| To help speed up Web browsing,
Windows XP comes with a local cache containing any DNS addresses that
have been looked up recently. (For more detail on how Internet addresses
work, see this article.)
Once an URL has been resolved by an Internet name server into a numerical IP,
the information is stored locally. Anytime your browser requests an URL, Windows
XP first looks in the local cache to see if it is there before querying the external
name server used by your ISP. If it finds the resolved URL locally it uses that
IP. This is supposed to save time and cut down on Web traffic
Problems
The default time period for keeping an address in the cache is 24
hours. Thus, a problem can arise if the IP for an URL changes before
the 24 hour period is up. In this case an error message will result
if you try to connect to the URL. It is not a frequent occurrence but
is not unknown. Another more common problem can arise from URLs that
are temporarily busy or from congested Internet traffic. If a negative
response is received from an attempt to connect, that result is also
kept in the local cache. The default time period for retaining a negative
response is five minutes. In other words, once a negative response
is received you will not be able to connect to the site for at least
five more minutes. Since temporary congestion lasting a few seconds
is often the cause of a momentary inability to connect to a site, this
delay of five minutes can be a nuisance.
Clear the DNS resolver cache
Fortunately, it is possible to clear out the local DNS cache in order to
get around these types of problems. Windows XP comes with a useful
command-line utility ipconfig (discussed
here)
that has a switch, flushdns,
for removing the entries in the cache.
Open a command prompt and enteripconfig /flushdnsand the cache will emptied. Personally, I find that it sometimes speeds
up Internet browsing to do this flush even when there are no obvious error messages.
If you are curious about the contents of the DNS cache, they can also be seen
by using ipconfig.
In this case open a command prompt and enteripconfig /displaydns
If you are experiencing trouble connecting to a particular Web site, it may be
useful to see if its URL is in the cache and what IP is being used.
Configure the DNS resolver cache
A way to minimize problems from the long default wait times that are
used for holding data in the DNS cache is to reduce the times (known
as Time to Live or TTL). This requires a Registry edit so should
be done only by those who know how to restore their Registry. The
Registry key that is involved is
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNSCache\Parameters
The DWORD value MaxCacheTtl can be used
to control how long positive responses are kept. Values are in seconds
and the default value in the absence of an entry (the usual case) is
86,400 seconds or one day. If you wish to shorten the TTL, create the
entry and enter your preferred time in seconds. Most PC users will probably
not gain much this way, however.
Negative responses are another story. I often encounter Web sites that
do not respond immediately but which are available after a short wait and
a retry. A wait of five minutes would be neither convenient nor necessary.
I see no reason to store negative responses in the DNS cache and I have
tweaked my own Registry to prevent them from being entered. In this case
create a DWORD value for the above Registry key and name it MaxNegativeCacheTtl. Set the value to “0” (zero). If you prefer to keep
the negative responses, but with a shorter TTL, enter the appropriate number
of seconds. The default TTL is 300 seconds.
More information
Documentation on DNS caching can be found at this Microsoft
site
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