Resolver API calls

Application programs invoke resolver functions by using resolver API calls such as gethostbyname() and getaddrinfo(). The z/OS® resolver supports a number of IBM® APIs, although not all APIs support all possible resolver API calls. The z/OS resolver is invoked by the following resolver API calls:

Application programs that use the gethostbyaddr() and gethostbyname() resolver calls from the following IBM APIs:
Application programs that use the getaddrinfo(), getnameinfo(), and freeaddrinfo() resolver calls from the following IBM APIs:
Application programs that use the sethostent(), gethostent(), and endhostent() resolver calls from the following IBM APIs:
- z/OS XL C/C++ runtime library functions in z/OS XL C/C++ Runtime Library Reference
- z/OS Communications Server C/C++ API in z/OS Communications Server: IP Sockets Application Programming Interface Guide and Reference

The z/OS Communications Server SMTP server and the dig, nslookup, and nsupdate z/OS UNIX commands provide their own unique resolver services. When their resolver initializes, it uses the appropriate TCPIP.DATA information, including information from the global TCPIP.DATA file, if one is specified. For more information, see The resolver and the global TCPIP.DATA file.

Restrictions for the SMTP resolver:

The SMTP resolver does not support the EDNS0 standards.
The SMTP resolver uses only the first value of the SEARCH TCPIP.DATA statement when it resolves host names.
The SMTP resolver does not support IPv6 addresses on a NAMESERVER or NSINTERADDR statement.
The SMTP resolver does not support caching.
The SMTP resolver cannot use the name server responsiveness monitor function.