📡 TCP vs UDP
- TCP — reliable। HTTP, SSH, DB।
- UDP — fast, no guarantee। DNS, video, gaming।
🚪 Common Ports
22 SSH 25 SMTP 53 DNS 80 HTTP 443 HTTPS 3306 MySQL 5432 PostgreSQL 6379 Redis 27017 MongoDB 3000 Node.js dev 8000 Django dev
🛡️ UFW Firewall
sudo ufw default deny incoming sudo ufw default allow outgoing sudo ufw allow 22/tcp sudo ufw allow 80,443/tcp sudo ufw allow from 10.0.0.0/24 to any port 3306 sudo ufw enable sudo ufw status verbose sudo ufw delete allow 80
🌐 NAT
One Public IP → multiple Private IP। Home router এ এটাই হয়।
🏠 IP Ranges
Public: Internet routable
Private: 10.0.0.0/8
172.16.0.0/12
192.168.0.0/16
Loopback: 127.0.0.1
🔍 Debug Commands
ss -tulnp # listening ports lsof -i :80 # who is on 80 nmap -sV server.com # port scan (own only!) nc -zv example.com 443 # test port open tcpdump -i eth0 port 80 # packet capture
🛡️ Production Checklist
- SSH key only, port 22 changed
- DB port — internal only
- Redis password set, never public
- UFW deny default
- Cloud security group layered
- Fail2ban for SSH
🎓 মূল কথা ⭐
Default deny, allow specific। DB port public = guaranteed hack। UFW + Cloud firewall = layered defense।