D

Data blocks

Units of disk space that are used to store data. Regular files, directories, and symbolic links make use of data blocks.

De-encapsulation

When a header is removed from each segment received on the way up the layers.

Default printer

The printer designated to accept print jobs when a destination printer is not specified.

Default shell

The shell that is specified for each user account in the /etc/passwd file. When the user logs in, they are automatically placed in their default shell. If no shell is specified for the user, the /sbin/sh shell will be their default shell.

Device alias

Device pathnames can be long and complex to enter. The concept of device aliases, like Unix aliases, allows a short name to be substituted for a long name. An alias represents an entire device pathname, not a component of it.

Device autoconfiguration

This offers many advantages over the manual configuration method used in earlier versions of Unix, in which device drivers were manually added to the kernel, the kernel was recompiled, and the system had to be restarted. See also reconfiguration boot.

Device driver

A low-level program that allows the kernel to communicate with a specific piece of hardware.

Device hierarchy

During a reconfiguration restart, this is created in the /devices directory to represent the devices connected to the system.

Device tree

Each device has a unique name representing both the type of device and the location of that device in the system addressing structure called the device tree. The OpenBoot firmware builds a device tree for all devices from information gathered at the power-on self-test (POST).

DHCP (Dynamic Host Configuration Protocol)

An application-layer protocol that enables individual computers, or clients, on a TCP/IP network to extract an IP address and other network configuration information from a designated and centrally maintained DHCP server or servers. This facility reduces the overhead of maintaining and administering a large IP network.

Disk array

A subsystem that contains multiple disk drives, designed to provide performance, high availability, serviceability, or other benefits.

Direct map

A type of automount map that lists the mount points as absolute path names. This type of map explicitly indicates the mount point on the client.

Disk block

The smallest addressable unit on a disk platter. One sector holds 512 bytes of data. Sectors are also known as disk blocks.

Disk label

A special area of every disk that is set aside for storing information about the disk's controller, geometry, and slices.

Disk partition

See Disk slice.

Disk quota

Enables system administrators to control the size of UFSs by limiting the amount of disk space and the number of I-nodes (which roughly corresponds to the number of files) that individual users can acquire.

Disk set

A grouping of two hosts and disk drives in which all the drives are accessible by each host in the set.

Disk slice

Groupings of cylinders that are commonly used to organize data by function.

Disk-based file system

Any file system created on a local disk. Disk-based file systems include UFS, HSFS, PCFS, and UDFs.

Diskless client

A client that has no local disk or file systems. The diskless client boots from the server; remotely mounts its root (/), /usr, and /export/home file systems from a server; allocates swap space on the server; and obtains all its data from the server. Any files created are stored on the server.

DNS resolver

DNS clients use the dynamic library routines, collectively called the resolver, to locate a remote host. The resolver queries the DNS database on a name server, which eventually returns the host name or IP address of the machine requested by the resolver.

DNS

The name service provided by the Internet for Transmission Control Protocol/Internet Protocol (TCP/IP) networks. It was developed so that workstations on the network can be identified by common names instead of Internet addresses.

Domain

A part of the Internet naming hierarchy. A domain represents a group of systems on a local network that share administrative files.

Dynamic failover (as it relates to NFS)

When high availability for read-only NFS resources is needed, dynamic failover provides an alternate NFS mount point if the primary mount point fails.