This articles talks about how the connection monitoring works on the interface, including what it checks 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. See our help topic, Adding new interface IP addresses. 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 lookup for smoothwall.net to 8.8.8.8 or 8.8.4.4 every 15 seconds, in a round-robin fashion. If six consecutive lookups 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 Filter and Firewall 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.
Notes:
- Connection monitoring should only be used for those Smoothwall Filter and Firewalls that have multiple gateways, or those that allow direct access to the Internet.
- With a single gateway configuration, (and connection monitoring disabled) the Smoothwall Filter and Firewall assumes the gateway is always available.