Understanding search orders of configuration information

It is important to understand the search order for configuration files used by TCP/IP functions, and when you can override the default search order with environment variables, JCL, or other variables you provide. This knowledge allows you to accommodate your local data set and HFS file naming standards, and it is helpful to know the configuration data set or HFS file in use when diagnosing problems.

Another important point to note is that when a search order is applied for any configuration file, the search ends with the first file found. Therefore, unexpected results are possible if you place configuration information in a file that never gets found, either because other files exist earlier in the search order, or because the file is not included in the search order chosen by the application.

When searching for configuration files, you can explicitly tell TCP/IP where most configuration files are by using DD statements in the JCL procedures or by setting environment variables. Otherwise, you can let TCP/IP dynamically determine the location of the configuration files, based on search orders documented in Communications Server: IP Configuration Guide (SC31-8775).

The TCP/IP stack’s configuration component uses TCPIP.DATA during TCP/IP stack initialization to determine the stack’s HOSTNAME. To get its value, the z/OS® UNIX environment search order is used.

Note: Use the trace resolver facility to determine what TCPIP.DATA values are being used by the resolver and where they were read from. For information on dynamically starting the trace, refer to Communications Server: IP Diagnosis Guide (GC31-8782). Once the trace is active, issue a TSO NETSTAT HOME command and a z/OS UNIX shell netstat –h command to display the values. Issuing a PING of a host name from TSO and from the z/OS UNIX shell also shows activity to any DNS servers that might be configured.