The root wrapper enables an unprivileged user to run a number of Compute actions as the root user in the safest manner possible. Historically, Compute used a specific sudoers file that listed every command that the Compute user was allowed to run, and used sudo to run that command as root. However this was difficult to maintain (the sudoers file was in packaging), and did not enable complex filtering of parameters (advanced filters). The rootwrap was designed to solve those issues. Instead of calling sudo make me a sandwich, Compute services start with a nova-rootwrap call; for example, sudo nova-rootwrap /etc/nova/rootwrap.conf make me a sandwich. A generic sudoers entry lets the Compute user run nova-rootwrap as root. The nova-rootwrap code looks for filter definition directories in its configuration file, and loads command filters from them. Then it checks if the command requested by Compute matches one of those filters, in which case it executes the command (as root). If no filter matches, it denies the request. To use nova-rootwrap, you must be aware of the issues with using NFS and root-owned files. The NFS share must be configured with the no_root_squash option enabled. The escalation path is fully controlled by the root user. A sudoers entry (owned by root) allows Compute to run (as root) a specific rootwrap executable, and only with a specific configuration file (which should be owned by root). nova-rootwrap imports the Python modules it needs from a cleaned (and system-default) PYTHONPATH. The configuration file (also root-owned) points to root-owned filter definition directories, which contain root-owned filters definition files. This chain ensures that the Compute user itself is not in control of the configuration or modules used by the nova-rootwrap executable. You configure nova-rootwrap in the rootwrap.conf file. Because it's in the trusted security path, it must be owned and writable by only the root user. The file's location is specified both in the sudoers entry and in the nova.conf configuration file with the rootwrap_config=entry. The rootwrap.conf file uses an INI file format with these sections and parameters:

rootwrap.conf configuration options

Configuration option=Default value	(Type) Description
[DEFAULT] filters_path=/etc/nova/rootwrap.d,/usr/share/nova/rootwrap	(ListOpt) Comma-separated list of directories containing filter definition files. Defines where filters for root wrap are stored. Directories defined on this line should all exist, be owned and writable only by the root user.

Filters definition files contain lists of filters that nova-rootwrap will use to allow or deny a specific command. They are generally suffixed by .filters. Since they are in the trusted security path, they need to be owned and writable only by the root user. Their location is specified in the rootwrap.conf file. Filter definition files use an INI file format with a [Filters] section and several lines, each with a unique parameter name (different for each filter that you define):

.filters configuration options

Configuration option=Default value	(Type) Description
[Filters] filter_name=kpartx: CommandFilter, /sbin/kpartx, root	(ListOpt) Comma-separated list containing first the Filter class to use, followed by that Filter arguments (which vary depending on the Filter class selected).