Ubuntu Network Configuration

Introduction

Network configuration is a fundamental skill for any Ubuntu system administrator or user. Whether you're setting up a home server, configuring a workstation in a corporate environment, or deploying cloud instances, understanding how to properly configure network settings is essential.

In this guide, we'll explore Ubuntu's networking system, focusing on how to configure network interfaces, set up static IP addresses, manage DNS settings, and troubleshoot common network issues. Ubuntu uses Netplan as its network configuration tool starting from version 18.04, which we'll cover in detail.

Network Configuration Basics

Understanding Network Interfaces

Before diving into configuration, let's understand the basic network interfaces you might encounter on an Ubuntu system:

• eth0, eth1, etc. - Traditional names for Ethernet interfaces (older systems)
• ens33, enp0s3, etc. - Modern predictable network interface names (newer systems)
• lo - Loopback interface (always present)
• wlan0, wlp2s0, etc. - Wireless network interfaces

You can view your network interfaces using the ip command:

bash

ip addr show

Example output:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 08:00:27:a9:b2:d1 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.100/24 brd 192.168.1.255 scope global dynamic enp0s3
       valid_lft 85559sec preferred_lft 85559sec
    inet6 fe80::a00:27ff:fea9:b2d1/64 scope link 
       valid_lft forever preferred_lft forever

Netplan - Ubuntu's Network Configuration Tool

Since Ubuntu 18.04, Netplan has become the standard way to configure networking. Netplan uses YAML files to define network configuration, which are then rendered into configurations for the chosen backend (either NetworkManager or systemd-networkd).

Netplan Configuration Files

Netplan configuration files are stored in the following locations:

• /etc/netplan/*.yaml

The main configuration file is usually named:

• /etc/netplan/01-netcfg.yaml or
• /etc/netplan/50-cloud-init.yaml (on cloud instances) or
• /etc/netplan/00-installer-config.yaml (on desktop installations)

Basic Netplan Structure

Netplan YAML files follow this basic structure:

yaml

network:
  version: 2
  renderer: networkd  # or NetworkManager
  ethernets:
    # Ethernet configuration
  wifis:
    # WiFi configuration
  bridges:
    # Bridge configuration
  bonds:
    # Bond configuration

Configuring a Static IP Address

Let's look at how to configure a static IP address using Netplan:

1. Edit the Netplan configuration file:

bash

sudo nano /etc/netplan/01-netcfg.yaml

2. Add the following configuration (adjust according to your network interface and desired IP):

yaml

network:
  version: 2
  renderer: networkd
  ethernets:
    enp0s3:
      addresses:
        - 192.168.1.100/24
      routes:
        - to: default
          via: 192.168.1.1
      nameservers:
        addresses: [8.8.8.8, 8.8.4.4]

This configuration:

• Sets a static IP of 192.168.1.100 with a subnet mask of 255.255.255.0 (/24)
• Sets the default gateway to 192.168.1.1
• Uses Google's DNS servers (8.8.8.8 and 8.8.4.4)

3. Apply the configuration:

bash

sudo netplan apply

Using DHCP

If you prefer to use DHCP instead of a static IP:

yaml

network:
  version: 2
  renderer: networkd
  ethernets:
    enp0s3:
      dhcp4: true

Configuring Wi-Fi Networks

For wireless network configuration, use the wifis section in Netplan:

yaml

network:
  version: 2
  renderer: networkd
  wifis:
    wlp2s0:
      dhcp4: true
      access-points:
        "My-WiFi-SSID":
          password: "my-wifi-password"

For security reasons, you might want to protect the file containing your WiFi password:

bash

sudo chmod 600 /etc/netplan/01-netcfg.yaml

Advanced Network Configurations

Bonding Network Interfaces

Bonding allows you to combine multiple network interfaces into a single logical "bonded" interface for redundancy or increased throughput:

yaml

network:
  version: 2
  renderer: networkd
  ethernets:
    enp0s3:
      dhcp4: false
    enp0s8:
      dhcp4: false
  bonds:
    bond0:
      interfaces: [enp0s3, enp0s8]
      addresses: [192.168.1.100/24]
      gateway4: 192.168.1.1
      nameservers:
        addresses: [8.8.8.8, 8.8.4.4]
      parameters:
        mode: active-backup
        primary: enp0s3

