privilege separation, potentially introducing security holes each time. HAL was created to deal with this, but is now deprecated in most Linux distributions. Overview Unlike traditional Unix systems, where the device nodes in the directory have been a static set of files, the Linux udev device manager dynamically provides only the nodes for the devices actually present on a system. Although devfs used to provide similar functionality, Greg Kroah-Hartman cited a number of reasons for preferring udev over devfs: udev supports persistent device naming, which does not depend on, for example, the order in which the devices are plugged into the system. The default udev setup provides persistent names for storage devices. Any hard disk is recognized by its unique filesystem id, the name of the disk and the physical location on the hardware it is connected to. udev executes entirely in user space, as opposed to devfs's kernel space. One consequence is that udev moved the naming policy out of the kernel and can run arbitrary programs to compose a name for the device from the device's properties, before the node is created; there, the whole process is also interruptible and it runs with a lower priority. The udev, as a whole, is divided into three parts: Library libudev that allows access to device information; it was incorporated into the systemd 183 software bundle. User space daemon that manages the virtual . Administrative command-line utility for diagnostics. The system gets calls from the kernel via netlink socket. Earlier versions used hotplug, adding a link to themselves in with this purpose. Operation udev is a generic device manager running as a daemon on a Linux system and listening (via a netlink socket) to uevents the kernel sends out if a new device is initialized or a device is removed from the system. The udev package comes with an extensive set of rules that match against exported values of the event and properties of the discovered device. A matching rule will possibly name and create a device node and run configured programs to set up and configure the device. udev rules can match on properties like the kernel subsystem, the kernel device name, the physical location of the device, or properties like the device's serial number. Rules can also request information from external programs to name a device or specify a custom name that will always be the same, regardless of