To use DNS connection optimization with IIOP, you need to define a DNSGROUP name in the IIOP TCPIPSERVICE resource definition. All CICS regions providing the same TCPIPSERVICE, with the same DNSGROUP name are registered with MVS™ Workload Management (WLM) with the same group-name, as candidates for client requests requiring the same service. This registration also includes the region's Host name, obtained by the TCP/IP function gethostbyaddr, and a unique Server name, which CICS obtains from the specific APPLID of the region as specified by the APPLID system initialization parameter.
Listener regions need to be configured to talk to the same DNS name server on z/OS® that the MVS Workload Manager is configured to use. You can define the name server to be used by TCP/IP by providing a SYSTCPD DD statement in the CICS startup JCL, as described in CICS® Transaction Server for z/OS Installation Guide.