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      • Network Basics

      Overview

      This chapter introduces Linux network configuration and management fundamentals, including network interfaces, IP address configuration, DNS settings, and more.

      Network Configuration Viewing

      ip Command

      ip is the main modern Linux network configuration tool.

      # View all network interfaces
$ ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 ...
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 ...
3: wlan0: <BROADCAST,MCAST,UP,LOWER_UP> mtu 1500 ...

# View IP addresses
$ ip addr
$ ip a
2: eth0: <BROADCAST,MCAST,UP,LOWER_UP>
    inet 192.168.1.100/24 brd 192.168.1.255 scope global eth0
    inet6 fe80::1/64 scope link

# View specific interface
$ ip addr show eth0

# View routing table
$ ip route
$ ip r
default via 192.168.1.1 dev eth0 proto dhcp
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.100

# View ARP cache
$ ip neigh
192.168.1.1 dev eth0 lladdr aa:bb:cc:dd:ee:ff REACHABLE

      ifconfig (Traditional Tool)

      # Install
$ sudo apt install net-tools

# View all interfaces
$ ifconfig

# View specific interface
$ ifconfig eth0

      Network Interface Information

      # View network interface statistics
$ ip -s link

# View interface details
$ ethtool eth0

# View wireless information
$ iwconfig wlan0

      Configuring IP Addresses

      Temporary Configuration

      # Add IP address
$ sudo ip addr add 192.168.1.100/24 dev eth0

# Delete IP address
$ sudo ip addr del 192.168.1.100/24 dev eth0

# Enable interface
$ sudo ip link set eth0 up

# Disable interface
$ sudo ip link set eth0 down

# Add default gateway
$ sudo ip route add default via 192.168.1.1

# Delete default gateway
$ sudo ip route del default

      Permanent Configuration (Netplan - Ubuntu)

      Ubuntu 18.04+ uses Netplan for network configuration.

      # /etc/netplan/01-netcfg.yaml
network:
  version: 2
  renderer: networkd
  ethernets:
    eth0:
      dhcp4: true
      # Static IP configuration
network:
  version: 2
  renderer: networkd
  ethernets:
    eth0:
      addresses:
        - 192.168.1.100/24
      gateway4: 192.168.1.1
      nameservers:
        - 8.8.8.8
        - 8.8.4.4
      # Apply configuration
$ sudo netplan apply

# Test configuration
$ sudo netplan try

      Permanent Configuration (NetworkManager)

      # View connections
$ nmcli connection show

# View devices
$ nmcli device status

# Configure static IP
$ sudo nmcli connection modify "Wired connection 1" \
    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

# Re-activate connection
$ sudo nmcli connection up "Wired connection 1"

# Configure DHCP
$ sudo nmcli connection modify "Wired connection 1" ipv4.method auto

      Permanent Configuration (Traditional Method - Debian)

      # /etc/network/interfaces
auto eth0
iface eth0 inet static
    address 192.168.1.100
    netmask 255.255.255.0
    gateway 192.168.1.1
    dns-nameservers 8.8.8.8 8.8.4.4

# Or DHCP
auto eth0
iface eth0 inet dhcp
      # Restart network
$ sudo systemctl restart networking

      DNS Configuration

      /etc/resolv.conf

      $ cat /etc/resolv.conf
nameserver 8.8.8.8
nameserver 8.8.4.4
search example.com

      systemd-resolved

      # View DNS status
$ resolvectl status

# View current DNS
$ resolvectl dns

      /etc/hosts

      Local hostname resolution:

      $ cat /etc/hosts
127.0.0.1       localhost
127.0.1.1       mycomputer
192.168.1.10    server.local server

# Add custom resolution
$ sudo echo "192.168.1.20 myserver" >> /etc/hosts

      Hostname Configuration

      # View hostname
$ hostname
$ hostnamectl

# Set hostname
$ sudo hostnamectl set-hostname newhostname

# Or edit file
$ sudo vim /etc/hostname

      Network Diagnostic Tools

      ping - Test Connectivity

      # Basic usage
$ ping google.com
PING google.com (142.250.185.206) 56(84) bytes of data.
64 bytes from 142.250.185.206: icmp_seq=1 ttl=115 time=10.5 ms

