Linux Networking¶

Overview¶

Understanding Linux networking is essential for DevOps, system administration, and troubleshooting connectivity issues. This guide covers network configuration, diagnostics, and common networking tools.

Network Interfaces¶

Understanding Interface Names¶

Modern Linux uses predictable network interface names:

Common interface types:

lo          - Loopback interface (localhost)
eth0        - Traditional Ethernet naming
enp4s0      - Ethernet (PCI bus 4, slot 0)
wlp3s0      - Wireless (PCI bus 3, slot 0)
docker0     - Docker bridge network
br-*        - Docker custom bridge networks

Network Interface Naming Convention¶

Format: <type><bus><slot> - en = Ethernet - wl = Wireless - p = PCI bus number - s = Slot number

Examples: - enp4s0 - Ethernet, PCI bus 4, slot 0 - wlp3s0 - Wireless, PCI bus 3, slot 0

Network Configuration Commands¶

`ifconfig` - Traditional Network Configuration¶

Display and configure network interfaces (older tool).

ifconfig                      # Show all active interfaces
ifconfig -a                   # Show all interfaces (including down)
ifconfig eth0                 # Show specific interface

Configure interface:

sudo ifconfig eth0 up                        # Bring interface up
sudo ifconfig eth0 down                      # Bring interface down
sudo ifconfig eth0 192.168.1.100             # Set IP address
sudo ifconfig eth0 netmask 255.255.255.0     # Set netmask

`ip` - Modern Network Configuration¶

Recommended replacement for ifconfig (more powerful).

View interfaces:

ip addr                       # Show all interfaces with IP addresses
ip addr show                  # Same as above
ip addr show eth0             # Show specific interface
ip -4 addr                    # Show only IPv4
ip -6 addr                    # Show only IPv6
ip -br addr                   # Brief output

Configure interfaces:

sudo ip link set eth0 up                     # Bring interface up
sudo ip link set eth0 down                   # Bring interface down
sudo ip addr add 192.168.1.100/24 dev eth0   # Add IP address
sudo ip addr del 192.168.1.100/24 dev eth0   # Remove IP address

View statistics:

ip -s link                    # Show interface statistics
ip -s link show eth0          # Show stats for specific interface

Common Network Interfaces Explained¶

1. Loopback Interface (lo)¶

Purpose: Internal communication within the local machine

# Typical configuration
lo: flags=73<UP,LOOPBACK,RUNNING>
    inet 127.0.0.1  netmask 255.0.0.0
    inet6 ::1  prefixlen 128

Usage: - Testing network applications locally - Inter-process communication - Always available, never goes down

Examples:

ping localhost                # Test loopback
ping 127.0.0.1                # Same as above
curl http://localhost:8080    # Access local service

2. Ethernet Interface (enp4s0)¶

Purpose: Wired network connection

# Typical configuration
enp4s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>
    inet 192.168.1.100  netmask 255.255.255.0  broadcast 192.168.1.255

Usage: - Primary network connectivity - Stable, high-speed connection - Preferred for servers

Management:

# Check status
ip link show enp4s0

# Bring up/down
sudo ip link set enp4s0 up
sudo ip link set enp4s0 down

# Configure IP
sudo ip addr add 192.168.1.100/24 dev enp4s0

3. Wireless Interface (wlp3s0)¶

Purpose: Wi-Fi network connection

# Typical configuration
wlp3s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>
    inet 192.168.1.101  netmask 255.255.255.0

Usage: - Wireless network connectivity - Mobile devices and laptops - Managed by NetworkManager or wpa_supplicant

Management:

# Check wireless status
iwconfig wlp3s0

# Scan for networks
sudo iwlist wlp3s0 scan

# Using NetworkManager
nmcli device wifi list
nmcli device wifi connect SSID password PASSWORD

4. Docker Bridge (docker0)¶

Purpose: Default Docker container networking

# Typical configuration
docker0: flags=4099<UP,BROADCAST,MULTICAST>
    inet 172.17.0.1  netmask 255.255.0.0

Usage: - Connects Docker containers - Provides NAT for container internet access - Default network for containers

Management:

# View Docker networks
docker network ls

# Inspect docker0 bridge
docker network inspect bridge
ip addr show docker0

# View connected containers
docker network inspect bridge | grep -A 5 Containers

5. Custom Docker Bridge (br-*)¶

Purpose: User-defined Docker networks

# Example: br-34a35fdfe682
br-34a35fdfe682: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>
    inet 172.18.0.1  netmask 255.255.0.0

Usage: - Isolated container networks - Custom network configurations - Better container communication control

Management:

# Create custom network
docker network create mynetwork

# Connect container to network
docker network connect mynetwork container_name

# Inspect custom network
docker network inspect mynetwork

Network Diagnostics¶

`ping` - Test Connectivity¶

Test network connectivity to a host.

ping example.com              # Ping continuously (Ctrl+C to stop)
ping -c 4 example.com         # Send 4 packets
ping -i 2 example.com         # 2 second interval
ping -W 5 example.com         # 5 second timeout

