Network Interfaces

A Linux network interface connects a network namespace to a physical device, loopback path, bridge, tunnel, virtual device, or other link. Interface state, addresses, routes, DNS, and persistent configuration are related but distinct.

Discovering Interfaces

Use the modern iproute2 tools:

$ ip -brief link show
$ ip -brief address show

Interface names can be predictable hardware-derived names such as enp1s0, traditional names such as eth0, or administrator-defined names. Never assume eth0 exists or identifies a particular adapter.

Administrative and Operational State

UP means the interface is administratively enabled. LOWER_UP commonly indicates that the lower layer reports operational readiness, such as Ethernet carrier. Either flag alone does not prove an IP address, route, DNS, firewall, or application path works.

$ ip -details link show dev enp1s0
$ ip -s link show dev enp1s0

The statistics view can reveal errors, drops, and counters, but counters need a time interval and baseline to become meaningful.

Changing Runtime State

Runtime commands include:

$ sudo ip link set dev enp1s0 up
$ sudo ip address add 192.0.2.10/24 dev enp1s0

These changes affect current kernel state and can conflict with a network manager that later reapplies its profile. Bringing down a remote-management interface can immediately end access. Before changing it, verify the exact device, preserve console access, record current state, and prepare a timed or tested rollback.

Identifying Configuration Ownership

Persistent paths differ across distributions and installations. Possibilities include NetworkManager profiles, systemd-networkd units, netplan input, /etc/network/interfaces, cloud-init, or orchestration. Determine which service manages the device before editing files:

$ systemctl --type=service --state=running | grep -E 'NetworkManager|networkd|networking'
$ networkctl status
$ nmcli device status

Use only commands present for the identified manager. Two managers controlling the same link can race and overwrite each other's state.

Verifying a Change

Verify link state, assigned addresses and lifetimes, selected routes, resolver state, neighbor reachability, and the actual application. For a persistent change, test a controlled service restart or reboot only when a recovery path exists.

Summary

You can now inspect and change an interface without confusing runtime state with persistent policy.

1. Discover real interface names and addresses.
2. Separate administrative state from operational connectivity.
3. Treat direct ip changes as current kernel state.
4. Identify the active configuration owner before persistence changes.
5. Verify routing, resolution, and application behavior afterward.