Password Agents

systemd 12 and newer support lightweight password agents which can be used to query the user for system-level passwords or passphrases. These are passphrases that are not related to a specific user, but to some kind of hardware or service. This is used for encrypted hard-disk passphrases or to query passphrases of SSL certificates at web server start-up time. The basic idea is that a system component requesting a password entry can simply drop a simple .ini-style file into /run/systemd/ask-password/ which multiple different agents may watch via inotify(), and query the user as necessary. The answer is then sent back to the querier via an AF_UNIX/SOCK_DGRAM socket. Multiple agents might be running at the same time in which case they all should query the user and the agent which answers first wins. Right now systemd ships with the following passphrase agents:

• A Plymouth agent used for querying passwords during boot-up
• A console agent used in similar situations if Plymouth is not available
• A wall(1) agent which sends wall messages as soon as a password shall be entered.
• A simple tty agent which is built into “systemctl start” (and similar commands) and asks passwords to the user during manual startup of a service
• A simple tty agent which can be run manually to respond to all queued passwords

Implementing Agents

It is easy to write additional agents. The basic algorithm to follow looks like this:

• Create an inotify watch on /run/systemd/ask-password/, watch for IN_CLOSE_WRITE|IN_MOVED_TO
• Ignore all events on files in that directory that do not start with “ask.”
• As soon as a file named “ask.xxxx” shows up, read it. It’s a simple .ini file that may be parsed with the usual parsers. The xxxx suffix is randomized.
• Make sure to ignore unknown .ini file keys in those files, so that we can easily extend the format later on.
• You’ll find the question to ask the user in the Message= field in the [Ask] section. It is a single-line string in UTF-8, which might be internationalized (by the party that originally asks the question, not by the agent).
• You’ll find an icon name (following the XDG icon naming spec) to show next to the message in the Icon= field in the [Ask] section
• You’ll find the PID of the client asking the question in the PID= field in the [Ask] section (Before asking your question use kill(PID, 0) and ignore the file if this returns ESRCH; there’s no need to show the data of this field but if you want to you may)
• Echo= specifies whether the input should be obscured. If this field is missing or is Echo=0, the input should not be shown.
• The socket to send the response to is configured via Socket= in the [Ask] section. It is a AF_UNIX/SOCK_DGRAM socket in the file system.
• Ignore files where the time specified in the NotAfter= field in the [Ask] section is in the past. The time is specified in usecs, and refers to the CLOCK_MONOTONIC clock. If NotAfter= is 0, no such check should take place.
• Make sure to hide a password query dialog as soon as a) the ask.xxxx file is deleted, watch this with inotify. b) the NotAfter= time elapses, if it is set != 0.
• Access to the socket is restricted to privileged users. To acquire the necessary privileges to send the answer back, consider using PolicyKit. For convenience, a reference implementation is provided: “/usr/bin/pkexec /usr/lib/systemd/systemd-reply-password 1 /path/to/socket” or “/usr/bin/pkexec /usr/lib/systemd/systemd-reply-password 0 /path/to/socket” and writing the password to its standard input. Use ‘1’ as argument if a password was entered by the user, or ‘0’ if the user canceled the request.
• If you do not want to use PK ensure to acquire the necessary privileges in some other way and send a single datagram to the socket consisting of the password string either prefixed with “+” or with “-” depending on whether the password entry was successful or not. You may but don’t have to include a final NUL byte in your message.

Again, it is essential that you stop showing the password box/notification/status icon if the ask.xxxx file is removed or when NotAfter= elapses (if it is set != 0)!

It may happen that multiple password entries are pending at the same time. Your agent needs to be able to deal with that. Depending on your environment you may either choose to show all outstanding passwords at the same time or instead only one and as soon as the user has replied to that one go on to the next one.

If you write a system level agent, a smart way to activate it is using systemd .path units. This will ensure that systemd will watch the /run/systemd/ask-password/ directory and spawn the agent as soon as that directory becomes non-empty. In fact, the console, wall and Plymouth agents are started like this. If systemd is used to maintain user sessions as well you can use a similar scheme to automatically spawn your user password agent as well.

Implementing Queriers

It’s also easy to implement applications that want to query passwords this way (i.e. client for the agents above). Simply bind an AF_UNIX/SOCK_DGRAM socket somewhere (suggestion: you can do this in /run/systemd/ask-password/ under a randomized socket name, not beginning with ask.). Then, create an /run/systemd/ask-password/ask.xxxx (replace the xxxx by some randomized string) file, with the appropriate Message=, PID=, Icon=, Echo=, NotAfter= fields in the [Ask] section. Most importantly, include Socket= pointing to your socket entrypoint. Then, just wait until the password is delivered to you on the socket. Finally, don’t forget to remove the file and the socket once done.

Testing

You may test agents by manually invoking the “systemd-ask-password” tool from a shell. Pass --no-tty to ensure the password is asked via the agent system.

You may test queriers by manually invoking the “systemd-tty-ask-password-agent” from a shell.

Unprivileged Per-User Password Agents

Starting with systemd v257 the scheme is extended to per-user password agents. A second per-user directory $XDG_RUNTIME_DIR/systemd/ask-password/ is now available, with the same protocol as the system-wide counterpart. Unprivileged, per-directory agents should watch this directory in parallel to the system-wide one. Unprivileged queriers (i.e. clients to these agents) should pick the per-user directory to place their password request files in.