Polkit

Polkit is an authorization manager which is used for allowing unprivileged processes to speak to privileged processes through some form of inter-process communication mechanism like D-Bus.

Prerequisites

Install and configure D-Bus.

Installation

For graphical applications, polkit relies on elogind or Seatd to determine the identity of the user making a request. To use the full features of polkit, using polkit with elogind is recommended.

Using polkit with elogind

For a feature-rich desktop experience, use polkit with elogind. Features like authentication agents can be used only with elogind. Install the polkit-elogind package and enable the polkit service using OpenRC.

# apk add polkit-elogind # rc-update add polkit # rc-service polkit start

Proceed to configure elogind, if not done already.

Using polkit with seatd

For a minimal desktop, polkit can be used with seatd with certain limitations. With Seatd, polkit rules can only evaluate group membership, resulting in a 'yes' or 'no' decision. Graphical, session aware authentication agents are not supported.

To proceed to use polkit with seatd, install the polkit package and enable the polkit service using OpenRC:

# apk add polkit # rc-update add polkit # rc-service polkit start

Authentication agents

Polkit authentication agent integration helps coordinate the display of a password prompt to the active and local users. When an unprivileged user attempts to access a privileged location (such as by typing admin:// in the address bar of a File Manager), when the appropriate polkit policy requires administrative authentication, a password dialogue will typically appear.

Note: Authentication agents will work only when polkit is used with elogind.

Some of the authentication agents available in Alpine linux is listed below:

For Xfce, install xfce-polkit as follows:

# apk add xfce-polkit

Enabling privilege escalation GUI

To enable the privilege escalation GUI for an admin user, install the appropriate authentication agent for the desktop environment and ensure that polkit is used with elogind.

Ensure that the authentication agent is autostarted along with the desktop environment.

If the above steps are completed, When admin:// is typed in the address bar of File Manager, a password dialogue will appear.

Polkit rule files

The following example rule files have been provided to show the limitations of seatd.

Example1

A sample polkit rule file /etc/polkit-1/rules.d/50-udisks.rules which allow automatic mounting of removable storage based on being a member of disk or storage group. This rule depends only on group membership which works with seatd:

Contents of /etc/polkit-1/rules.d/50-udisks.rules

polkit.addRule(function(action, subject) {
    if (subject.isInGroup("disk") || subject.isInGroup("storage")) &&
        (action.id == "org.freedesktop.udisks2.filesystem-mount" ||
         action.id == "org.freedesktop.udisks2.filesystem-mount-system" ||
	action.id == "org.freedesktop.udisks2.filesystem-unmount-others" ||  
         action.id == "org.freedesktop.udisks2.drive-eject" ||
         action.id == "org.freedesktop.udisks2.encrypted-unlock" ||
         action.id == "org.freedesktop.udisks2.power-off-drive")) {
        return polkit.Result.YES; // 
    }
});

The above polkit rule file is fully supported when used with both seatd and Elogind.

Example2

Elogind is required for "subject.active" rules and no AUTH_ADMIN, since polkit agents need POLKIT_IS_SUBJECT. Given below is a sample polkit rule file /etc/polkit-1/rules.d/51-require-active-session.rules which allow only active local sessions to suspend:

Contents of /etc/polkit-1/rules.d/51-require-active-session.rules

 
    polkit.addRule(function(action, subject) {
        if (action.id == "org.freedesktop.login1.suspend" &&
            subject.active) {
            return polkit.Result.YES;
        } else if (action.id == "org.freedesktop.login1.suspend") {
            return polkit.Result.NO; // Or polkit.Result.AUTH_ADMIN to prompt for password
        }
    });

The above rule file depends on subject.active which is supported only when polkit is used with Elogind.