Information Technology Reference
In-Depth Information
master copy of all DNS records for that domain. A single server can be authoritative for multi-
ple domains.
Each shape in Figure 9-1 has one or more DNS servers managing the names associated with
it. For example, the root of the tree has several DNS servers called
root servers
, which keep a
database of addresses of other DNS servers managing top-level domain names. These other
servers, aptly named, are called
top-level domain (TLD) servers
. Each top-level domain has
servers that maintain addresses of other DNS servers. For example, the .com TLD servers main-
tain a database containing addresses of DNS servers for each domain name ending with .com,
such as yahoo.com and microsoft.com. These second-level DNS servers contain hostname/IP
address pairs for computers in their networks.
The DNS Database
DNS servers maintain a database of information that contains zones.
A
zone
is a grouping of DNS information that represents one or more domains and possibly sub-
domains. Each zone contains a variety of record types called resource records. A
resource record
contains information about network resources, such as hostnames, other DNS servers, domain
controllers, and so forth, and is identified by a letter code. Table 9-1 lists resource record types,
the identifying codes, and a description of the resource record.
Table 9-1
DNS resource record types
Record type (code)
Description
9
Start of Authority (SOA)
Less a resource than an informational record, the SOA identifies the name
server that's authoritative for the domain.
Host (A)
The most common resource record; consists of a computer name and IPv4
address.
IPv6 Host (AAAA)
Like an A record, but uses an IPv6 address.
Name Server (NS)
The FQDN of a name server that has authority over the domain. NS records are
used by DNS servers to refer queries to another server that's authoritative for
the requested domain.
Canonical Name (CNAME)
A record that contains an alias for another record and enables you to refer to
the same resource with different names yet maintain only one host record. For
example, you could create an A record for a computer named “web” and a
CNAME record that points to the A record but allows users to access the host
with the name “www.”
Mail Exchanger (MX)
Contains the address of an e-mail server for the domain. Because e-mail
addresses are typically specified as user@domain.com, the mail server's name is
not part of the e-mail address. To deliver a message to the mail server, an MX
record query supplies the address of a mail server in the specified domain.
Pointer (PTR)
Used for reverse DNS lookups. Although DNS is mainly used to resolve a name
to an address, it can also resolve an address to a name by using a reverse
lookup. PTR records can be created automatically on Windows DNS servers.
Service Records (SRV)
Allows DNS clients to request the address of a server that provides a specific
service instead of querying the server by name. This type of record is useful
when an application doesn't know the name of the server it needs but does
know what service is required. For example, in Windows domains, DNS servers
contain SRV records with the addresses of domain controllers so that clients
can request the logon service to authenticate to the domain.
DNS records can be added and changed by using one of two methods:
•
Static updates
—With this method, an administrator must enter DNS record information
manually. This method is reasonable with a small network of only a few resources
accessed by name, but in a large network, static updates can be an administrative burden.
•
Dynamic updates
—Referred to as
Dynamic DNS (DDNS)
, computers in the domain can
register or update their own DNS records, or DHCP can update DNS on the clients' behalf
when a computer leases a new IP address. Both the client computer and the DHCP server
must be configured to use this feature.
Search WWH ::
Custom Search