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CUNP: Unix Network Configuration and Interface Management

CUNP · Work through your exam, one topic at a time.

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Unix Network Configuration and Interface Management

This domain tests how to configure and inspect Unix/Linux interfaces, addresses, routes, name resolution, and persistent network settings. Modern exams expect iproute2 knowledge, not deprecated ifconfig habits, and an understanding of how configuration is applied at boot.

Core rules and concepts

  • Use ip link for interface state, ip addr for addresses, and ip route for routes. Addresses are written in CIDR form, such as 192.0.2.10/24.
  • Adding an address normally creates a connected route for that prefix. The default route is 0.0.0.0/0 for IPv4 and ::/0 for IPv6.
  • A gateway must be reachable through a connected route. If it is not on-link, the kernel rejects or cannot use the default route.
  • Multiple default routes are resolved by metric and policy rules; lower metric wins in the main table.
  • MTU must match the path. Jumbo frames require every segment to support them, or large packets will fail while small pings succeed.
  • Persistence differs by distribution: NetworkManager, systemd-networkd, ifupdown, or cloud-init. Runtime ip commands usually vanish after reboot.
  • DNS configuration lives in /etc/resolv.conf and may be managed by a resolver service. DNS failure is not the same as routing failure.

Worked example

A server must use 192.0.2.10/24 on eth0 with gateway 192.0.2.1. The runtime commands are ip addr add 192.0.2.10/24 dev eth0 and ip route add default via 192.0.2.1. Verify with ip -br addr, ip route, and ip route get 198.51.100.20. The last command shows the selected source address and gateway. If it fails, check that 192.0.2.1 is within 192.0.2.0/24 and that eth0 is up. Then persist the same settings in the distribution's network configuration and reload that manager.

Traps that cost marks

Do not specify a netmask where a prefix length is required, forget the connected route, or assume a default route fixes DNS. Watch for interface naming changes and for multiple addresses on one interface. Remember that ip addr add is not persistent unless a configuration manager records it.

Exam focus

Read the routing decision with ip route get, verify link state and addresses, then check resolver configuration separately.