Managing Network Interfaces

1. Enabling or Disabling Network Interfaces

Bring a network interface up or down using the ip command:

sudo ip link set eth0 up     # Enable interface
sudo ip link set eth0 down   # Disable interface

Useful for activating or deactivating a device without rebooting.

2. Assigning a Static IP Address

Set a manual IP address and default gateway:

sudo ip addr add 10.0.0.5/24 dev eth0
sudo ip route add default via 10.0.0.1

• ip addr add: Assigns the IP address with subnet mask.

• ip route add default: Sets the default gateway for outbound traffic.

These settings are temporary and will be lost on reboot unless made persistent via network configuration files or a network manager.

3. Viewing the Routing Table

Inspect how packets are routed through the system:

route -n          # Legacy tool
ip route show     # Preferred modern command

• Displays default gateway, connected networks, and routing priorities.

4. DNS Configuration and Lookup

View Current DNS Servers

cat /etc/resolv.conf

• Shows the DNS servers used for resolving domain names.

• Typically managed by systemd-resolved, NetworkManager, or other services.

Querying DNS Records

dig @8.8.8.8 example.com A
nslookup example.com 1.1.1.1

• dig: Powerful and script-friendly DNS query tool.

• nslookup: Simple DNS query tool, still widely used.

5. Checking Active Network Services

Inspect open ports and services:

sudo ss -tulpn

• Lists all listening TCP/UDP sockets with program names.

• Replaces the older netstat -tulpn.

6. Monitoring Network and ARP Activity

View ARP Cache

arp -n
ip neigh

• Shows resolved MAC addresses for local IPs.

• ip neigh is the modern replacement for arp.

Live Packet Capture

sudo tcpdump -i eth0 -n

• Captures packets on eth0 and displays header-level data.

• Useful for low-level network diagnostics.

7. Firewall Configuration

List Firewall Rules

sudo iptables -L -nv

sudo ufw status verbose

• iptables: Low-level firewall tool.

• ufw: User-friendly frontend for iptables.

Add Basic Rules

sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT
sudo ufw allow 443/tcp

• Allows specific traffic (e.g., SSH on port 22, HTTPS on port 443).

8. Essential Networking and Security Tools

Tool	Purpose
nmap	Network scanning, port detection, vulnerability checks
netcat (nc)	TCP/UDP connectivity testing, file transfers
tcpdump	Packet capture and inspection
wireshark	GUI-based packet analysis
traceroute	Displays route to a remote host
whois	Domain and IP ownership information
dig, nslookup	DNS record querying