By default, resolver first attempts to use any configured domain name servers for resolution requests. If the resolution request cannot be satisfied, local host tables are used. Resolver behavior is controlled by the following factors:
The TCPIP.DATA resolver statements define if and how domain name servers are to be used. The LOOKUP TCPIP.DATA statement can also be used to control how domain name servers and local host tables are used. For more information on TCPIP.DATA statements, see z/OS Communications Server: IP Configuration Reference.
If your application program uses the TCP/IP-provided C⁄C++ API and the XL C⁄C++ RESOLVE_VIA_LOOKUP symbol was defined, only local host tables will be used. For information on the use of the RESOLVE_VIA_LOOKUP symbol, see z/OS Communications Server: IP Sockets Application Programming Interface Guide and Reference and z/OS XL C/C++ Programming Guide.
The local host table supplies sitename information for, as one example, resolving hostnames to host or network addresses. The local host table can also supply address information, for example, for resolving addresses to hostname or network names. There are different search orders used for selecting the local host table for these different purposes. The search order to use is based on certain resolver setup statements, the type of API invocation, and possibly the type of host address (IPv4 versus IPv6) being requested or being resolved.