Linux Networking Guide - Configuration, DNS & Troubleshooting

Network Configuration Basics

Modern Linux distributions use either NetworkManager (desktop systems) or systemd-networkd / Netplan (servers) to manage network connections. Understanding both approaches is essential for any Linux administrator.

Viewing Network Interfaces

# Show all network interfaces and their IP addresses

ip addr show

# Short version

ip a

# Show only active interfaces

ip link show up

# Show routing table

ip route show

Configuring a Static IP with Netplan (Ubuntu Server)

# /etc/netplan/01-static.yaml

network:

  version: 2

  ethernets:

    eth0:

      addresses:

        - 192.168.1.100/24

      gateway4: 192.168.1.1

      nameservers:

        addresses:

          - 1.1.1.1

          - 8.8.8.8

# Apply the configuration

sudo netplan apply

NetworkManager CLI (nmcli)

# List all connections

nmcli connection show

# Show active Wi-Fi networks

nmcli device wifi list

# Connect to a Wi-Fi network

nmcli device wifi connect "NetworkName" password "password123"

# Set a static IP

nmcli connection modify "Wired connection 1" \

  ipv4.addresses 192.168.1.100/24 \

  ipv4.gateway 192.168.1.1 \

  ipv4.dns "1.1.1.1,8.8.8.8" \

  ipv4.method manual

nmcli connection up "Wired connection 1"

DNS Configuration

DNS (Domain Name System) translates human-readable domain names into IP addresses. On modern Linux systems, DNS is typically managed through systemd-resolved or configured directly in /etc/resolv.conf.

# Check current DNS settings

resolvectl status

# Test DNS resolution

dig linuxland.nl

nslookup linuxland.nl

# Edit hosts file for local overrides

sudo nano /etc/hosts

# 192.168.1.50  myserver.local

Tip: For improved privacy and speed, consider using DNS over HTTPS (DoH) or DNS over TLS (DoT). Both Cloudflare (1.1.1.1) and Google (8.8.8.8) support encrypted DNS.

Network Troubleshooting Tools

Tool	Purpose	Example
`ping`	Test basic connectivity	`ping -c 4 google.com`
`traceroute`	Trace the path packets take	`traceroute google.com`
`dig`	DNS lookup and diagnostics	`dig example.com A`
`ss`	Show open sockets and ports	`ss -tulnp`
`curl`	HTTP requests and testing	`curl -I https://example.com`
`wget`	Download files	`wget https://example.com/file.zip`
`nmap`	Network and port scanning	`nmap -sV 192.168.1.1`
`tcpdump`	Capture and analyse packets	`sudo tcpdump -i eth0 port 80`
`mtr`	Combined ping and traceroute	`mtr google.com`

Common Troubleshooting Workflow

# Step 1: Check if interface is up

ip link show

# Step 2: Check if you have an IP address

ip addr show

# Step 3: Test local network (gateway)

ping -c 3 192.168.1.1

# Step 4: Test external connectivity

ping -c 3 1.1.1.1

# Step 5: Test DNS resolution

ping -c 3 google.com

# Step 6: Check open ports and listening services

sudo ss -tulnp

This step-by-step approach isolates the problem layer by layer: interface, IP, local network, internet, then DNS.

Managing Network Services

Checking Open Ports

# Show all listening TCP and UDP ports

sudo ss -tulnp

# Find what process is using port 80

sudo ss -tlnp | grep :80

# Scan your own server for open ports

sudo nmap -sT localhost

SSH Tunnelling and Port Forwarding

SSH tunnels let you securely forward traffic through an encrypted connection, which is useful for accessing remote services or bypassing firewalls:

# Local port forwarding: access remote service locally

# Forwards local port 8080 to remote port 80

ssh -L 8080:localhost:80 user@remote-server

# Remote port forwarding: expose local service remotely

ssh -R 9090:localhost:3000 user@remote-server

# SOCKS proxy through SSH

ssh -D 1080 user@remote-server

VPN on Linux

Virtual Private Networks encrypt all traffic between your machine and the VPN server. Linux supports several VPN protocols natively.

WireGuard

A modern, fast, and lightweight VPN protocol built into the Linux kernel since version 5.6. WireGuard uses state-of-the-art cryptography and is significantly simpler to configure than older alternatives. It is now the recommended choice for most Linux VPN setups.

OpenVPN

The most widely supported VPN protocol with broad compatibility across platforms and commercial VPN providers. OpenVPN is battle-tested and runs in userspace, making it highly portable. Most commercial VPN services provide OpenVPN configuration files.

WireGuard Quick Setup

# Install WireGuard

sudo apt install wireguard -y

# Generate key pair

wg genkey | tee privatekey | wg pubkey > publickey

# Create configuration

sudo nano /etc/wireguard/wg0.conf

[Interface]

PrivateKey = YOUR_PRIVATE_KEY

Address = 10.0.0.1/24

ListenPort = 51820

[Peer]

PublicKey = PEER_PUBLIC_KEY

AllowedIPs = 10.0.0.2/32

Endpoint = peer.example.com:51820

# Start the VPN interface

sudo wg-quick up wg0

# Enable on boot

sudo systemctl enable wg-quick@wg0

Network Monitoring

# Monitor bandwidth in real time

sudo apt install iftop -y

sudo iftop -i eth0

# Monitor network connections

sudo apt install nethogs -y

sudo nethogs eth0

# Network statistics per interface

ip -s link show eth0

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