# Specify count
$ ping -c 4 google.com

# Specify interval
$ ping -i 2 google.com

# Specify packet size
$ ping -s 1000 google.com

      traceroute - Route Tracing

      # Install
$ sudo apt install traceroute

# Trace route
$ traceroute google.com

# Use ICMP
$ sudo traceroute -I google.com

# Use TCP
$ sudo traceroute -T google.com

      mtr - Comprehensive Diagnosis

      # Install
$ sudo apt install mtr

# Run (interactive mode)
$ mtr google.com

# Report mode
$ mtr -r -c 10 google.com

      dig - DNS Query

      # Install
$ sudo apt install dnsutils

# Query A record
$ dig google.com

# Query specific record type
$ dig google.com MX
$ dig google.com NS
$ dig google.com TXT

# Short output
$ dig +short google.com

# Specify DNS server
$ dig @8.8.8.8 google.com

# Reverse query
$ dig -x 8.8.8.8

      nslookup - DNS Query

      $ nslookup google.com
$ nslookup google.com 8.8.8.8

      host - Simple DNS Query

      $ host google.com
$ host -t MX google.com

      Ports and Connections

      ss - Socket Statistics

      # View all connections
$ ss

# View listening ports
$ ss -l

# View TCP connections
$ ss -t

# View UDP connections
$ ss -u

# Display process information
$ ss -p

# Display numeric format
$ ss -n

# Common combinations
$ ss -tlnp    # TCP listening ports, show process
$ ss -tunap   # All TCP/UDP, show process

      netstat (Traditional Tool)

      # Install
$ sudo apt install net-tools

# View listening ports
$ netstat -tlnp

# View all connections
$ netstat -anp

# View routing table
$ netstat -r

      lsof - View Port Usage

      # View port usage
$ sudo lsof -i :80
$ sudo lsof -i :22

# View specific process's network connections
$ sudo lsof -i -p 1234

# View port usage by user
$ sudo lsof -i -u maxwell

      Network Test Tools

      curl - HTTP Request

      # GET request
$ curl https://example.com

# Show response headers
$ curl -I https://example.com

# Show detailed information
$ curl -v https://example.com

# POST request
$ curl -X POST -d "key=value" https://example.com

# JSON data
$ curl -X POST -H "Content-Type: application/json" \
    -d '{"key":"value"}' https://example.com

# Download file
$ curl -O https://example.com/file.zip
$ curl -o newname.zip https://example.com/file.zip

# Follow redirects
$ curl -L https://example.com

# Save cookies
$ curl -c cookies.txt https://example.com

# Use cookies
$ curl -b cookies.txt https://example.com

      wget - Download Tool

      # Download file
$ wget https://example.com/file.zip

# Specify filename
$ wget -O newname.zip https://example.com/file.zip

# Background download
$ wget -b https://example.com/file.zip

# Resume broken download
$ wget -c https://example.com/file.zip

# Limit download speed
$ wget --limit-rate=1m https://example.com/file.zip

# Recursive download
$ wget -r https://example.com/

# Directory download
$ wget -r https://example.com/ -P /path/to/dir

      nc (netcat) - Network Swiss Army Knife

      # Port scan
$ nc -zv 192.168.1.1 1-1000

# Test port
$ nc -zv google.com 80

# As server
$ nc -l 8080

# As client
$ nc 192.168.1.1 8080

# Transfer file
# Receiver
$ nc -l 8080 > received_file
# Sender
$ nc 192.168.1.1 8080 < file_to_send

      Firewall

      UFW (Ubuntu)

      # Enable firewall
$ sudo ufw enable

# Disable firewall
$ sudo ufw disable

# View status
$ sudo ufw status
$ sudo ufw status verbose

# Allow ports
$ sudo ufw allow 22
$ sudo ufw allow 80/tcp
$ sudo ufw allow 443
$ sudo ufw allow 80

# Allow services
$ sudo ufw allow ssh
$ sudo ufw allow http
$ sudo ufw allow https

# Deny ports
$ sudo ufw deny 23

# Delete rules
$ sudo ufw delete allow 80

# Allow specific IP
$ sudo ufw allow from 192.168.1.100

# Allow subnet
$ sudo ufw allow from 192.168.1.0/24

      iptables

      # View rules
$ sudo iptables -L
$ sudo iptables -L -n -v

# Allow port
$ sudo iptables -A INPUT -p tcp --dport 80 -j ACCEPT

# Deny port
$ sudo iptables -A INPUT -p tcp --dport 23 -j DROP

# Save rules
$ sudo iptables-save > /etc/iptables.rules

# Restore rules
$ sudo iptables-restore < /etc/iptables.rules

      Network Configuration Files

      FileDescription
      /etc/hostsLocal hostname resolution
      /etc/hostnameHostname
      /etc/resolv.confDNS configuration
      /etc/network/interfacesNetwork interface configuration (Debian)
      /etc/netplan/*.yamlNetplan configuration (Ubuntu)
      /etc/sysconfig/network-scripts/Network configuration (RHEL)

      Summary

      This chapter introduced Linux network basics:

      • Viewing configuration: ip, ifconfig
      • Configuring network: Netplan, NetworkManager
      • DNS configuration: resolv.conf, hosts
      • Diagnostic tools: ping, traceroute, dig
      • Port viewing: ss, netstat, lsof
      • Network testing: curl, wget, nc
      • Firewall: ufw, iptables

      Network configuration is an important skill for server administration. Mastering these tools will help you diagnose and solve network problems.

      Previous chapter: Service Management

      Next chapter: SSH Remote Connection