Some notes on Linux networking

1. Network Interfaces

• List interfaces: ip link, ip addr
• Bring up/down: ip link set eth0 up|down
• Assign IP: ip addr add 192.168.1.10/24 dev eth0
• Loopback: lo (127.0.0.1)

2. IP Addressing

• IPv4 CIDR: /24 = 255.255.255.0
• View routes: ip route
• Default gateway: default via 192.168.1.1
• ARP cache: ip neigh

3. Routing

• Add route: ip route add 10.0.0.0/24 via 192.168.1.1
• Delete route: ip route del …
• Policy routing: ip rule, multiple routing tables

4. DNS

• Resolver config: /etc/resolv.conf
• Test DNS: dig, nslookup
• Local hosts override: /etc/hosts
• systemd-resolved status: resolvectl status

5. Ports & Sockets

• List listening ports: ss -lntup
• Old tool: netstat -tulnp
• TCP vs UDP:
  • TCP: reliable, ordered
  • UDP: fast, no guarantee

6. Firewall (iptables / nftables)

• Modern backend: nftables

• Allow port (iptables):

  iptables -A INPUT -p tcp --dport 22 -j ACCEPT

• Check rules: iptables -L -n

• UFW (simple):

  ufw allow 80 ufw enable 

7. Network Debugging

• Ping: ping 8.8.8.8
• Path tracing: traceroute, tracepath
• Packet capture: tcpdump -i eth0
• Test port: nc -zv host port, telnet

8. Bandwidth & Traffic

• Interface stats: ip -s link
• Live traffic: iftop, nload
• Per-process: ss, lsof -i

9. Network Services

• SSH: port 22
• HTTP/HTTPS: 80 / 443
• DNS: 53

• Check service bind:

  ss -lntp | grep 80

10. Namespaces & Containers (Important)

• Network namespace: isolated network stack
• Docker:
  • bridge (default)
  • host (no isolation)
  • overlay (multi-node)
• View namespaces: ip netns

11. Common Files

• /etc/hosts
• /etc/resolv.conf
• /etc/network/interfaces (Debian legacy)
• /etc/netplan/*.yaml (Ubuntu)

12. Quick Commands Cheat

ip a ip r ss -tulnp ping dig tcpdump curl