Configure IPv4 and IPv6 Networking and Hostname Resolution¶

Overview¶

This guide covers essential commands and configurations for managing network interfaces, IP addressing (both IPv4 and IPv6), and hostname resolution in Linux systems.

Network Interface Management¶

`ip` Command¶

The modern standard for network configuration in Linux, replacing older tools like ifconfig.

Display Network Interfaces¶

# Show all network interfaces
ip link show

# Show specific interface
ip link show eth0

# Show interface statistics
ip -s link show eth0

Enable/Disable Interfaces¶

# Bring interface up
ip link set eth0 up

# Bring interface down
ip link set eth0 down

IPv4 Configuration¶

Viewing IPv4 Addresses¶

# Show all IPv4 addresses
ip -4 addr show

# Show IPv4 for specific interface
ip addr show eth0

# Brief output
ip -br addr show

Adding/Removing IPv4 Addresses¶

# Add IPv4 address
ip addr add 192.168.1.100/24 dev eth0

# Add with broadcast
ip addr add 192.168.1.100/24 broadcast 192.168.1.255 dev eth0

# Remove IPv4 address
ip addr del 192.168.1.100/24 dev eth0

# Flush all addresses from interface
ip addr flush dev eth0

Default Gateway¶

# Show routing table
ip route show

# Add default gateway
ip route add default via 192.168.1.1

# Delete default gateway
ip route del default via 192.168.1.1

# Add route to specific network
ip route add 10.0.0.0/8 via 192.168.1.254

IPv6 Configuration¶

Viewing IPv6 Addresses¶

# Show all IPv6 addresses
ip -6 addr show

# Show IPv6 only for specific interface
ip -6 addr show eth0

# Show IPv6 routing table
ip -6 route show

Adding/Removing IPv6 Addresses¶

# Add IPv6 address
ip -6 addr add 2001:db8::1/64 dev eth0

# Remove IPv6 address
ip -6 addr del 2001:db8::1/64 dev eth0

# Add IPv6 default gateway
ip -6 route add default via 2001:db8::ff

# Delete IPv6 default gateway
ip -6 route del default via 2001:db8::ff

IPv6 Link-Local Addresses¶

# View link-local addresses (fe80::/10)
ip -6 addr show scope link

# Ping using link-local (must specify interface)
ping6 fe80::a00:27ff:fe4e:66a1%eth0

Network Configuration Files¶

NetworkManager (RHEL/CentOS/Fedora)¶

Using `nmcli` Command¶

# Show all connections
nmcli connection show

# Show active connections
nmcli connection show --active

# Show device status
nmcli device status

# Create new connection with static IPv4
nmcli connection add \
    type ethernet \
    con-name eth0-static \
    ifname eth0 \
    ipv4.addresses 192.168.1.100/24 \
    ipv4.gateway 192.168.1.1 \
    ipv4.dns "8.8.8.8 8.8.4.4" \
    ipv4.method manual

# Create DHCP connection
nmcli connection add \
    type ethernet \
    con-name eth0-dhcp \
    ifname eth0 \
    ipv4.method auto

# Modify existing connection
nmcli connection modify eth0-static ipv4.addresses 192.168.1.101/24

# Add secondary IP address
nmcli connection modify eth0-static +ipv4.addresses 192.168.1.102/24

# Configure IPv6
nmcli connection modify eth0-static \
    ipv6.addresses 2001:db8::100/64 \
    ipv6.gateway 2001:db8::1 \
    ipv6.method manual

# Enable/disable IPv6
nmcli connection modify eth0-static ipv6.method auto
nmcli connection modify eth0-static ipv6.method ignore

# Activate connection
nmcli connection up eth0-static

# Deactivate connection
nmcli connection down eth0-static

# Delete connection
nmcli connection delete eth0-static

# Reload configuration
nmcli connection reload

Configuration Files (RHEL/CentOS)¶

Location: /etc/sysconfig/network-scripts/ifcfg-<interface>

Example static IPv4 configuration:

# /etc/sysconfig/network-scripts/ifcfg-eth0
TYPE=Ethernet
BOOTPROTO=none
NAME=eth0
DEVICE=eth0
ONBOOT=yes
IPADDR=192.168.1.100
PREFIX=24
GATEWAY=192.168.1.1
DNS1=8.8.8.8
DNS2=8.8.4.4

Example DHCP configuration:

TYPE=Ethernet
BOOTPROTO=dhcp
NAME=eth0
DEVICE=eth0
ONBOOT=yes

Example IPv6 configuration:

IPV6INIT=yes
IPV6ADDR=2001:db8::100/64
IPV6_DEFAULTGW=2001:db8::1

Netplan (Ubuntu/Debian)¶

Location: /etc/netplan/*.yaml

Example static configuration:

# /etc/netplan/01-netcfg.yaml
network:
  version: 2
  renderer: networkd
  ethernets:
    eth0:
      addresses:
        - 192.168.1.100/24
        - 2001:db8::100/64
      gateway4: 192.168.1.1
      gateway6: 2001:db8::1
      nameservers:
        addresses:
          - 8.8.8.8
          - 8.8.4.4
          - 2001:4860:4860::8888

Example DHCP configuration:

network:
  version: 2
  ethernets:
    eth0:
      dhcp4: true
      dhcp6: true

Apply Netplan configuration:

