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.

bash

# 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)

bash

# Install
$ sudo apt install net-tools

# View all interfaces
$ ifconfig

# View specific interface
$ ifconfig eth0

Network Interface Information

bash

# View network interface statistics
$ ip -s link

# View interface details
$ ethtool eth0

# View wireless information
$ iwconfig wlan0

Configuring IP Addresses

Temporary Configuration

bash

# 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.

yaml

# /etc/netplan/01-netcfg.yaml
network:
  version: 2
  renderer: networkd
  ethernets:
    eth0:
      dhcp4: true

yaml

# 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

bash

# Apply configuration
$ sudo netplan apply

# Test configuration
$ sudo netplan try

Permanent Configuration (NetworkManager)

bash

# 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)

bash

# /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

bash

# Restart network
$ sudo systemctl restart networking

DNS Configuration

/etc/resolv.conf

bash

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

systemd-resolved

bash

# View DNS status
$ resolvectl status

# View current DNS
$ resolvectl dns

/etc/hosts

Local hostname resolution:

bash

$ 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

bash

# View hostname
$ hostname
$ hostnamectl

# Set hostname
$ sudo hostnamectl set-hostname newhostname

# Or edit file
$ sudo vim /etc/hostname

Network Diagnostic Tools

ping - Test Connectivity

bash

# 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

bash

# 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

bash

# Install
$ sudo apt install mtr

# Run (interactive mode)
$ mtr google.com

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

dig - DNS Query

bash

# 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

bash

$ nslookup google.com
$ nslookup google.com 8.8.8.8

host - Simple DNS Query

bash

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

Ports and Connections

ss - Socket Statistics

bash

# 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)

bash

# 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

bash

# 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

bash

# 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

bash

# 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

bash

# 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)

bash

# 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

bash

# 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

File	Description
`/etc/hosts`	Local hostname resolution
`/etc/hostname`	Hostname
`/etc/resolv.conf`	DNS configuration
`/etc/network/interfaces`	Network interface configuration (Debian)
`/etc/netplan/*.yaml`	Netplan 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

Network Basics ​

Overview ​

Network Configuration Viewing ​

ip Command ​

ifconfig (Traditional Tool) ​

Network Interface Information ​

Configuring IP Addresses ​

Temporary Configuration ​

Permanent Configuration (Netplan - Ubuntu) ​

Permanent Configuration (NetworkManager) ​

Permanent Configuration (Traditional Method - Debian) ​

DNS Configuration ​

/etc/resolv.conf ​

systemd-resolved ​

/etc/hosts ​

Hostname Configuration ​

Network Diagnostic Tools ​

ping - Test Connectivity ​

traceroute - Route Tracing ​

mtr - Comprehensive Diagnosis ​

dig - DNS Query ​

nslookup - DNS Query ​

host - Simple DNS Query ​

Ports and Connections ​

ss - Socket Statistics ​

netstat (Traditional Tool) ​

lsof - View Port Usage ​

Network Test Tools ​

curl - HTTP Request ​

wget - Download Tool ​

nc (netcat) - Network Swiss Army Knife ​

Firewall ​

UFW (Ubuntu) ​

iptables ​

Network Configuration Files ​

Summary ​

Network Basics

Overview

Network Configuration Viewing

ip Command

ifconfig (Traditional Tool)

Network Interface Information

Configuring IP Addresses

Temporary Configuration

Permanent Configuration (Netplan - Ubuntu)

Permanent Configuration (NetworkManager)

Permanent Configuration (Traditional Method - Debian)

DNS Configuration

/etc/resolv.conf

systemd-resolved

/etc/hosts

Hostname Configuration

Network Diagnostic Tools

ping - Test Connectivity

traceroute - Route Tracing

mtr - Comprehensive Diagnosis

dig - DNS Query

nslookup - DNS Query

host - Simple DNS Query

Ports and Connections

ss - Socket Statistics

netstat (Traditional Tool)

lsof - View Port Usage

Network Test Tools

curl - HTTP Request

wget - Download Tool

nc (netcat) - Network Swiss Army Knife

Firewall

UFW (Ubuntu)

iptables

Network Configuration Files

Summary