Interpreting results:

64 bytes from example.com (93.184.216.34): icmp_seq=1 ttl=56 time=15.2 ms

- ttl: Time to live (hops remaining) - time: Round-trip time (latency)

Common scenarios:

# Test local network
ping 192.168.1.1              # Gateway/router

# Test internet connectivity
ping 8.8.8.8                  # Google DNS
ping 1.1.1.1                  # Cloudflare DNS

# Test DNS resolution
ping google.com               # If this fails but 8.8.8.8 works, DNS issue

`traceroute` / `tracepath` - Trace Network Path¶

Show the route packets take to destination.

traceroute example.com        # Trace route
tracepath example.com         # Alternative (no root needed)
mtr example.com               # Continuous traceroute (requires install)

`netstat` - Network Statistics¶

Display network connections and statistics (older tool).

netstat -tuln                 # TCP/UDP listening ports
netstat -tulnp                # Include process names (requires root)
netstat -r                    # Routing table
netstat -i                    # Interface statistics
netstat -s                    # Protocol statistics

Options explained: - -t = TCP connections - -u = UDP connections - -l = Listening sockets - -n = Numeric (don't resolve names) - -p = Show process/program

`ss` - Socket Statistics¶

Modern replacement for netstat (faster and more detailed).

ss -tuln                      # TCP/UDP listening ports
ss -tulnp                     # Include process names
ss -s                         # Summary statistics
ss -t state established       # Show established TCP connections
ss -o                         # Show timer information

Practical examples:

# Find what's using port 80
ss -tulnp | grep :80
sudo lsof -i :80

# Show all established connections
ss -t state established

# Show listening ports only
ss -tuln | grep LISTEN

`nslookup` / `dig` - DNS Lookup¶

Query DNS servers for domain information.

# nslookup (simple)
nslookup example.com
nslookup example.com 8.8.8.8  # Use specific DNS server

# dig (detailed)
dig example.com               # Full DNS query
dig example.com +short        # Brief output
dig @8.8.8.8 example.com      # Use specific DNS server
dig -x 8.8.8.8                # Reverse DNS lookup
dig example.com ANY           # All record types

`host` - Simple DNS Lookup¶

host example.com              # Basic lookup
host -t MX example.com        # Mail servers
host -t NS example.com        # Name servers

Network Connectivity Testing¶

Check Network Interface Status¶

# Modern method
ip link show

# Check if interface is up
ip link show eth0 | grep "state UP"

# Traditional method
ifconfig eth0

Test Local Network¶

# Ping gateway
ping -c 4 $(ip route | grep default | awk '{print $3}')

# Check routing table
ip route
route -n

Test Internet Connectivity¶

# Test DNS resolution
ping -c 4 google.com

# Test without DNS
ping -c 4 8.8.8.8

# Test HTTP connectivity
curl -I https://www.google.com
wget --spider https://www.google.com

SSH - Secure Shell¶

Basic SSH Usage¶

ssh username@hostname         # Connect to remote server
ssh username@192.168.1.100    # Connect using IP
ssh -p 2222 username@host     # Custom port

SSH Key Authentication¶

# Generate SSH key pair
ssh-keygen -t rsa -b 4096
ssh-keygen -t ed25519         # Modern, more secure

# Copy public key to server
ssh-copy-id username@hostname

# Manual copy
cat ~/.ssh/id_rsa.pub | ssh username@hostname "cat >> ~/.ssh/authorized_keys"

SSH Configuration¶

# User SSH config: ~/.ssh/config
Host myserver
    HostName 192.168.1.100
    User admin
    Port 2222
    IdentityFile ~/.ssh/id_rsa

# Connect using alias
ssh myserver

SSH Tunneling¶

# Local port forwarding
ssh -L 8080:localhost:80 username@remote

# Remote port forwarding
ssh -R 8080:localhost:80 username@remote

# Dynamic port forwarding (SOCKS proxy)
ssh -D 1080 username@remote

SCP - Secure Copy¶

# Copy file to remote
scp file.txt username@remote:/path/

# Copy file from remote
scp username@remote:/path/file.txt .

# Copy directory recursively
scp -r directory/ username@remote:/path/

# Copy with custom port
scp -P 2222 file.txt username@remote:/path/

SFTP - Secure FTP¶

# Connect to remote server
sftp username@hostname

# SFTP commands
put file.txt                  # Upload file
get file.txt                  # Download file
ls                            # List remote files
lls                           # List local files
cd /path                      # Change remote directory
lcd /path                     # Change local directory

Firewall Management¶

`ufw` - Uncomplicated Firewall (Ubuntu/Debian)¶

# Enable/disable firewall
sudo ufw enable
sudo ufw disable

# Check status
sudo ufw status
sudo ufw status verbose

# Allow/deny ports
sudo ufw allow 22             # Allow SSH
sudo ufw allow 80/tcp         # Allow HTTP
sudo ufw deny 23              # Deny Telnet