# Test configuration
netplan try

# Apply configuration
netplan apply

# Generate backend configuration
netplan generate

Hostname Configuration¶

Viewing Hostname¶

# Show hostname
hostname

# Show FQDN (Fully Qualified Domain Name)
hostname -f

# Show short hostname
hostname -s

# Show all hostname information
hostnamectl

Setting Hostname¶

Using `hostnamectl` (systemd systems)¶

# Set hostname
hostnamectl set-hostname server1.example.com

# Set static hostname only
hostnamectl set-hostname server1 --static

# Set pretty hostname (descriptive name)
hostnamectl set-hostname "Production Web Server" --pretty

# Set transient hostname (temporary)
hostnamectl set-hostname temp-host --transient

Manual Configuration Files¶

# RHEL/CentOS/Fedora
echo "server1.example.com" > /etc/hostname

# Also update /etc/hosts
echo "192.168.1.100 server1.example.com server1" >> /etc/hosts

DNS and Name Resolution¶

DNS Configuration File¶

Location: /etc/resolv.conf

# Configure DNS servers
nameserver 8.8.8.8
nameserver 8.8.4.4
nameserver 2001:4860:4860::8888

# Search domains
search example.com local.domain

# Options
options timeout:2
options attempts:3

`/etc/hosts` File¶

Static hostname to IP mapping:

# IPv4
127.0.0.1       localhost localhost.localdomain
192.168.1.100   server1.example.com server1
192.168.1.101   server2.example.com server2

# IPv6
::1             localhost localhost.localdomain
2001:db8::100   server1.example.com server1

`/etc/nsswitch.conf`¶

Controls the order of name resolution sources:

# Important line for hostname resolution
hosts: files dns myhostname

# This means: check /etc/hosts first, then DNS, then systemd hostname

DNS Testing Commands¶

`nslookup`¶

# Query DNS for domain
nslookup example.com

# Query specific DNS server
nslookup example.com 8.8.8.8

# Reverse lookup
nslookup 8.8.8.8

`dig`¶

# Basic query
dig example.com

# Query specific record type
dig example.com A      # IPv4
dig example.com AAAA   # IPv6
dig example.com MX     # Mail exchange
dig example.com NS     # Name servers

# Query specific DNS server
dig @8.8.8.8 example.com

# Short output
dig +short example.com

# Reverse lookup
dig -x 8.8.8.8

# Trace DNS resolution path
dig +trace example.com

`host`¶

# Simple lookup
host example.com

# Reverse lookup
host 8.8.8.8

# Query specific record type
host -t MX example.com
host -t AAAA example.com

# Use specific DNS server
host example.com 8.8.8.8

`getent`¶

# Query using system resolution (respects /etc/nsswitch.conf)
getent hosts example.com

# This uses the configured resolution order (files, DNS, etc.)
getent ahosts example.com  # All addresses

Network Manager Text UI¶

`nmtui`¶

Interactive text-based interface for NetworkManager:

# Launch network configuration UI
nmtui

# Directly launch specific function
nmtui edit       # Edit connection
nmtui connect    # Activate connection
nmtui hostname   # Set hostname

SystemD Network Configuration¶

`systemd-networkd`¶

Alternative to NetworkManager on systemd systems.

Configuration files: /etc/systemd/network/*.network

Example configuration:

# /etc/systemd/network/20-wired.network
[Match]
Name=eth0

[Network]
Address=192.168.1.100/24
Gateway=192.168.1.1
DNS=8.8.8.8
DNS=8.8.4.4

Enable and start:

systemctl enable systemd-networkd
systemctl start systemd-networkd
systemctl status systemd-networkd

Useful Network Information Commands¶

Show Network Configuration Summary¶

# Modern way
ip addr show

# Show only IPv4
ip -4 addr

# Show only IPv6
ip -6 addr

# Show with colors
ip -c addr

# Brief output
ip -br addr show

Check Connectivity¶

# Ping IPv4
ping -c 4 192.168.1.1

# Ping IPv6
ping6 -c 4 2001:db8::1

# Ping with specific interface
ping -I eth0 192.168.1.1

Persistent vs Temporary Configuration¶

Temporary (runtime only):

ip addr add 192.168.1.100/24 dev eth0
# Lost after reboot

Persistent (survives reboot): - Use nmcli commands - Edit configuration files in /etc/sysconfig/network-scripts/ (RHEL) - Edit Netplan files /etc/netplan/ (Ubuntu) - Use systemd-networkd configuration files

Common Troubleshooting Steps¶

Check interface status:
```
ip link show
ip addr show
```
Check routing:
```
ip route show
ip -6 route show
```

Test connectivity:

ping -c 4 8.8.8.8
ping6 -c 4 2001:4860:4860::8888

Check DNS resolution:

dig example.com
nslookup example.com
getent hosts example.com

Verify configuration files:

cat /etc/resolv.conf
cat /etc/hosts
cat /etc/nsswitch.conf

Check NetworkManager/networkd status:

systemctl status NetworkManager
systemctl status systemd-networkd

Key Exam Tips¶

Know both temporary (ip commands) and persistent (config files, nmcli) methods
Understand IPv4 and IPv6 addressing and configuration
Be comfortable with CIDR notation (e.g., /24, /64)
Practice DNS configuration and testing
Understand the hostname resolution order in /etc/nsswitch.conf
Know how to configure both DHCP and static IP addresses
Remember to restart/reload network services after configuration changes