Information Technology Reference
In-Depth Information
To add one more wrinkle to the DNS lookup process, DNS clients maintain a text file called
Hosts that can contain static DNS entries. On Windows, this file is stored in
%systemroot%\System32\drivers\etc. By default, it contains two entries on Windows Server
2008 and Vista computers for resolving the local loopback address for both IPv4 and IPv6. The
format of the file is simply IP address and hostname separated by one or more spaces. A typical
Hosts file in Windows Server 2008 looks like this:
127.0.0.1 localhost
::1 localhost
The entries in the Hosts file are cached at system startup and each time the file is changed.
Of course, you can add as many entries as you like to the Hosts file. Usually, however, the Hosts
file is left as it is because in a dynamic network, static DNS entries are likely to cause more harm
than good. Some people use the Hosts file as a sort of Web filter. You can add entries to this file
for hosts on domains that create pop-up ads and fill your Web pages with advertisements. For
each entry, simply use the address 127.0.0.1. Unless you're running a Web server locally, your
browser won't get a response from this address, and the ad will be blocked. You can even down-
load a Hosts file that's already loaded with hundreds of entries for well-known Web advertisers,
such as doubleclick.net.
DNS servers can perform one or more of the following roles for a zone:
•
Authoritative server
—As discussed, an
authoritative server
for a domain holds a complete
copy of a zone's resource records.
•
Forwarder
—A
forwarder
is a DNS server to which other DNS servers send requests they
can't resolve themselves. A forwarder is commonly used when a DNS server on an inter-
nal, private network receives a query for a domain on the public Internet. The internal
DNS server forwards the request recursively to a DNS server connected to the public
Internet. This method prevents the internal DNS server from having to contact root servers
and TLD servers directly because the forwarder does that on its behalf.
•
Conditional forwarder
—A
conditional forwarder
is a DNS server to which other DNS servers
send requests targeted for a specific domain. For example, computers in the coolgadgets.com
domain might send a DNS query for a computer named server1.niftytools.com. The DNS
server in the coolgadgets.com domain can be configured with a conditional forwarder that
in effect says “If you receive a query for niftytools.com, forward it to the DNS server han-
dling the niftytools.com domain.” Servers that are forwarders or conditional forwarders
require no special configuration, but the servers using them as forwarders must be config-
ured to do so.
•
Caching-only server
—A
caching-only DNS server
isn't configured with any zones. Its sole
job is to field DNS queries, do recursive lookups to root servers or send requests to for-
warders, and then cache the results. After the query results are cached, the caching server
can respond to a similar query directly. Caching servers are ideal for branch offices so that
local computers' queries are forwarded to an authoritative server at a main office.
9
As mentioned, a zone is a database containing resource and information records for a domain.
There are three different types of zones:
•
Primary zone
—The
primary zone
contains a read/write master copy of all resource records
for the zone. Updates to resource records can be made only on a server configured as a
primary zone server, referred to as the primary DNS server. A primary DNS server is con-
sidered authoritative for the zone it manages.
•
Secondary zone
—The
secondary zone
contains a read-only copy of all resource records
for the zone. Changes can't be made directly on a secondary DNS server, but because it
contains an exact copy of the primary zone, it's considered authoritative for the zone.
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