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 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 users other config files.
All messages that devd receives are forwarded to the Unix domain socket at /var/run/devd.pipe.