Month: July 2011

It seems I’ve been writing little daemons a lot lately–small things that don’t want to run as root but still need to be launched in the background as services. I’ve been noticing because it’s such a pain to integrate them into the system once they are written (or installed). I have to mess around as root creating /etc/init.d shell scripts (probably by copy and pasting–who out there can actually make those from scratch?) or maybe tweaking some upstart or systemd config file.

And then when I’m done it annoys me that I have to be root to start or stop a daemon that ends up running as my login user anyway.

Enter Daemon Manager

So last October (2010) I sat down and wrote daemon-manager. It started from a few core ideas:

1. It must be possible for a user to set up and configure their daemons—no root access must be required for a user to create a new daemon or restart one of their daemons.
2. It must be secure—users should not be allowed to control other users’ daemons (unless they are given explicit permission).
3. It should allow for good security practices—users should be allowed to launch a daemon as a user other than themselves if root has explicitly allowed it. This is so you can run your daemons as a “nobody” style user.¹
4. It should restart the daemons if they crash (I’m looking at you, php-cgi).
5. It should be easy to use—1 config file per daemon and a simple command line interface to interact with the running daemons.

There are programs out there that do parts of that list, but none that do everything:

• daemon tools: I’ve used it before and I really like its philosophy of being small and simple. But it seems to really want to run as root which means you have to be root to control it. Also, setting up new daemons is kind of a pain.
• Upstart: It’s very similar and it makes setting up a new daemon pretty easy but since it’s an “init” replacement it doesn’t seems very adept at running programs meant for non-root users. I’ve done it before but there was a lot of “sudo” configuration and it wasn’t easy. Also the config files are stored in /etc/init and only root can write new ones.
• Systemd: I really love systemd. Or the idea of it, really—some day it will make it into Debian and I’ll start actually using it. But its philosophy is great. But again, being an “init” replacement gives it most of the same downsides as upstart.

A Quick Tutorial Through Examples

Master config: /etc/daemon-manager.conf

[runs_as]
david: www-data
michaelc: www-data
amy: www-data
joann: www-data
jim:
greenfelt: greenfelt-daemon

[manages]
david: michaelc,amy,joann,greenfelt
michaelc: amy, joann
bill: joann
jim: greenfelt

The main section is the “runs_as” section. This section tells Daemon Manager which users are allowed to start daemons and which users the daemons can be run as. In the above example, “david”, “michaelc”, “amy”, and “joann” can launch daemons as themselves and also “www-data”. “greenfelt” can launch daemons as itself and the “greenfelt-daemon” user. “jim” is only allowed to launch daemons as himself. No other users on the system are allowed to launch daemons at all because they weren’t explicitly listed.

The “manages” section is a little experimental at this point, but the idea is that “david” is allowed to manage (start, stop, or restart) the daemons of “michaelc”, “amy”, “joann”, and “greenfelt” in addition to his own daemons. This is so you can have help desk type users who can stop or restart other users’ daemons even though they may not have read or write access to the users’ home directories. As you might expect, “root” is always allowed to start and stop anyone’s daemons.

Daemon: deluged.conf

dir=/home/david
start=exec deluged -d

This is a simple Daemon Manager config file that launches the deluge bittorrent daemon. “dir” and “start” are the only required entries in the config file. “dir” is the working directory and “start” is a one line shell script to run. Because it is a shell script it needs to “exec”, otherwise Daemon Manager can’t manage it properly.²

Daemon: wordpress.conf

dir=/home/amy/wordpress
user=www-data
start=exec php-cgi -q -b wordpress.socket

This starts a PHP FastCGI daemon in a WordPress directory and starts it running with as the “www-data” user (“amy” was given explicit permission to start daemons as the www-data user in “/etc/daemon-manager.conf”).

Daemon: greenfelt.conf

dir=/var/www/greenfelt.net
user=greenfelt-daemon
start=exec ./script/greenfelt_fastcgi.pl -l greenfelt.socket -n 1
output=log

This is a paraphrase of the config file that runs greenfelt.net which is a Catalyst app that talks to nginx via FastCGI through the “greenfelt.socket” unix domain socket. It runs as the user “greenfelt-daemon” so it can have less privileges than the main “greenfelt” user. The “output” parameter tells Daemon Manager to collect the stdout and stderr of the daemon and save it to a log file in the “~/.daemon-manager/logs/” directory (the default setting is to throw away stdout and stderr).

Controlling Daemon Manager

Daemon Manager is controlled using the “dmctl” command. It is relatively simple at this point, allowing you to start, stop and restart daemons. It also lets you scan for new config files and query for daemon statistics. Here’s an example output the “status” command:

$ dmctl status
daemon-id                      state              pid respawns cooldown   uptime    total
david/deluge-web               running           2948        0       0s     3w3d     3w3d
david/deluged                  running           2950        0       0s     3w3d     3w3d
david/greenfelt                stopped              0        0       0s       0s       0s
david/minecraft                stopped              0        0       0s       0s       0s
david/moviefile                running           2951        0       0s     3w3d     3w3d
david/pytivo                   running          22905        0       0s     4d7h     4d7h
david/streamium                running           2958        0       0s     3w3d     3w3d
david/wordpress                running          27012       33       0s   12h57m     3w3d

Notice that the wordpress server (good old php-cgi) has crashed 33 times (and has been automatically respawned by Daemon Manager).

Download and Use

Daemon Manager is licensed under the GPL and can be downloaded here (the source is also available on github). If you use Debian there’s a Debian branch available for building a package. The version as of this writing is 0.9 which means that there are some obvious things that need to be fixed³ and that I’m not sure it’s 100% secure or bug free yet, though I have been using it for months now and I absolutely love it–it filled a void I wasn’t even sure was really there when I started.

For a more detailed version of the history of Daemon Manager, see this blog post.