Domain Name

Enhanced Definition

A domain name is a human-readable identifier used to locate resources on a network, such as servers or services, and is resolved into an IP address by the Domain Name System (DNS). On z/OS, domain names are crucial for TCP/IP applications and services to establish connections with other systems, whether within the enterprise network or across the internet. A Domain Name is a human-readable identifier used to locate and identify resources on an IP network, such as a host, server, or service. In the z/OS environment, domain names are used by the TCP/IP stack and applications to resolve to specific IP addresses, enabling communication with other systems both within the enterprise and across the internet without needing to know their numerical IP addresses.

Key Characteristics

- Hierarchical Structure: Domain names are organized hierarchically (e.g., www.ibm.com), allowing for delegated administration and scalability.
- DNS Resolution: The z/OS TCP/IP stack uses configured DNS servers to translate domain names into IP addresses, enabling network communication.
- Configuration: DNS resolver configuration on z/OS is typically managed through the RESOLVER setup, often defined in the TCPIP.DATA dataset or within BPXPRMxx parmlib members for z/OS UNIX System Services (USS) environments.
- Application Agnostic: Various z/OS applications, including CICS, DB2 DDF, IMS Connect, FTP, and WebSphere Liberty, leverage domain names for outbound and inbound network connections.
- IP Version Support: Domain names can resolve to both IPv4 and IPv6 addresses, depending on the DNS server and the network configuration.

Use Cases

- Distributed Database Access: A CICS transaction or a COBOL batch job using DB2 DDF to connect to a distributed database (e.g., SQL Server, Oracle) by specifying its domain name rather than a hardcoded IP address.
- External Service Integration: A z/OS application (e.g., a Java application running in WebSphere Liberty on z/OS) making API calls to external web services or cloud resources using their respective domain names.
- FTP/SFTP Connectivity: A z/OS FTP client initiating a file transfer to an external FTP server by providing its domain name, which is then resolved to an IP address for the connection.
- TN3270 Client Access: TN3270 clients connecting to a z/OS LPAR using a symbolic domain name (e.g., zoslpar1.mycompany.com) rather than its direct IP address, simplifying access and allowing for IP address changes.
- Email Routing (SMTP): The z/OS SMTP server resolving the Mail Exchanger (MX) records for recipient domain names to route outgoing email correctly.

Related Concepts

Domain names are inextricably linked to the TCP/IP stack on z/OS, which provides the underlying networking capabilities. They rely on the Domain Name System (DNS) protocol for resolution, converting human-readable names into machine-readable IP addresses. The z/OS Resolver is the component responsible for performing these lookups, often configured via TCPIP.DATA datasets. This abstraction allows applications like CICS, DB2 DDF, and IMS Connect to connect to remote resources without needing to manage specific IP addresses, enhancing flexibility and maintainability.

Best Practices:

Redundant DNS Servers: Configure multiple, geographically dispersed DNS servers in your TCPIP.DATA to ensure high availability and fault tolerance for name resolution.
Resolver Cache Management: Utilize the z/OS Resolver's caching capabilities (CACHESERV in TCPIP.DATA) to minimize network traffic and improve performance by storing recently resolved names.
Search List Configuration: Define an appropriate SEARCH list in TCPIP.DATA to allow for the resolution of unqualified domain names within your enterprise, reducing the need for fully qualified names in application configurations.
Security Considerations: Implement secure DNS practices, potentially including DNSSEC, and ensure that DNS queries and responses are protected, especially for sensitive applications.
Monitoring and Logging: Monitor DNS resolution performance and log any errors or timeouts to quickly identify and troubleshoot network connectivity issues related to name resolution.