Table of Contents (3.3.22)

# Security

## Authentication

Bazaar’s philosophy on authentication is that it is best to reuse existing authentication technologies, rather than trying to reinvent potentially complicated methods for securely identifying users. As such, we describe two such uses of existing software for authentication purposes.

### Using SSH

SSH is a very well tested and featureful technology for authenticating users. For situations where all of the developers have local accounts on the server, it is trivial to provide secure, authenticated `bzr+ssh://` access. One concern with this method is that it may not be desirable to grant shell access to developers on the server machine. In this case, Bazaar provides `bzr_ssh_path_limiter`, a script that runs the Bazaar smart server on the server machine at a specified path, and allows no other access.

To set it up, specify:

    command="/path/to/bzr_ssh_path_limiter <path>" <typical key line>

in each user’s `~/.ssh/authorized_keys` file on the server, where <path> is the path to limit access to (and its subdirectories). For more documentation on the syntax of the `authorized_keys` file see the documentation of the SSH server. This will only permit Bazaar access to the specified path and no other SSH access for that user.

If it isn’t desired to give each user an account on the server, multiple private/public key pairs can be included under one single SSH account (say sshuser) in the `~sshuser/.ssh/authorized_keys` file and then each developer can be given their own private key. They can then use `bzr+ssh://sshuser@server.example.com/` URLs to access the server.

### Using HTTP authentication methods

## Access Control

Many projects need fine-grained access control on who may read and write to which branches. Incorporating these controls into OS-level user accounts using groups and filesystem permissions can be difficult or even not permitted in some instances. Bazaar provides a script called `bzr_access` that can be used to provide access control based on usernames, with authentication performed by SSH. To do so, we need to set up private-key authentication in SSH. This can be done using a single SSH user on the server, or one account per user. The idea is to use the SSH’s `authorized_keys` file to specify the `bzr_access` script as the only command that can be run by a user identified by a particular key pair.

First, you will need to generate a private/public key pair for each user who will be accessing the repository. The private key should be distributed to the user and the public key will be needed on the server to identify the user. On the server, in the SSH user’s `~/.ssh/authorized_keys` file, use the following line for each repository user and the corresponding public key:

    command="/path/to/bzr_access /path/to/bzr /path/to/repository <username>",no- port-forwarding,no-X11-forwarding,no-agent-forwarding ssh-<type> <key>

where <key> is the (possibly very long) public key, <type> is the type of SSH key and <username> is the username to associate with that public key.

The `bzr_access` script obtains its configuration information from the file `/path/to/repository/bzr_access.conf`. This file should not be placed under version control in a branch located at `/path/to/repository` since that would allow anyone with access to the repository to change the access control rules. The `bzr_access.conf` file is in a simple INI-style format with sections defined by `[groups]` and `[/]`. The options in the `[groups]` section are the names of groups and the values of those options should be the usernames in that group. Inside the `[/]` section, the options are usernames or group names (prefixed with `@`) and the values are either `rw`, `r` or nothing, representing read-write access, read-only access or no access at all. A sample of `bzr_access.conf` could be:

    [groups]
    admins = alpha
    devels = beta, gamma, delta

    [/]
    @admins = rw
    @devels = r
    upsilon =

where the user whose key is associated with alpha would have read-write access, the users beta, gamma and delta would have read-only access and user upsilon would not be able to access any branches under `/path/to/repository`.

### Additional Considerations with `bzr_access`

As currently written, `bzr_access` only allows each public key to be associated with a single repository location. This means that if developers need to access two or more different repositories, then each developer will need to have two or more private keys for SSH and be able to select between them (see `man ssh` for more information on configuring multiple private keys).

Also, each repository can only have a single configuration file, with access configured for all branches in the repository. This means that if different access rules are needed for different projects, then those projects must be in different repositories. This then necessitates the use of multiple private keys as just described.

Finally, as noted above under Using SSH all of the public keys may be included in the `authorized_keys` file of a single user on the server. It is also possible to use a single private/public key pair for all of the developers, but this only allows a single username for access control to the repository (since the username is associated with the public key in `authorized_keys`. While this is certainly possible it seems to defeat the purpose of fine-grained access control, although it does provide the same limited SSH access as that described above.