Using common INET

With Common INET (CINET), you have the capability to define up to 32 AF_INET or dual AF_INET/AF_INET6 transport providers or stacks. The stacks can all be active at the same time. The information for modifying BPXPRMxx and bringing up Common INET is in z/OS UNIX System Services Planning.

For a server that you want to be able to listen to all of the available stacks at the same time, specify INADDR_ANY and it will be listening to all at once. Note that for an IPv6 server, IN6ADDR_ANY can be specified allowing the server to listen for IPv4 and IPv6 connections from all stacks.

The z/OS® UNIX Common INET layer performs a multiplexing/demultiplexing function when more than one stack is activated under z/OS UNIX. Each stack has its own home IP addresses and when a program binds to a specific IP address that socket becomes associated with the one stack that is that IP address. When a program binds to NADDR_ANY (0.0.0.0) or IN6ADDR_ANY (::), the socket remains available to all the stacks.

There are three ways that an INADDR_ANY or IN6ADDR_ANY program can associate itself with a single stack:

Call setibmopt(IBMTCP_IMAGE) - This sets a process so all future socket() calls create sockets with only the one specified stack.
The _BPXK_SETIBMOPT_TRANSPORT environment variable can be used in the PARM= parameter of an MVS started proc to effectively issue a SETIBMOPT outside of the program.
Call ioctl(SIOCSETRTTD) - This associates an existing socket with the one specified stack, removing the others.

Also, you should be able to set up things so gethostbyname() or getaddrinfo() returns the home IP address of the local TCP/IP you are interested. With that, you can issue a specific bind() to that IP address. This may not be useful though, if that stack has multiple IP addresses and you really want to use INADDR_ANY to service all of them. Applications can bind to IN6ADDR_ANY to service both Ipv4 and IPv6 clients when TCP/IP is enabled for IPv6.