A DNS server is only necessary when a hostname is configured instead of an IP address as the endpoint URL on the client (the URL to the OPC UA server). As a result, DNS lookup is only enabled on the EIOC when a hostname is configured.
The EIOC supports redundant DNS servers. The calculated address is the x.x.x.1 address of the subnet and it is calculated specifically for the primary and secondary device network IP addresses. The DNS is disabled if the EIOC is configured at the x.x.x.1 address; but, this is handled independently on the primary and secondary networks (meaning that the DNS can be selectively disabled on each network). Also, a disabled network (configured as all zeros) does not have a defined DNS address.
Though it is possible to configure the primary and secondary networks using hostnames, care needs to be taken to ensure the hostnames resolve to appropriate addresses for each subnet. If the same hostname is used for both networks or if two different hostnames resolve to the same IP, all communications will be in the same subnet and therefore be sent over the same network. An improperly configured redundant setup does not actually communicate redundantly.
If redundant DNS servers are configured, it is assumed that both servers are identical. This requires both DNS servers be able to resolve all hostnames for both the primary and secondary networks. The EIOC will attempt to resolve all hostnames through the primary DNS, only contacting the secondary DNS if the connection to the primary fails. If the primary responds with unknown address, the EIOC will not contact the secondary as it expects the same response.
It is possible to configure two network paths to the same DNS server, though that configuration does not provide DNS node redundancy.
If the hostname resolves to an address outside of the configured subnets, the EIOC will attempt to reach the node through the default gateway on the primary network.
The IPv4 default gateway address for the EIOC is set to the x.x.x.1 IP address of the primary device network's subnet for that EIOC. This means the x.x.x.1 IP address is reserved for the default gateway address. If the primary device network is using the x.x.x.1 address, then the default gateway is disabled. Also, a disabled network (configured as all zeros) does not have a defined default gateway address.
Redundant IPv4 default gateways are not provided; therefore, the secondary device network does not have a default IPv4 gateway address.