Udev

The udev application manages the device nodes in. It is triggered by the Linux kernel through uevents and uses a rule-based configuration to decide what actions to undertake when such a uevent has been fired.

Requirements
Most Gentoo Linux systems use udev for the device node management. As such, the requirements for udev are (most of the time) already fulfilled:
 * A Linux kernel 2.6.32 or higher
 * The  kernel setting

Installing udev
The package is part of most Gentoo Linux profiles' default set (called the system set), so should be installed automatically when you install Gentoo Linux.

However, if this isn't the case, you can always install it by simply emerging it:

Next, validate that the udev init script is automatically started at boot (through the sysinit runlevel):

Also check if the udev-postmount init script is started through the default runlevel:

uevents
The Linux kernel is of course in charge of handling device drivers and events. When it detects that a change occurs within the systems' device tree, it will eventually submit a uevent to whatever process is interested. Udev, through the udevd daemon, is one of those processes. It registers itself as an interested party by reading messages from a netlink socket (an inter-process communication method for transferring information between kernel and user-space processes).

When a uevent is received, the udevd daemon then goes through its rule definitions to find out which action(s) it has to take.

rules.d
The rule definitions for udev are stored in (for end-user specified rules) and  (for system-specified rules). In these directories, multiple rule files (with suffix ) are traversed in alphanumerical order. Inside the rules files, udev will find expressions that might match a uevent together with the state to match (is the uevent because a device is added or removed) and the command to execute.

The event matching is based on information such as:
 * the SUBSYSTEM of the uevent (for which type of device is the uevent fired)
 * the ACTION that is taken (add, change or remove)
 * one or more attributes (through ATTR or ATTRS), such as the device class, vendor or other device information
 * the kernel-provided name (through KERNEL), such as sd* (for SCSI/SATA disks) or input* (for input devices such as mice and keyboards)
 * one or more environment settings (through ENV), used to send information between multiple rules

Based on this information, the rule can then state if
 * some information needs to be shared with later events (through environment variables)
 * links need to be created in
 * commands need to be executed

Udev does this for every rule that matches (so it does not stop after the first match) to allow a flexible device management approach.