Scope: Debian-based and RHEL-based Linux distributions
Topics: Interface inspection, links, routes, neighbors, sockets, hostname configuration, netplan, NetworkManager, and verification.

1. Inspect Network Interfaces#

List all interfaces and addresses:

ip addr show

Use the compact view:

ip -br addr

Expected output:

lo               UNKNOWN        127.0.0.1/8 ::1/128
eth0             UP             192.0.2.10/24 fe80::a00:27ff:fe1a:2b3c/64

2. Inspect and Change the Link State#

Show link state and MAC addresses:

ip link show

Bring an interface up:

sudo ip link set <INTERFACE> up

3. Inspect Routes#

Show the routing table:

ip route show

Expected output:

default via 192.0.2.1 dev eth0
192.0.2.0/24 dev eth0 proto kernel scope link src 192.0.2.10

Check which route a destination uses:

ip route get <TARGET_IP>

4. Inspect Neighbors#

Show the ARP/NDP neighbor table:

ip neigh show

Expected output:

192.0.2.1 dev eth0 lladdr 00:11:22:33:44:55 REACHABLE

5. Inspect Listening Ports#

List listening TCP and UDP sockets with owning processes:

ss -tulpn

Expected output:

Netid State  Local Address:Port  Process
tcp   LISTEN 0.0.0.0:22          users:(("sshd",pid=812,fd=3))
tcp   LISTEN 0.0.0.0:80          users:(("nginx",pid=950,fd=6))

Options:

• -t: Shows TCP sockets.
• -u: Shows UDP sockets.
• -l: Shows listening sockets only.
• -p: Shows the owning process. Requires root for processes of other users.
• -n: Shows numeric port numbers instead of service names.

6. Test Connectivity#

Test reachability and latency:

ping -c 4 <GATEWAY_IP>
ping -c 4 <TARGET_IP>

Options:

• -c 4: Sends four packets and stops.

7. Inspect the Hostname#

Show the hostname and related settings:

hostnamectl

Set a static hostname:

sudo hostnamectl set-hostname <HOSTNAME>

The static hostname is stored in /etc/hostname and is applied at boot on both distribution families.

Part 1: Debian-Based Systems#

Examples: Debian, Ubuntu

1. Identify the Netplan Configuration#

List existing netplan files:

ls /etc/netplan/

Expected output:

00-installer-config.yaml

File names end in .yaml and are applied in lexical order.

2. Write a Static Address Configuration#

Edit the file:

sudo vim /etc/netplan/00-installer-config.yaml

Example configuration:

network:
  version: 2
  renderer: networkd
  ethernets:
    <INTERFACE>:
      dhcp4: no
      addresses:
        - 192.0.2.10/24
      routes:
        - to: default
          via: 192.0.2.1
      nameservers:
        addresses:
          - 192.0.2.53
          - 8.8.8.8

For DHCP, set dhcp4: true and drop the addresses, routes, and nameservers keys.

3. Validate and Apply#

Validate the syntax and generate the backend configuration:

sudo netplan generate

Apply with automatic rollback:

sudo netplan try

netplan try applies the configuration and rolls it back after 120 seconds unless it is confirmed. Use it for remote sessions.

Apply without rollback:

sudo netplan apply

4. Verify the Result#

Check the address and the default route:

ip -br addr show <INTERFACE>
ip route show

Confirm the gateway is reachable:

ping -c 4 192.0.2.1

Part 2: RHEL-Based Systems#

Examples: RHEL, Rocky Linux, AlmaLinux

1. Inspect Devices and Connections#

Show device states:

nmcli device status

Expected output:

DEVICE  TYPE      STATE      CONNECTION
eth0    ethernet  connected  eth0
lo      loopback  unmanaged  --

Show connection profiles:

nmcli device show <INTERFACE>
nmcli connection show

2. Configure a Static Address#

Modify the active connection profile:

sudo nmcli connection modify <CONNECTION> \
  ipv4.method manual \
  ipv4.addresses 192.0.2.10/24 \
  ipv4.gateway 192.0.2.1 \
  ipv4.dns "192.0.2.53 8.8.8.8"

Activate the changes:

sudo nmcli connection up <CONNECTION>

Switch back to DHCP:

sudo nmcli connection modify <CONNECTION> ipv4.method auto
sudo nmcli connection up <CONNECTION>

Options:

• ipv4.method manual: Configures a static address. Use auto for DHCP.
• ipv4.addresses: Sets the address and prefix length.
• ipv4.gateway: Sets the default gateway.
• ipv4.dns: Sets the DNS servers for this connection.

3. Verify the Result#

Check the applied configuration:

ip -br addr show <INTERFACE>
ip route show
nmcli device show <INTERFACE>

Confirm that IP4.ADDRESS and IP4.GATEWAY contain the expected values.

Part 3: Important Notes#

Legacy Interface Configuration#

On older Debian systems, interfaces are configured in /etc/network/interfaces with the ifupdown tools. Example:

auto eth0
iface eth0 inet static
    address 192.0.2.10/24
    gateway 192.0.2.1

Apply with ifup eth0 or systemctl restart networking. On current releases, netplan is the supported configuration path. Do not mix ifupdown and netplan configuration for the same interface.

On RHEL 7 and earlier, interface files were stored in /etc/sysconfig/network-scripts/. RHEL 9 manages these profiles through NetworkManager. Prefer nmcli over editing keyfiles directly.

Configuration File Locations#

Verification Steps#

Run these checks after every configuration change:

1. Confirm the interface is up: ip link show <INTERFACE>.
2. Confirm the expected address: ip -br addr show <INTERFACE>.
3. Confirm the default route: ip route show.
4. Confirm the gateway is reachable: ping -c 4 <GATEWAY_IP>.
5. Confirm external connectivity: ping -c 4 8.8.8.8.
6. Confirm that name resolution works.

Command Reference#

Security Guidelines#

• Do not edit and apply network configuration blindly over SSH. Keep console access or use netplan try before netplan apply.
• Replace placeholders such as <INTERFACE>, <CONNECTION>, and <HOSTNAME> with actual values. Do not include the angle brackets in commands.
• Make one change at a time and verify connectivity after each change.
• Persist configuration through netplan or NetworkManager. Do not leave long-lived addresses configured only with ip addr add.
• Document static addresses centrally and avoid conflicts with active DHCP ranges.
• Restrict management access to dedicated networks where possible. Do not expose management ports on public interfaces.
• Keep an out-of-band or console path available before changing network settings on a remote host.