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Patches to OpenSSH to allow it to authorize access to version control systems
(like SVN and Git) in a flexible way. This allows you to run a daemon which
gives users access to application resources without real system accounts,
similar to how GitHub allows access to Git repositories over SSH without
creating real system accounts for each user (instead, users upload public SSH
keys).
If you're just using Git and don't have all that many user accounts, Gitosis
is probably a much better solution to this problem (you could also try Gitolite
or Gitorious). sshd-vcs is a much more raw solution; while it is more powerful,
it is far more difficult to set up.
In more detail, the problem this solves is:
- "Application" is whatever you're building, like GitHub. "Application
Users" are people who use your application, like GitHub users.
- "System" is the underlying Lunix system. "System Users" are real user
accounts that have UIDs.
- You have an arbitrary number of application users (like GitHub users)
who need to access some application resource (like Git repositories).
You don't want to create system accounts for each user. sshd-vcs allows
all the application users to act as a single system user, while using
application credentials (instead of system credentials) to authenticate
them.
- You either want a more flexible configuration than Gitosis provides,
or want to support something other than git. Gitosis is a much simpler
solution to this problem if it is suitable for your needs (it should
be suitable in nearly all cases).
Specifically, there are two new config parameters available:
- AuthorizedKeysScript: Public keys can now be loaded from an external
source instead of ~/.ssh/authorized_keys.
- ForceUser: Ignore the login user and always use a specific account.
Basically this allows you to set up sshd something like this:
- Write a script which accepts an SSH public key on stdin, and produces
either a nonzero exit code to deny login, or a zero exit code with
an optional option string on stdout to accept login. Suppose this script
is called "auth.sh".
- Write a script which accepts a user as argv[1], a command from
environmental variable SSH_ORIGINAL_COMMAND, and uses stdin/stdout to
communicate with the remote client. Suppose this script is called
"serve.sh". You can adapt something like Gitosis to accomplish this, if
your backing service is git but you're choosing sshd-vcs for additional
flexibility.
- Have "auth.sh" emit something like 'command="serve.sh <user>",
no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty' when it
finds a valid public key.
- Set AuthorizedKeysScript to "auth.sh".
- Create a real system user account like "git" which all users will act as
when logged into the system.
- Set ForceUser to "git".
- Have application users put their public SSH keys into a database.
- Now, when users login, sshd invokes "auth.sh" and sends it the public key
on stdin. "auth.sh" checks the database and either sees that the key is
not present (in which case it exits nonzero), or sees that the key is
valid and identifies an application user, in which case it emits
'command="serve.sh <appuser>",...' to stdout and exits 0. sshd now invokes
the command, and puts the user's original command in the environmental
variable SSH_ORIGINAL_COMMAND. "serve.sh" runs with the valid user
in argv[1]. It can now execute Gitosis, or wrap raw git commands, or
whatever else.
You should run sshd-vcs on port 22 and run a real copy of sshd on some other
port if you need shell access to the box. It is strongly recommended you do not
try to run a copy of sshd-vcs as your real sshd. Instead, run sshd-vcs with
every possible access setting locked down and real sshd elsewhere.
For some very basic examples, see example/.
Building
./configure --prefix=/opt/sshd-vcs
make
Installing
sudo make install
Once off setup
sudo mkdir /var/empty
sudo adduser --system --no-create-home --home /var/run/sshd --shell /usr/sbin/nologin sshd
sudo apt-get install -y php5
sudo mkdir /opt/sshd-vcs/etc
sudo ssh-keygen -t rsa -f /opt/sshd-vcs/etc/ssh_host_rsa_key
sudo ssh-keygen -t dsa -f /opt/sshd-vcs/etc/ssh_host_dsa_key
Setup paths to auth script in ./example/sshd_config and path to serve.php in auth.php
Running
sudo /opt/sshd-vcs/sbin/sshd-vcs -f ./example/sshd_config
Testing
ssh git@127.0.0.1
sshd-vcs is based extensively on prior work:
https://github.com/wuputahllc/openssh-for-git
http://www.openssh.com/
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