Hosting repositories

This sections shows how to securely provide repository storage for users. Optionally, each user can have a storage quota.

Repositories are accessed through SSH. Each user of the service should have her own login which is only able to access the user’s files. Technically it would be possible to have multiple users share one login, however, separating them is better. Separate logins increase isolation and are thus an additional layer of security and safety for both the provider and the users.

For example, if a user manages to breach borg serve then she can only damage her own data (assuming that the system does not have further vulnerabilities).

Use the standard directory structure of the operating system. Each user is assigned a home directory and repositories of the user reside in her home directory.

The following ~user/.ssh/authorized_keys file is the most important piece for a correct deployment. It allows the user to login via their public key (which must be provided by the user), and restricts SSH access to safe operations only.

command="borg serve --restrict-to-repository /home/<user>/repository",restrict
<key type> <key> <key host>

Note

The text shown above needs to be written on a single line!

Warning

If this file should be automatically updated (e.g. by a web console), pay utmost attention to sanitizing user input. Strip all whitespace around the user-supplied key, ensure that it only contains ASCII with no control characters and that it consists of three parts separated by a single space. Ensure that no newlines are contained within the key.

The restrict keyword enables all restrictions, i.e. disables port, agent and X11 forwarding, as well as disabling PTY allocation and execution of ~/.ssh/rc. If any future restriction capabilities are added to authorized_keys files they will be included in this set.

The command keyword forces execution of the specified command line upon login. This must be borg serve. The --restrict-to-repository option permits access to exactly one repository. It can be given multiple times to permit access to more than one repository.

The repository may not exist yet; it can be initialized by the user, which allows for encryption.

Storage quotas can be enabled by adding the --storage-quota option to the borg serve command line:

restrict,command="borg serve --storage-quota 20G ..." ...

The storage quotas of repositories are completely independent. If a client is able to access multiple repositories, each repository can be filled to the specified quota.

If storage quotas are used, ensure that all deployed Borg releases support storage quotas.

Refer to Storage quotas for more details on storage quotas.

Specificities: Append-only repositories

Running borg init via a borg serve --append-only server will not create a repository that is configured to be append-only by its repository config.

But, --append-only arguments in authorized_keys will override the repository config, therefore append-only mode can be enabled on a key by key basis.

Refer to the sshd(8) man page for more details on SSH options. See also borg serve