ubuntu防火墻如何測試連接

Testing Connections with Ubuntu Firewall (UFW)
When managing Ubuntu firewalls (typically UFW—Uncomplicated Firewall), testing connections ensures your rules work as intended. Below are key methods to validate connectivity, organized by tool and use case.

1. Check Firewall Status First

Before testing, confirm UFW is active and view its rules to understand which ports/IPs are allowed/denied:

sudo ufw status

• Active/Inactive: Status: active means UFW is enforcing rules; inactive means it’s not blocking traffic.
• Rules List: Shows allowed/denied ports (e.g., 22/tcp ALLOW Anywhere). If a port isn’t listed, it’s denied by default.

2. Test TCP Port Connectivity

Using telnet (Simple TCP Check)

Telnet tests basic TCP connectivity. Install it if missing (sudo apt install telnet), then run:

telnet <server-address> <port>

• Success: You’ll see Connected to <server-address> (e.g., Connected to www.example.com).
• Failure: You’ll get Unable to connect to remote host: Connection refused (firewall/port closed) or Connection timed out (network issues).

Using nc (Netcat, More Flexible)

Netcat (nc) checks both TCP/UDP connectivity. Install it (sudo apt install netcat-openbsd), then:

nc -zv <server-address> <port>

• Flags: -z scans without sending data, -v enables verbose output.
• Success: Connection to <server-address> <port> port [tcp/udp] succeeded.
• Failure: Connection refused (firewall/port closed) or Timeout (network issues).

3. Test UDP Port Connectivity

UDP is connectionless, so tools like nc are ideal. Use:

nc -zuv <server-address> <port>

• Flag: -u specifies UDP.
• Note: Some services (e.g., DNS) use UDP; test these if your app relies on them.

4. Test HTTP/HTTPS Connectivity

For web services, use curl to check if the firewall allows HTTP/HTTPS traffic:

curl -I http://<server-address>  # HTTP
curl -I https://<server-address> # HTTPS

• Success: Returns HTTP headers (e.g., HTTP/1.1 200 OK).
• Failure: Connection refused (firewall blocking) or Timeout (server issue).

5. Advanced Testing with nmap

nmap scans ports to detect open/closed/filtered states (filtered = blocked by firewall). Install it (sudo apt install nmap), then:

nmap -p <port> <server-address>

• Example: nmap -p 22 localhost checks if port 22 is open locally.
• Output:
  • open: Port is accessible.
  • closed: Port is not listening (service not running).
  • filtered: Port is blocked by a firewall.

Key Troubleshooting Tips

• Service Not Running: If a port is closed but UFW allows it, ensure the service (e.g., Apache for port 80) is active (sudo systemctl status apache2).
• SSH Issues: If you’re locked out after enabling UFW, allow SSH again: sudo ufw allow ssh (or sudo ufw allow 22/tcp).
• External Testing: Test from another machine (e.g., your laptop) to rule out local network issues.

By combining these tools, you can systematically verify if your Ubuntu firewall is correctly configured and not blocking legitimate traffic.