommand /usr/bin/github-ssh-auth -u %u
AuthorizedKeysCommandUser nobody
GitHub token requirements
Since this application is dealing with some sensitive data (users and their team memberships) within an organization, we will need to create a so-called Personal access tokens
.
To do that, fire up your GitHub organization dashboard, look for Settings
then Developer settings
.
Then click on Generate new token
and set its permissions to:
read:org
This is the only requirement so that the API can be queried for users and teams memberships. All users keys are public by default an can be accessed from the outside world without authentication against GitHub API.
See for yourself, go to https://github.com/<yourhandle>.keys
. :rocket:
GitHub SSH Auth configuration file
It resides by default in /etc/github-ssh/conf
but of course you can change it using -c
flag when calling (see above).
The format is a standard INI style, as per configparser
.
Configuration file template
Below is the complete grammar with inline comments:
[global]
access_token = <token> # This is mandatory
organization = <org> # This is also mandatory
# In case of connectivity lost and to prevent too many connections to GitHub API,
# it is strongly recommended to set it to an absolute filepath.
# Default is set to /etc/github-ssh/cache.json
# If you want to disable, set it to 'false'
cache_file = [/path/to/file | false]
# If not overridden after, users (whether individual or teams) will have these configurations applied.
# By default, nothing is set so basically no one will be granted access
# The '<' case means that if a local exists with the same name as a GitHub user,
# it will be granted access. It is a shorthand to avoid a too
# complex, verbose yet common use case where every developer would
# like to have his/her own shell account.
teams_default = [ list,of,local,users,or,< ]
users_default = [ list,of,local,users,or,< ]
# Configuration below will override defaults
[teams]
<team_name> = [ list,of,local,users,or,< ]
...
# And same for users specific configuration
[users]
<user_name> = [ list,of,local,users,or,< ]
...
Special note on the '<'
Just a quick focus on that special caracter, designed to allow any user
to connect provided that:
user
is also the Github handleuser
exists in the local user base (getent passwd
)
Real world example
Let's say Acme Corp. wants all its developers connect with their GitHub accounts. Let's say users jdoe
bob
alice
are such handles. Also these user handles are also the respective handlers in GitHub.
Then with this simple configuration, we allow globally all these users to login with their handles:
[global]
...other config skipped for brevety...
users_default = <
Now if you only want bob
to connect using login bob
, the configuration file would look like this one:
[global]
...other config skipped for brevety...
users_default =
[users]
bob = <
Other real world examples
Some other ready-to-be-deployed-or-almost can be found in the example
directory.
Testing
As this is my first Python module, and even my first Python program ever, I tried different methods to handle testing.
Having a lot of shortcomings with some of the tools in the ecosystem (tox
, etc.) due to complexity and such I decided to give a go to Docker with Python installed and everything is done inside containers, which is super fast.
After facing same issues elsewhere and again hours digging the Internet, I eventually found that this was related to IPV6 ... Still, it is available for those who want.
Nevertheless, testing is done using Travis CI which is great anyway and does what tox
would have done, at least for my needs (no troll here please).
So once you have git clone
d the repository locally, you can see it for yourself by issuing a:
$ make help
rage artifacts
lint - check style with flake8
test - run tests quickly with the default Python
coverage - check code coverage quickly with the default Python
docs - generate Sphinx HTML documentation, including API docs
release - package and upload a release
dist - package
install - install the package to the active Python's site-packages
And play around with it :wink:
Contributions
Comments, issues, PR as :beer: will be warmly welcomed !
License
GPLv3+
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