Creating Network Bridges

Bridges are useful for virtual machine setups like KVM or container technologies:

yaml

network:
  version: 2
  renderer: networkd
  ethernets:
    enp0s3:
      dhcp4: false
  bridges:
    br0:
      interfaces: [enp0s3]
      addresses: [192.168.1.100/24]
      gateway4: 192.168.1.1
      nameservers:
        addresses: [8.8.8.8, 8.8.4.4]
      parameters:
        stp: true
        forward-delay: 4

Network Configuration Workflow

Here's a diagram of the typical Ubuntu network configuration workflow:

DNS Configuration

Setting DNS Servers

DNS servers can be specified in the Netplan configuration file:

yaml

network:
  version: 2
  renderer: networkd
  ethernets:
    enp0s3:
      addresses: [192.168.1.100/24]
      routes:
        - to: default
          via: 192.168.1.1
      nameservers:
        search: [example.com, local.lan]
        addresses: [8.8.8.8, 8.8.4.4]

The search domains allow you to use short hostnames instead of fully qualified domain names within those domains.

Checking DNS Resolution

You can verify DNS resolution using the dig or nslookup commands:

bash

dig google.com

bash

nslookup ubuntu.com

Troubleshooting Network Issues

Common Commands for Network Troubleshooting

Here are some essential commands for diagnosing network problems:

1. Check interface status:

bash

ip addr show

2. Test connectivity:

bash

ping -c 4 8.8.8.8

3. Trace network route:

bash

traceroute google.com

4. Check open ports:

bash

sudo netstat -tuln

5. Verify DNS resolution:

bash

nslookup google.com

6. Restart networking:

bash

sudo netplan apply

7. Check network logs:

bash

journalctl -u systemd-networkd

Common Network Issues and Solutions

Issue	Possible Cause	Solution
No IP address	DHCP failure or misconfiguration	Check DHCP server or verify static IP settings
Cannot reach internet	Default gateway issue	Verify gateway configuration and connectivity
DNS resolution failure	Incorrect DNS servers	Check nameserver configuration
Interface not up	Hardware or driver issue	Use ip link set dev INTERFACE up
Inconsistent connectivity	Network interface issues	Try sudo ethtool INTERFACE to check link status

Netplan Command Reference

Command	Description
sudo netplan generate	Generate configuration files for the renderer
sudo netplan apply	Apply configuration changes
sudo netplan try	Try configuration with a timeout for rollback
netplan --debug generate	Debug Netplan configuration generation

Network Interface Naming Scheme

Ubuntu uses predictable network interface names following the systemd/udev scheme:

• en - Ethernet
• wl - WLAN (Wireless LAN)
• ww - WWAN (Wireless WAN)

Followed by:

• o - On-board device
• s - Hotplug slot
• p - PCI geographic location
• x - MAC address

For example, enp0s3 indicates an Ethernet device on PCI bus 0, slot 3.

If you prefer the old naming scheme (eth0, wlan0), you can disable predictable names by adding net.ifnames=0 biosdevname=0 to your kernel command line in GRUB.

Summary

In this guide, we've covered how to configure networking on Ubuntu systems using Netplan. We've explored:

• Basic network interface management
• Static IP configuration
• DHCP setup
• Wi-Fi configuration
• Advanced settings like bonding and bridging
• DNS configuration
• Troubleshooting techniques

Understanding network configuration is crucial for managing Ubuntu systems effectively. The switch to Netplan has made configuration more structured and consistent, with the power of YAML and the ability to apply configurations before committing to them.

Exercises

1. Configure your Ubuntu system with a static IP address using Netplan.
2. Set up a bridge network interface for virtual machine connectivity.
3. Create a network bond for redundancy using two network interfaces.
4. Configure a system to use both a wired and wireless connection with proper routing.
5. Write a Netplan configuration that uses one DNS server for one domain and another DNS server for everything else.

Additional Resources

• Ubuntu Server Guide - Networking
• Netplan Reference Documentation
• Ubuntu Wiki - Network Configuration
• The Linux Command Line: A Complete Introduction