gitolite/doc/gitolite-and-ssh.mkd

F=gl_ssh how gitolite uses ssh

Although other forms of authentications exist (see the document on [authentication versus authorisation][auth]), ssh is the one that most git users use.

Therefore, gitolite is (usually) heavily dependent on ssh.

Most people didn't realise this, and even if they did they don't know ssh well enough to help themselves. If you don't understand how ssh public key authentication works, or how the ~/.ssh/authorized_keys file can be used to restrict users, etc., you will have endless amounts of trouble getting gitolite to work, because you'll be attacking the wrong problem.

So please please please understand this before tearing your hair out and blaming git/gitolite for whatever is going wrong with your setup :-)

ssh basics

Let's start with some basics, focusing only on the pieces relevant to gitolite. If this is not detailed enough, please use google and learn more from somewhere, or maybe buy the OReilly ssh book.

• You can login to an ssh server by typing a password, but ssh can also use public-private keys (also called "key pairs") for authentication. gitolite requires you to use this mechanism for your users -- they cannot log in using passwords. Hopefully by the time you finish reading this document you will understand why :-)

  The way you set this up is you generate a key pair on your workstation, and give the server the public key. (I need not add that the "private" key must be, well, kept private!)

• generating a key pair on your workstation is done by running the command ssh-keygen -t rsa. This produces two files in ~/.ssh. One is id_rsa; this is the private key -- never let it out of your machine. The other is id_rsa.pub, which is the corresponding public key. This public key is usually just one long line of text.

  • on Windows machines with msysgit installed, you should do this from within a "git bash" window. The command will report the full path where the files have been written; make a note of this, and use those files in any of the description that follows

• adding your public key to the server's ~/.ssh/authorized_keys file is how ssh uses pubkeys to authenticate users. Let's say sita@work.station is trying to log in as git@serv.er. What you have to do is take the ~/.ssh/id_rsa.pub file for user sita on work.station and append its contents (remember it's only one line) to ~/.ssh/authorized_keys for user git on serv.er.

  The authorized_keys file can have multiple public keys (from many different people) added to it so any of them can log in to git@serv.er.

  In the normal case (not gitolite, but your normal everyday shell access), there's a command that does this, ssh-copy-id, which also fixes up permissions etc., as needed, since sshd is a little picky about allowing pubkey access if permissions on the server are loose. Or you can do it manually, as long as you know what you're doing and you're careful not to erase or overwrite the existing contents of ~/.ssh/authorized_keys on the server!

  But in the gitolite case, it's different; we'll get to that in a minute.

  • troubleshooting pubkey authentication failures: if you are unable to get ssh access to the server after doing all this, you'll have to look in /var/log/secure or /var/log/auth.log or some such file on the server to see what specific error sshd is complaining about.

• restricting users to specific commands is very important for gitolite. If you read man sshd and look for authorized_keys file format, you'll see a lot of options you can add to the public key line, which restrict the incoming user in various ways. In particular, note the command= option, which means "regardless of what the incoming user is asking to do, forcibly run this command instead".

  Also note that when there are many public keys (i.e., lines) in the authorized_keys file, each line can have a different set of options and command= values.

  Without this command= option, the ssh daemon will simply give you a shell, which is not what we want for our gitolite keys (although we may well have other keys which we use to get a shell).

  This is the backbone of what makes gitolite work; please make sure you understand this.

how does gitolite use all this ssh magic?

These are two different questions you ought to be having by now:

• how does it distinguish between me and someone else, since we're all logging in as the same remote user "git"
• how does it restrict what I can do within a repository

restricting shell access/distinguishing one user from another

The answer to the first question is the command= we talked about before. If you look in the authorized_keys file, you'll see entries like this (I chopped off the ends of course; they're pretty long lines):

command="[path]/gl-auth-command sitaram",[more options] ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEA18S2t...
command="[path]/gl-auth-command usertwo",[more options] ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEArXtCT...

First, it finds out which of the public keys in this file match the incoming login. That's crypto stuff, and I won't go into it. Once the match has been found, it will run the command given on that line; e.g., if I logged in, it would run [path]/gl-auth-command sitaram. So the first thing to note is that such users do not get "shell access", which is good!

Before running the command, however, sshd sets up an environment variable called SSH_ORIGINAL_COMMAND which contains the actual git command that your workstation sent out. This is the command that would have run if you did not have the command= part in the authorised keys file.

When gl-auth-command gets control, it looks at the first argument ("sitaram", "usertwo", etc) to determine who you are. It then looks at the SSH_ORIGINAL_COMMAND variable to find out which repository you want to access, and whether you're reading or writing.

Now that it has a user, repository, and access requested (read/write), gitolite looks at its config file, and either allows or rejects the request.

But this cannot differentiate between different branches within a repo; that has to be done separately.

restricting branch level actions

[If you look inside the git source tree, there's a file among the "howto"s in there called update-hook-example.txt, which was the inspiration for this part of gitolite.]

Git allows you to specify many "hooks", which get control as various events happen -- see git help hooks for details. One of those hooks is the update hook, which, if it is present, is invoked just before a branch or a tag is about to be updated. The hook is passed the name of the branch or tag, the old SHA1 value, and the new SHA1 value, as arguments. Hooks that are called before an action happens are allowed to prevent that action from happening by returning an error code.

When gitolite is told to create a new repository (by the admin), it installs a special update hook. This hook takes all the information presented, looks at the config file, and decides to allow or reject the update.

And that's basically it.