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1.3 The Domain Name System in a Nutshell
The Domain Name System is a
distributed database. This structure allows local control of the
segments of the overall database, yet data in each segment is
available across the entire network through a client/server scheme.
Robustness and adequate performance are achieved through replication
and caching.Programs called name servers
constitute the server half of
DNS's client/server mechanism. Name servers contain
information about some segments of the database and make that
information available to clients, called
resolvers. Resolvers are often just library
routines that create queries and send them across a network to a name
server.The structure of the DNS database, shown in Figure 1-1, is similar to the structure of the Windows
filesystem. The whole database (or filesystem) is pictured as an
inverted tree, with the root node at the top. Each node in the tree
has a text label, which identifies the node relative to its parent.
This is roughly analogous to a "relative
pathname" in a filesystem, like
Temp. One labelthe null label, or
" "is reserved for the root
node. In text, the root node is written as a single dot (.). In the
Windows filesystem, the root is written as a backslash (\ ).
Figure 1-1. The DNS database versus a Windows filesystem
Each node is also the root of a new subtree of the overall tree. Each
of these subtrees represents a partition of the overall
databasea "directory" or
"folder" in the Windows filesystem,
or a domain in the Domain Name System. Each
domain or directory can be further divided into additional
partitions, called subdomains in DNS, like a
filesystem's subdirectories. Subdomains, like
subdirectories, are drawn as
children of their parent domains.Every domain has a unique name, like every directory. A
domain's domain name identifies
its position in the database, much as a directory's
"absolute pathname" specifies its
place in the filesystem. In DNS, the domain name is the sequence of
labels from the node at the root of the domain to the root of the
whole tree, with dots (.) separating the labels. In the Windows
filesystem, a directory's absolute pathname is the
list of relative names read from root to leaf (the opposite direction
from DNS, as shown in Figure 1-2), using a slash to
separate the names.
Figure 1-2. Reading names in DNS and in a Windows filesystem
In DNS, each domain can be broken into a number of subdomains, and
responsibility for those subdomains can be doled out to different
organizations. For example, an organization called EDUCAUSE manages
the edu (educational) domain, but delegates
responsibility for the berkeley.edu subdomain to
U.C. Berkeley (see Figure 1-3). This is similar to
remotely mounting a filesystem: certain directories in a filesystem
may actually be filesystems on other hosts, mounted from remote
hosts. The administrator on host winken, for
example (again, Figure 1-3), is responsible for the
filesystem that appears on the local host as the directory
/usr/nfs/winken.
Figure 1-3. Remote management of subdomains and of filesystems
Delegating authority for berkeley.edu to U.C.
Berkeley creates a new zone, an autonomously
administered piece of the namespace. The zone
berkeley.edu is now independent from
edu, and contains all domain names that end in
berkeley.edu. The zone edu,
on the other hand, contains only domain names that end in
edu but aren't in delegated
zones like berkeley.edu.
berkeley.edu may be further divided into
subdomains,
like cs.berkeley.edu, and some of these
subdomains may themselves be separate zones, if the
berkeley.edu administrators delegate
responsibility for them to other organizations. If
cs.berkeley.edu is a separate zone, the
berkeley.edu zone doesn't
contain domain names that end in cs.berkeley.edu
(see Figure 1-4).
Figure 1-4. The edu, berkeley.edu, and cs.berkeley.edu zones
Domain names are used as indexes into the DNS database. You might
think of data in DNS as "attached"
to a domain name. In a filesystem, directories contain files and
subdirectories. Likewise, domains can contain both hosts and
subdomains. A domain contains those hosts and subdomains whose domain
names are within the domain's subtree of the
namespace.Each host on a network
has
a domain name, which points to information about the host (see Figure 1-5). This information may include IP addresses,
information about mail routing, etc. Hosts may also have one or more
domain name aliases, which are simply pointers
from one domain name (the alias) to another (the official or
canonical domain name). In Figure 1-5, mailhub.nv . . . is an
alias for the canonical name rincon.ba.ca . . .
.
Figure 1-5. An alias in DNS pointing to
a canonical name
Why all the complicated structure? To solve the problems that
HOSTS.TXT had. For example, making domain names
hierarchical eliminates the pitfall of name collisions. Each domain
has a unique domain
name,
so the organization that runs the domain is free to name hosts and
subdomains within its domain. Whatever name they choose for a host or
subdomain won't conflict with other
organizations' domain names, since it will end in
their unique domain name. For example, the organization that runs
hic.com can name a host
puella (as shown in Figure 1-6), since it knows that the
host's domain name will end in
hic.com, a unique domain name.
Figure 1-6. Solving the name collision problem
