Networks

Hostname

The hostname identifies a server on a network. Choose a name that reflects the server’s purpose and develop a consistent naming scheme, such as webserver-01, webserver-02. The hostname is stored in both /etc/hostname and /etc/hosts. Update both files when you change it: hostnamectl only updates /etc/hostname.

The default shell prompt displays the hostname up to the first period (.). Use these commands to view and change the hostname:

hostname                                        # view the current hostname
hostnamectl set-hostname <hostname>             # update /etc/hostname
vim /etc/hostname                               # edit /etc/hostname manually
vim /etc/hosts                                  # edit /etc/hosts manually

The /etc/hosts file maps IP addresses to hostnames for local resolution:

127.0.0.1 localhost                         # allows the server to reach itself through the network stack
127.0.1.1 ubuntu-server24                   # additional local address and optional FQDN (<servername>.<org-domain-name>)

Network interfaces

A server uses its network interface to connect to a network.

Naming convention

Ubuntu uses a predictable naming convention for network interfaces based on the physical location of the hardware. A name like enp0s3 reflects the card’s position on the system bus and does not change between reboots unless you physically move the hardware.

Ethernet interfaces begin with en. Wireless interfaces begin with wl.

The name enp0s3 identifies an Ethernet card on the system’s first PCI bus in slot 3:

ip

ip is part of the iproute2 utility suite, which replaced the older net-tools package and its ifconfig command. Use these commands to inspect and manage interfaces:

ip addr show                # view network interfaces and current status
ip a                        # shorthand for ip addr show
ip link set enp0s3 down     # bring the enp0s3 interface down
ip link set enp0s3 up       # bring the enp0s3 interface up

ifconfig

ifconfig is deprecated. It is part of the net-tools package, which iproute2 has replaced. Install it only if you need to support legacy scripts:

apt install net-tools       # install the package
ifconfig                    # view network interfaces
ifconfig enp0s3 down        # bring the enp0s3 interface down
ifconfig enp0s3 up          # bring the enp0s3 interface up

Assign static IP address

After installation, a server receives a dynamically assigned address from the DHCP server. For servers that need a consistent address, choose one of these two approaches:

When using a static IP, choose an address outside the DHCP server’s assignment range to avoid conflicts with dynamically assigned addresses.

Netplan

Netplan manages network configuration on Ubuntu Server using YAML files in /etc/netplan/. NetworkManager is available on Ubuntu Desktop only. Netplan processes configuration files in lexical order, so 99_config.yaml is applied after the default 50-cloud-init.yaml.

The DNS server address (nameservers) is often the same as the default gateway, but not always. Check your network configuration to confirm.

When configuring networking over SSH, run netplan apply inside a tmux session. If the connection drops during the change, the command continues running in the background.

See the Configuring networks Ubuntu docs for Ubuntu 24.04.

Apply the configuration after editing the file:

netplan apply

/etc/netplan/99_config.yaml

network:
  version: 2
  renderer: networkd
  ethernets:
    enp0s3:
      dhcp4: true
    enp0s8:
      dhcp4: no
      addresses:
        - 192.168.56.52/24

Name resolution

Linux does not always use the DNS server to resolve hostnames. The /etc/nsswitch.conf file controls which sources are checked and in what order.

/etc/hosts maps IP addresses to hostnames locally. You can add entries to this file, but a central DNS server is easier to maintain at scale.

Do not edit /etc/resolv.conf manually. Since Ubuntu 22.04, it redirects lookups to systemd-resolved, which reads its DNS configuration from Netplan or the DHCP server. The file is generated automatically.

Use resolvectl to view your current DNS servers. The nsswitch.conf entry for hosts shows the resolution order:

resolvectl                      # view DNS servers your system points to

cat /etc/nsswitch.conf
hosts:          files dns       # checks /etc/hosts first, then the DNS server

ss

ss displays socket statistics. Use it to list open and listening ports:

ss -tunlp           # list listening ports