Communications Server for z/OS V1R7 TCP/IP Implementation, Volume 1: Base Functions, Connectivity, and Routing

18 Communications Server for z/OS V1R7 TCP/IP Implementation, Volume 1 - Base Functions, Connectivity, and Routing

2.1 Overview

A Resolver is a set of routines that act as a client on behalf of an application to read a local

host file or to access one or more Domain Name System (DNS) for name-to-IP address or IP

address-to-name resolution.

Where the Resolver looks for name-address resolution and how it handles requests, is

defined in a file called the Resolver configuration file. The Resolver can exist on any system

that supports sockets programs, using name resolution.

On a z/OS system, the Resolver configuration data is located either in an z/OS dataset

(TCPIP.DATA) or in a file (resolv.conf) in the z/OS UNIX file system.

The Resolver configuration on z/OS can be somewhat complex, because of the flexibility

provided to control how the Resolver behaves based on the socket environment and local

preferences. The Resolver function in a z/OS environment is performed by a component

called the

Resolver address space, which was introduced to simplify the configuration tasks.

2.1.1 Basic concepts

In most systems, for the application to reach a remote partner, it uses two commands to ask

the Resolver what the IP address is for a host name, or vice versa. The commands are

gethostbyname(xxx) and gethostbyaddress(nnn.nnn.nnn.nnn). Figure 2-1 shows the

information request and response flows as the Resolver gets a request, and based on its own

configuration file will either look at a local hosts file or send a request to a DNS server. Once

the relationship between the host name and IP address is made, the Resolver returns the

response to the application.

Figure 2-1 How the Resolver works

Note: The z/OS V1R7.0 Communications Server: IP Configuration Guide, SC31-8775,

contains useful information about the characteristics that are required for the z/OS data

sets or file system files that contain resolver setup and configuration statements. The

Guide also points out the security characteristics and file system permission settings that

are needed.

--

--

gethostbyname(hostx)

--

connect(192.168.1.1)

--

--

DomainOrigin abc.com

NSInterAddr 10.1.1.1

--

Get resolver info

send UDP query

receive UDP reply

return IP address

DNS server

IP address

is 192.168.1.1

Client Application

Resolver Routines

resolv.confv

Give me IP address

for hostx.abc.com

Find an IP address that

corresponds to the name