Network Administration Fundamentals: A Complete Guide for Linux Sysadmins

Network administration is a critical skill for any Linux system administrator. This comprehensive guide covers the fundamental concepts, tools, and techniques you need to effectively manage network infrastructure on Linux systems.

Understanding Network Architecture

Before diving into specific tools and configurations, it is essential to understand the OSI model and TCP/IP stack that form the foundation of modern networking.

The OSI Model Layers

• Layer 7 – Application: HTTP, HTTPS, FTP, SSH, DNS
• Layer 4 – Transport: TCP, UDP
• Layer 3 – Network: IP, ICMP, routing
• Layer 2 – Data Link: Ethernet, MAC addresses
• Layer 1 – Physical: Cables, connectors, signals

Essential Network Configuration Commands

IP Address Management

# View all network interfaces
ip addr show

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

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

# Bring interface up/down
sudo ip link set eth0 up
sudo ip link set eth0 down

NetworkManager Commands

# Show all connections
nmcli connection show

# Show device status
nmcli device status

# Create new connection
sudo nmcli connection add type ethernet con-name "My Connection" ifname eth0

# Modify connection
sudo nmcli connection modify "My Connection" ipv4.addresses 192.168.1.100/24
sudo nmcli connection modify "My Connection" ipv4.gateway 192.168.1.1
sudo nmcli connection modify "My Connection" ipv4.dns "8.8.8.8"
sudo nmcli connection modify "My Connection" ipv4.method manual

# Activate connection
sudo nmcli connection up "My Connection"

Routing and Gateway Configuration

# View routing table
ip route show
route -n

# Add default gateway
sudo ip route add default via 192.168.1.1

# Add static route
sudo ip route add 10.0.0.0/8 via 192.168.1.1 dev eth0

# Delete route
sudo ip route del 10.0.0.0/8

DNS Configuration

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

# Test DNS resolution
nslookup google.com
dig google.com

# Query specific DNS server
dig @8.8.8.8 google.com

# Reverse DNS lookup
dig -x 8.8.8.8

Network Troubleshooting Tools

# Test connectivity
ping -c 4 google.com

# Trace route
traceroute google.com
tracepath google.com

# Check open ports
ss -tuln
netstat -tuln

# Test specific port
nc -zv server.com 80
telnet server.com 80

# Capture network traffic
sudo tcpdump -i eth0
sudo tcpdump -i eth0 port 80

Firewall Management

centos-fedora">firewalld (RHEL/CentOS/Fedora)

# Check status
sudo firewall-cmd --state

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

# Add service
sudo firewall-cmd --add-service=http --permanent

# Add port
sudo firewall-cmd --add-port=8080/tcp --permanent

# Reload firewall
sudo firewall-cmd --reload

ubuntu-debian">UFW (Ubuntu/Debian)

# Enable UFW
sudo ufw enable

# Allow service
sudo ufw allow ssh
sudo ufw allow http

# Allow specific port
sudo ufw allow 8080/tcp

# Check status
sudo ufw status verbose

Best Practices for Network Administration

1. Document all network configurations and changes
2. Use configuration management tools like Ansible for consistency
3. Implement monitoring and alerting for network services
4. Regularly audit firewall rules and network access
5. Keep network services and firmware updated
6. Use VLANs to segment network traffic
7. Implement proper backup procedures for network configs

Conclusion

Mastering network administration on Linux requires understanding both the theoretical concepts and practical tools. Practice these commands in a lab environment and gradually apply them to production systems as you gain confidence.