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Chapter 5 Using the Command-Line Interface 97
5
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The following table describes the options:
Option Description
Lookup Timeout Specifies the timeout period in seconds for the IP address
lookup operation in the host database.
Foreground
Timeout
Specifies how long DNS entries can remain in the
database before they are flagged as stale. For example, if
foreground timeout is 24 hours, and a user requests an
entry that has been in the database for 24 hours or longer,
the entry is refreshed before being served.
You can set the background timeout (see next item) to
refresh entries in the background, before objects become
stale.
Be careful that you don’t set the foreground timeout too
low as you might slow response time. Also, setting this
time too high risks accumulation of incorrect information.
Setting the foreground timeout to greater than or equal to
the background timeout disables background refresh.
Background
Timeout
Specifies how long DNS entries can remain in the
database before they are flagged as entries to refresh in
the background. These entries are still fresh, so they can
be refreshed after they are served, rather than before.
For example, suppose the foreground timeout is 24 hours
and the background timeout is 12 hours. A user requests
an object from my.com and 16 hours later, a user makes a
second request for an object from my.com. The DNS entry
for my.com has not been refreshed in the foreground
because the entry is not yet 24 hours old. But since the
background timeout has expired, the appliance will first
serve the user’s request, then refresh the entry in the
background.
Invalid Host
Timeout
Specifies how long the proxy software should remember
that a host name is invalid. This is often called negative
DNS caching.
For example, if a user specifies an invalid host name, the
appliance informs the user that it could not resolve the
name, and the appliance gets another request for the
same host name. If the appliance still remembers the bad
name, it won’t try to look it up again, but will send another
“invalid host name” message to the user.
Re-DNS On
Reload
Re-resolves host names whenever clients reload pages.