Networking Overview

Configuration files are located in /etc/network/interfaces. To test networking, use nmcli.

Dynamic IP vs Static IP

• Dynamic IP: Assigned by a DHCP server.
• Static IP: Manually set by the administrator.

Managing Network Services

systemctl status servicename
systemctl start | stop servicename

Finding Network Connection Information

• List all adapters: ip a
• Predictable interface names:
  • Onboard: eno1, eno2
  • PCI hotplug: ens1, ens2
  • PCI card: enp0s3, enpp1s2
  • WLAN/WiFi: wlp1s3
  • MAC address-based: enx1a2b3c4d5e6f

Configuring Dynamic Address with Network Manager

• Check network manager status: nmcli -d
• Delete a connection: nmcli c del "Connection name"
• Edit or create a connection: nmcli c e [connection name]

nmcli UI Commands

• help
• Show properties: print ipv4
• Show property details: describe method
• Set automatic IP: set ipv4.method auto
• Set DNS server: set ipv4.dns 8.8.8.8
• Save changes: save
• List all current connections: print connection
• Assign connection to adapter: set connection.interface-name enp0s3
• Exit UI: quit

Configuring Static IP Address

• Manually create a connection: nmcli c e

nmcli UI Commands

set ipv4.method manual
set ipv4.addresses 10.0.2.10/24
set ipv4.gateway 10.0.0.1

• Edit /etc/network/interfaces for static IP:

auto enp0s3
iface enp0s3 inet static
    address 10.0.2.20/24
    gateway 10.0.2.1
    dns-nameservers 8.8.8.8

• Restart the network service:

systemctl restart NetworkManager
systemctl restart networking

Configuring a Firewall

• Enable firewall: sudo ufw enable
• Check firewall status: sudo ufw status verbose
• Reject traffic on port 3000: sudo ufw reject 3000
• Allow traffic on port 3000: sudo ufw allow 3000
• Delete a rule: sudo ufw delete allow 3000
• Allow traffic from a specific machine: ufw allow proto tcp from 10.0.2.6 to 10.0.2.20 port 3000

Firewall rules are stored in /etc/ufw/.

Configuring IP Forwarding

cat /proc/sys/net/ipv4/ip_forward
sudo sysctl -w net.ipv4.ip_forward=1

• Permanently enable IP forwarding in /etc/sysctl.conf

Network Address Translation (NAT)

• Reserved private networks:
  • 10.0.0.0/8
  • 172.16.0.0/12
  • 192.168.0.0/16

Configuring NAT in the Firewall

• Modify /etc/default/ufw: Change DEFAULT_FORWARD_POLICY="DROP" to "ACCEPT".
• Edit /etc/ufw/before.rules:

*nat
:POSTROUTING ACCEPT [0:0]
-A POSTROUTING -s 10.0.2.0/24 -o enp0s8 -j MASQUERADE
COMMIT

• Restart the firewall:

sudo ufw disable
sudo ufw enable

Routing Traffic

• Check system routes: ip route
• Create a temporary route: ip route add 10.0.3.0/24 via 10.0.2.6
• Permanent route in /etc/network/interfaces:

up route add 10.0.3.0/24 via 10.0.2.6

• Permanent route using NetworkManager:

nmcli connection modify ConnectionName ipv4.routes "10.0.3.0/24 10.0.2.6"

Configuring Network Tunneling

• GRE Tunnel Example:

Router 1:
ip tunnel add mytunnel0 mode gre remote 87.65.43.21 local 12.34.56.78 ttl 255
ip link set mytunnel0 up
ip address add 10.1.0.1/24 dev mytunnel0

Router 2:
ip tunnel add mytunnel0 mode gre remote 12.34.56.78 local 87.65.43.21 ttl 255
ip link set mytunnel0 up

Time Synchronization

• Check NTP status: chronyc sources
• Check system time: timedatectl
• Modify /etc/chrony/chrony.conf to allow other servers
• Restart chrony service: sudo systemctl restart chrony
• Open NTP port in firewall: sudo ufw allow 123

Network Monitoring Tools

• iftop - Monitor traffic per host
• nethogs - Monitor traffic per process
• ss - Show open ports and connections

ss Utility Examples

ss -u   # Show active connections
ss -tl  # Show listening TCP connections
ss -ul  # Show listening UDP connections
ss -ulnp  # Show which process is responsible for the connection

Conclusion

This guide covers essential Linux network administration, including IP configuration, firewall management, NAT, routing, tunneling, and network monitoring. Mastering these concepts ensures efficient and secure network management on Linux systems.