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Chapter 2 Command Descriptions 205
dns
DNS Lookup Test. This test uses a Domain Name Server (DNS)
lookup as a test to verify the integrity of the mobile connection. The test
is successful if a valid reply is received from a DNS server. Typically,
this means the hostname is successfully “resolved” to an IP address
by a DNS server. But even a reply such as “not found” or “name does
not exist” is acceptable as a successful test result, since that
demonstrates successful two-way communication over the mobile
connection. When a valid reply is received, the test completes
successfully and immediately.
The DNS servers used in this test for the hostname lookup, are the
primary and secondary DNS servers obtained from the mobile network
when the mobile PPP connection is first established. These addresses
may be viewed in your web browser on the Administration | System
Information | Mobile page.
Note that this DNS test is independent of the normal DNS client
configuration and lookup cache, which is used for other hostname
lookups. This test has been specifically designed to require
communication over the mobile connection for each lookup, and to
avoid being “short-circuited” by previously cached information. Also,
this test does not interfere in any way with the normal DNS client
configuration of this device.
Two hostnames may be configured for this test. If the first hostname
fails to get a reply, the same test is attempted for the second
hostname. If no reply is received for either hostname, the test fails.
The primary and secondary DNS names should be fully qualified
domain names. Note that the reverse lookup of an IP address is
possible, but that is usually unlikely to succeed in returning a name.
Still, such a reverse lookup can be used to demonstrate the integrity
of the mobile connection.
trigger={interval|idle}
The conditions under which link integrity monitoring tests are performed.
interval
Link integrity monitoring tests are repeated at the interval specified by
the “interval” option.
idle
Link integrity monitoring tests are performed only when idle; that is, if
no data is received for the period of time specified by the “interval”
option.
This value changes the behavior of the test, in that the test interval
varies according to the presence of other data received from the
mobile connection.
Although using "trigger=idle" may result in less data being exchanged
over the mobile connection, it also prevents the link integrity tests from
running as often to verify the true bi-directional state of that
connection.