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Refer to devd.conf(5) and devd(8) man pages for the details on how to run and configure devd. devd.conf is the configuration file for devd(8)

What is devd(8)

The devd daemon provides a way to have userland programs run when certain kernel events happen.

The devd utility is a system daemon that runs in the background all the time. Whenever a device is added to or removed from the device tree, devd will execute actions specified in devd.conf(5). For example, devd might execute dhclient(8) when an Ethernet adapter is added to the system, and kill the dhclient(8) instance when the same adapter is removed.
Another example would be for devd to use a table to locate and load via kldload(8) the proper driver for an unrecognized device that is added to the system.

The devd utility hooks into the devctl(4) device driver. This device driver has hooks into the device configuration system. When nodes are added or deleted from the tree, this device will deliver information about the event to devd. Once devd has parsed the message, it will search its action list for that kind of event and perform the action with the highest matching value. For most mundane uses, the default handlers are adequate. However, for more advanced users, the power is present to tweak every aspect of what happens.

The devd utility reads /etc/devd.conf or the alternate configuration file specified with a -f option and uses that file to drive the rest of the process. While the format of this file is described in devd.conf(5), some basics are covered here. In the options section, one can define multiple directories to search for config files. All files in these directories whose names match the pattern *.conf are parsed. These files are intended to be installed by third party vendors that wish to hook into the devd system without modifying the user's other config files.

Since [devctl(4) allows only one active reader, devd multiplexes it, forwarding all events to any number of connected clients. Clients connect by opening the SOCK_SEQPACKET UNIX domain socket at /var/run/devd.seqpacket.pipe.