When configuring and communicating with End Station Components in an industrial network, many end users wish to use HostNames rather than IP addresses. Since the underlying transport protocols (e.g., TCP and UDP) use IP addresses, a HostName needs to first be resolved to an IP address before initiating communications. In both IT and OT networks, the Domain Name System (DNS) is commonly used when HostName resolution is required.
By virtue of using OPC UA transport mechanisms, UAFX makes provisions for using HostNames, as described in the following sections. Requirements to support HostName resolution vary by UAFX Profile, as defined in OPC 10000-84.
There are two UAFX communication scenarios where HostName resolution could be used: with UA Client Server communications, such as during UAFX connection establishment, and PubSub communications to exchange the real-time data for a Connection.
Figure 2 illustrates the use of DNS (as specified in RFC 1034 and its related RFCs) in the establishment of a UAFX Connection between two UAXF Controllers.
Figure 2 – Client/server with DNS resolution
Controller A, with ConnectionManager (CM), has a DNS client and has been configured via DHCP or vendor-specific means with the IP address of the DNS server(s). The DNS server is contacted to resolve the HostName of Controller B, if not already resolved as part of UA session establishment, before issuing the EstablishConnections Method.
Figure 3 illustrates the use of DNS in conjunction with PubSub for controller-to-controller communications.
Figure 3 – PubSub with DNS resolution
In this case, via the EstablishConnections Method, each Controller has been given a PubSub configuration containing the other Controller's HostName. In order to send and receive publications, the HostNames need to be resolved to IP addresses. Each Controller has a DNS client and, if not already resolved, needs to contact the DNS server before publishing to the subscribing Controller.
Multicast DNS (mDNS), as specified in IETF RFC 6762, provides HostName resolution without needing a DNS server. Hosts needing to resolve HostNames issue mDNS queries to the well-known IP multicast address for mDNS.
In discovering UAFX Servers, Clients may use mDNS directly or the local discovery server-multicast extension (mDNS-ME), as described in OPC 1000012.
Figure 4 illustrates the use of mDNS for HostName resolution during UAFX connection establishment.
