Network services registry: On UNIX/Linux systems using inetd (or xinetd historically), which file lists service names and their corresponding TCP/UDP port numbers?

Introduction / Context:
Classic UNIX networking stacks map human-readable service names (like http, smtp, ssh) to well-known port numbers. System libraries consult a canonical file for these mappings, enabling tools and daemons to specify services symbolically.

Given Data / Assumptions:

• The host uses traditional name service files in /etc.
• inetd/xinetd or network libraries resolve service-to-port mappings from a local database.
• No special name service switch overrides are considered.

Concept / Approach:

The file /etc/services lists services with their port/protocol pairs (for example, ssh 22/tcp). C library functions like getservbyname consult this database. While many systems now prefer systemd socket activation or stand-alone daemons, /etc/services remains the canonical reference for service names and ports.

Step-by-Step Solution:

Verification / Alternative check:

Check nsswitch behavior in /etc/nsswitch.conf to ensure the services database is consulted locally. Validate resolution using getent services servicename.

Why Other Options Are Wrong:

b: /etc/nsorder is not standard across modern systems.

c: /etc/nsswitch.conf configures name service lookup order; it does not list service-port pairs.

d: /etc/hosts maps hostnames to IP addresses, not ports.

e: Not applicable because /etc/services is correct.

Common Pitfalls:

Confusing service names with daemon binary names; assuming /etc/services changes active daemon ports (it is informational, not authoritative for running daemons); forgetting protocol distinctions (tcp vs udp).

Final Answer:

/etc/services