Dns On Windows Server 1002003 [Electronic resources] نسخه متنی

6.3 Advanced Resolver Features

The Windows 2000, Windows XP, and Windows Server 2003 resolvers have
some advanced features that are worth describing here.

6.3.1 Caching

The
Windows 2000, Windows
XP, and Windows Server 2003 resolvers store every record they receive
in a shared cache available to all programs on the system. The
Windows NT 4.0 resolver caches, but only on a per-process basis. For
example, if you have two different web browsers running (say,
Internet Explorer and Netscape Navigator), each has its own copy of
the resolver with a separate cache. Windows 98, 95, and 3.1 resolvers
don't do any caching.

The Windows 2000, Windows XP, and Windows Server 2003 resolvers obey
the


TTL (time to live) field on resource
records they cache, up to a maximum of 24 hours by default. So if a
record specifies a TTL longer than that, the resolver rounds down to
24 hours. This maximum TTL is configurable with a Registry setting:

MaxCacheTtl
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNSCache\Parameters
Data type: REG_DWORD
Default value: 86,400 seconds (= 24 hours)

(On Windows 2000, this Registry value is called
MaxCacheEntryTtlLimit.)

The Windows 2000 and Windows Server 2003 resolvers also support
negative caching. Windows Server 2003 caches negative responses for
fifteen minutes by default, while Windows 2000 caches them for only
five. This negative caching timeout is also configurable with a
Registry setting:

MaxNegativeCacheTtl
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNSCache\Parameters
Data type: REG_DWORD
Default value: 900 seconds (= 15 minutes)

(On Windows 2000, this Registry value is called
NegativeCacheTime.) To disable negative caching
altogether, set this value to zero.

To view the resolver's cache, use ipconfig
/displaydns. To clear the cache, type ipconfig
/flushdns. To disable caching on Windows XP or Windows
Server 2003, you can use the command:

C:\> net stop dnscache

However, this only lasts until the next reboot. To disable
caching permanently, go to Services
(in the Administrative Tools program
group) and set the DNS Client service's Startup type to Disabled.

6.3.2 Subnet Prioritization

Subnet
prioritization




is analogous to the BIND resolver's address-sorting
feature. When the resolver receives multiple address records for the
same domain name, it examines the IP address in each record and
adjusts the order of the records before returning the list to the
calling application: any records with IP addresses on the same
subnets as the host on which the resolver is running are moved to the
top of the list. Since most applications use addresses in the order
returned by the resolver, this behavior causes traffic to remain on
local networks.

For example, Movie University has two mirrored web servers on two
different subnets:

www.movie.edu.   IN  A  192.253.253.101
www.movie.edu.   IN  A  192.249.249.101

Let's say the resolver on
terminator.movie.edu (192.249.249.3) sends a
query and receives these records. It sorts the record with address
192.249.249.101 to the top of the list because
terminator shares a network with that address.

Note that this behavior defeats the round-robin feature implemented
by most name servers. Round robin refers to the
name server behavior of rotating the order of multiple address
records in successive responses to distribute the load among the
servers (again taking advantage of the behavior of most applications
to use the first address in the list returned by the resolver). With
subnet prioritization enabled, the order of the records is subject to
shuffling by the resolver. You can disable subnet prioritization with
a Registry setting:

PrioritizeRecordData
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNSCache\Parameters
Data type: REG_DWORD
Range: 0 - 1
Default value: 1 (Subnet prioritization enabled)