How does connection monitoring work on the interface?
What does it really check for the connection to be considered up?
When allocating static or DHCP IP addresses to a network interface card (NIC) (Network > Configuration > Interface), you have the option to turn off / on connection monitoring. The user interface advises that "It is recommended you do not disable this as <Smoothwall> Series will always assume the gateway is connected". But what does it actually do?
Connection monitoring uses Google's public DNS servers to confirm whether your external interfaces have a working connection to the Internet.
The relevant interface performs a DNS look-up for
22.214.171.124 every 15 seconds, in a round-robin fashion. If six consecutive look-ups fail, the gateway is considered down, and the interface that uses it taken out of action. In less than 90 seconds, it can be reliably determined that a link has failed.
The Smoothwall continues to poll the DNS servers throughout this time. If one look-up succeeds, the gateway is determined to be up, and the interface marked as available.
In a multiple gateway configuration, should a look-up fail where the gateway being "tested" is the only one left active, it will not be flagged as down.
Note:Connection monitoring should only be used for those Smoothwalls that have multiple gateways, or those that allow direct access to the Internet.
Note:With a single gateway configuration, (and connection monitoring disabled) the Smoothwall assumes the gateway is always available.