Destination port

4.7 Identify key elements in an intrusion from a given PCAP file

📘Cisco Certified CyberOps Associate (200-201 CBROPS)

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1. What is a Destination Port?

• A destination port is a number assigned to a service on a device.
• It works together with the destination IP address to make sure traffic reaches the correct application on a server.
• Think of the IP address as the building number, and the port as the apartment number inside that building. The data needs both to get to the right place.

Example in IT terms:

• If a PC sends a web request to a server:
  • Destination IP: 192.168.1.10 → server’s address
  • Destination Port: 80 → server’s HTTP web service

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2. How Destination Ports are Used in IT Environments

1. Web Traffic
   • Port 80 → HTTP (web traffic)
   • Port 443 → HTTPS (secure web traffic)
2. Email Traffic
   • Port 25 → SMTP (sending email)
   • Port 110 → POP3 (retrieving email)
   • Port 143 → IMAP (email access)
3. File Transfer
   • Port 21 → FTP (file upload/download)
   • Port 22 → SFTP/SSH (secure file transfer)
4. Remote Access
   • Port 3389 → RDP (remote desktop connection)
   • Port 23 → Telnet (not secure, rarely used today)
5. Database Services
   • Port 1433 → Microsoft SQL Server
   • Port 3306 → MySQL

Why this matters: If you see traffic going to unusual ports, it could indicate an intrusion or malware trying to communicate.

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3. How to Identify Destination Port in PCAP Files

When analyzing a PCAP file using a tool like Wireshark, you will see columns for Source Port and Destination Port.

Steps:

1. Open the PCAP in Wireshark.
2. Look at a packet in the Packet Details pane:
   • Expand Transmission Control Protocol (TCP) or User Datagram Protocol (UDP).
   • Find Destination Port – it will show the port number and sometimes the service name.

Example:

Column	Value	Meaning
Destination IP	10.10.10.5	Server receiving the traffic
Destination Port	443	Secure web traffic (HTTPS)

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4. Common Patterns in Intrusions

1. Unusual Ports
   • Attackers sometimes use non-standard ports to bypass firewalls.
   • Example: HTTP normally uses port 80, but malware might send traffic over port 8080 or 9000.
2. Port Scanning
   • If you see many packets to different ports on the same IP, this could be a scan to find open services.
   • Tools like Nmap generate this kind of traffic.
3. Repeated Connection Attempts
   • If a destination port is repeatedly targeted, it may indicate brute force attacks on that service.
   • Example: Multiple login attempts to port 22 (SSH).
4. Suspicious Protocols
   • Some malware communicates over non-standard ports (for example, a malware C2 server using port 4444).
   • Recognizing these in PCAP is critical for incident response.

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5. Tips for the Exam

• Know common ports and services: 80, 443, 21, 22, 25, 110, 143, 3389, 3306, etc.
• Check both TCP and UDP: Some services like DNS use UDP (port 53).
• Compare source and destination ports:
  • Source port is usually ephemeral/random, temporary for your computer
  • Destination port is fixed for the service being accessed
• Look for anomalies: Any traffic to unusual ports should raise suspicion.

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6. Summary for Easy Understanding

1. Destination port = service on the receiving device.
2. Helps you know what type of communication is happening.
3. Critical in identifying malicious activity in PCAP analysis.
4. Recognize common ports, unusual ports, and repeated attempts.
5. Always check in Wireshark under TCP/UDP headers.

By understanding destination ports, you can identify attacks like port scans, brute force attempts, and malware communication, which is exactly what the exam expects you to know.