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DNS Lookup Test
The DNS Lookup Test measures the speed of looking up a particular
address, using the DNS system.
DNS is a globally distributed system comprising many thousands
of servers all sharing information. What is most important
to many companies is the speed at which their web (or any
other) address is looked up using the system as a whole, and
not the performance of a particular DNS server (which is covered
in the DNS Server Test).
All Internet addresses are defined on an authoritative server
first of all. The authoritative server for a particular address
is the only server that can say with 100% certainty what IP
address a particular host name has or vice-versa. Other DNS
servers will either take occasional copies of the authoritative
server tables, or they will ask the authoritative server when
they are themselves asked. Queries can move up a chain of
DNS servers in order to find the answer. Servers will normally
cache responses for a period (the "Time to Live", or "TTL").
When you configure the DNS Lookup Test, you are asked to
specify the address to lookup. This can be a fully qualified
hostname (for a forward lookup, to get the IP address), or
the IP address (for a reverse lookup, to get the host name).
As with all tests, Nexus Watch performs the DNS Lookup Test
simultaneously via multiple ISPs. The DNS server local to
each of the Nexus Watch pollers is queried for the address.
This is performed twice by each poller so that caching of
results does not produce inconsistent and misleading results.
The test then establishes the authoritative server for the
address provided, and queries it directly for the address.
Results are shown individually in the DNS Lookup reports.
The test configuration also allows you to specify what to
do if you don't get a response. DNS uses UDP as a transport
protocol. The very nature of UDP means that not getting a
response first time is acceptable (as it is a connectionless
protocol). You can configure the amount of time to wait for
a DNS response (the "timeout"). You can also configure how
many times to retry (the "retry" value), and how much to increase
the timeout by between retries (the "timeout multiplier").
Only after retrying the specified amount of times will Nexus
Watch generate an event if there is still no response.
Nexus Watch will repeat the test at intervals set by the
Polling Interval field on the test configuration page.
You may configure error and threshold events to watch out
for, and optionally have Nexus Watch send you an alert if
it sees them.
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