# Allow from specific IP
sudo ufw allow from 192.168.1.100

# Delete rule
sudo ufw delete allow 80

`firewall-cmd` - Firewalld (RHEL/CentOS)¶

# Check status
sudo firewall-cmd --state

# List rules
sudo firewall-cmd --list-all

# Allow port
sudo firewall-cmd --add-port=80/tcp --permanent
sudo firewall-cmd --reload

# Allow service
sudo firewall-cmd --add-service=http --permanent
sudo firewall-cmd --reload

Network Configuration Files¶

Debian/Ubuntu¶

# Network interfaces
/etc/network/interfaces

# DNS configuration
/etc/resolv.conf

# Hostname
/etc/hostname
/etc/hosts

RHEL/CentOS¶

# Network scripts
/etc/sysconfig/network-scripts/ifcfg-eth0

# DNS configuration
/etc/resolv.conf

# Hostname
/etc/hostname
/etc/hosts

NetworkManager¶

# Configuration directory
/etc/NetworkManager/

# Connection profiles
/etc/NetworkManager/system-connections/

Practical Scenarios¶

Troubleshoot No Internet Connection¶

# 1. Check interface status
ip link show

# 2. Check IP address
ip addr show

# 3. Ping gateway
ping -c 4 $(ip route | grep default | awk '{print $3}')

# 4. Ping external IP
ping -c 4 8.8.8.8

# 5. Test DNS
ping -c 4 google.com

# 6. Check DNS servers
cat /etc/resolv.conf

Find What's Using a Port¶

# Using ss
sudo ss -tulnp | grep :80

# Using netstat
sudo netstat -tulnp | grep :80

# Using lsof
sudo lsof -i :80

Monitor Network Traffic¶

# Install tools
sudo apt install iftop nethogs

# Monitor bandwidth by interface
sudo iftop -i eth0

# Monitor bandwidth by process
sudo nethogs eth0

# Simple packet monitoring
sudo tcpdump -i eth0
sudo tcpdump -i eth0 port 80

Configure Static IP¶

# Using ip command (temporary)
sudo ip addr add 192.168.1.100/24 dev eth0
sudo ip route add default via 192.168.1.1

# Using NetworkManager (permanent)
nmcli con mod eth0 ipv4.addresses 192.168.1.100/24
nmcli con mod eth0 ipv4.gateway 192.168.1.1
nmcli con mod eth0 ipv4.dns "8.8.8.8 8.8.4.4"
nmcli con mod eth0 ipv4.method manual
nmcli con up eth0

Best Practices¶

Use modern tools - Prefer ip over ifconfig, ss over netstat
Document network changes - Keep track of IP assignments and configurations
Test before permanent changes - Use temporary commands first
Secure SSH - Use key authentication, disable root login, change default port
Monitor network usage - Regular checks for unusual traffic
Keep firewall enabled - Only open necessary ports
Use DNS properly - Configure reliable DNS servers (8.8.8.8, 1.1.1.1)

System Monitoring - Network performance monitoring
Systemctl Services - Managing network services
Shell Scripting - Automating network tasks

Linux Networking¶

Overview¶

Network Interfaces¶

Understanding Interface Names¶

Network Interface Naming Convention¶

Network Configuration Commands¶

ifconfig - Traditional Network Configuration¶

ip - Modern Network Configuration¶

Common Network Interfaces Explained¶

1. Loopback Interface (lo)¶

2. Ethernet Interface (enp4s0)¶

3. Wireless Interface (wlp3s0)¶

4. Docker Bridge (docker0)¶

5. Custom Docker Bridge (br-*)¶

Network Diagnostics¶

ping - Test Connectivity¶

traceroute / tracepath - Trace Network Path¶

netstat - Network Statistics¶

ss - Socket Statistics¶

nslookup / dig - DNS Lookup¶

host - Simple DNS Lookup¶

Network Connectivity Testing¶

Check Network Interface Status¶

Test Local Network¶

Test Internet Connectivity¶

SSH - Secure Shell¶

Basic SSH Usage¶

SSH Key Authentication¶

SSH Configuration¶

SSH Tunneling¶

SCP - Secure Copy¶

SFTP - Secure FTP¶

Firewall Management¶

ufw - Uncomplicated Firewall (Ubuntu/Debian)¶

firewall-cmd - Firewalld (RHEL/CentOS)¶

Network Configuration Files¶

Debian/Ubuntu¶

RHEL/CentOS¶

NetworkManager¶

Practical Scenarios¶

Troubleshoot No Internet Connection¶

Find What's Using a Port¶

Monitor Network Traffic¶

Configure Static IP¶

Best Practices¶

Related Topics¶

`ifconfig` - Traditional Network Configuration¶

`ip` - Modern Network Configuration¶

`ping` - Test Connectivity¶

`traceroute` / `tracepath` - Trace Network Path¶

`netstat` - Network Statistics¶

`ss` - Socket Statistics¶

`nslookup` / `dig` - DNS Lookup¶

`host` - Simple DNS Lookup¶

`ufw` - Uncomplicated Firewall (Ubuntu/Debian)¶

`firewall-cmd` - Firewalld (RHEL/CentOS)¶