The Domain Name Server (DNS) is like a telephone book that contains a person's name, address, and telephone number. The name server maps a host name to an IP address, or an IP address to a host name. For each host, the name server can contain IP addresses, nicknames, mailing information, and available well-known services (for example, SMTP, FTP, or Telnet).

Translating host names into IP addresses is just one way of using the DNS. Other types of information related to hosts can also be stored and queried. The different possible types of information are defined through input data to the name server in the resource records.

Although the CICS® DNS cache function is optional, it is useful in a highly active CICS client environment. It combines the GETHOSTBYNAME() call that is supported in CICS sockets, and a cache that saves results from GETHOSTBYNAME() for future reference. If your system receives repeated requests for the same set of domain names, using the DNS can improve performance significantly. If you have specified that IP CICS sockets should use the Open Transaction Environment, and you link to the domain name service module, EZACIC25, your threadsafe program is switched to the QR TCB.

See z/OS Communications Server: IP Configuration Reference for information about caching issues, and see z/OS Communications Server: IP Configuration Guide for more information about system resolver caching.