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

Chapter 2. The Resolver 23

Figure 2-3 local host files search order using COMMONSEARCH

Once the correct Resolver configuration files and local files have been defined, the Resolver

address space will execute the next step to find the relationship between the name and the IP

address that has been requested by the application, which is based on what has been

defined in the Resolver configuration file, send a query to the name server, or look at the local

ipnode file to get the name/IP address being requested.

2.1.2 For additional information

For more specific information regarding the Resolver address space, refer to z/OS V1R7.0

Communications Server: IP Configuration Guide, SC31-8775.

2.2 Why the Resolver address space is important

The Resolver function allows applications to use names instead of IP addresses to connect to

other partners. Although the use of an IP address might seem to provide an easy method to

the connection, picture applications that must connect themselves with a larger number of

partners, or an application that is accessed by thousands of clients. In these cases, the use of

names is certainly a much easier and more reliable form for establishing access than IP

addressing.

Another important reason to use names instead of IP addressing is that a user or an

application is "insulated" from underlying network address changes.

To compare and decide which method would be more useful in your environment, refer to

Tabl e 2- 1 .

Table 2-1 Comparing the use of direct addressing with name resolution

LE C/C++ sockets,

NOCOMMONSEARCH and IPv4

host name or address search:

1. environment variable X_SITE or

X_ADDR

2. /etc/hosts

3. userID.HOSTS.SITEINFO or

userID.HOSTS.ADDRINFO

4. datasetprefix.HOSTS.SITEINFO

or

datasetprefix.HOSTS.ADDRINFO

Native TCP/IP or USS callable

sockets, NOCOMMONSEARCH

and IPv4 host name or address

search:

1. userID.HOSTS.SITEINFO or

userID.HOSTS.ADDRINFO

2. datasetprefix.HOSTS.SITEINFO

or

datasetprefix.HOSTS.ADDRINFO

COMMONSEARCH and IPv4 host

name or address search - or IPv6

host name or address search:

1. GLOBALIPNODES

2. environment variable

RESOLVER_IPNODES

3. userID.ETC.IPNODES

4. datasetprefix.ETC.IPNODES

5. DEFAULTIPNODES

6. /etc/ipnodes

Hard-coded IP

addresses

Local hosts file Domain Name System

(DNS)

Technology None - Use the entered

IP address directly on the

connect() or sendto()

socket call.

Use gethostbyname() and

let the Resolver find an

IP address in the locally

configured hosts file.

Use gethostbyname() and

let the Resolver contact

the configured name

server for an IP address.