What is Port Scanning?

Port scanning is a technique used to determine which ports on a network are open and capable of sending or receiving data. It also involves sending packets to specific ports on a host and analyzing the responses to identify vulnerabilities.

Before conducting port scanning, it’s necessary to identify a list of active hosts and map these hosts to their respective IP addresses. This process, known as host discovery, begins with a network scan.

The purpose of port and network scanning is to understand the structure of IP addresses, hosts, and ports to accurately identify open or vulnerable server locations and assess security levels. Both network and port scanning can reveal the presence of security measures such as firewalls between servers and user devices.

Once a thorough network scan is completed and a list of active hosts is compiled, port scanning can be conducted to identify open ports on the network that could potentially enable unauthorized access.

It’s important to note that network and port scanning can be used by both IT administrators and cybercriminals to assess network security policies and identify vulnerabilities. For attackers, host discovery through network scanning is often the initial step before launching an attack.

As network and port scanning continue to be essential tools for attackers, the results of these scans provide critical insights into network security levels for IT administrators working to protect networks from potential threats.

What are Ports and Port Numbers?

Computer ports serve as the primary interface for the exchange of information between programs, the Internet, devices, or other computers within a network. They function as designated points for data transfer, facilitated through electronic, software, or programming mechanisms.

Port numbers are crucial for consistency and programming purposes. When combined with an IP address, they constitute essential information retained by Internet Service Providers to facilitate communication. Port numbers span from 0 to 65,536 and are generally sorted by popularity.

The range from 0 to 1023 comprises well-known port numbers primarily designated for Internet use, though they may serve specialized functions as well. Managed by the Internet Assigned Numbers Authority (IANA), these ports are typically allocated to major companies such as Apple QuickTime, MSN, SQL services, and other significant organizations. Some notable ports and their associated services include:

• Port 20 (UDP) for File Transfer Protocol (FTP) used in data transfer
• Port 22 (TCP) for Secure Shell (SSH) protocol, facilitating secure logins, FTP, and port forwarding
• Port 53 (UDP) for the Domain Name System (DNS), responsible for translating names to IP addresses
• Port 80 (TCP) for World Wide Web HTTP

Ports numbered from 1024 to 49151 are categorized as “registered ports,” meaning they are officially assigned by software corporations. The remaining range, from 49152 to 65,536, encompasses dynamic and private ports, available for use by nearly any entity.

What are the Protocols Used in Port Scanning?

The primary protocols used for port scanning are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). These protocols represent different methods of data transmission over the internet.

TCP provides reliable, connection-oriented data transmission. It establishes a two-way connection between sender and receiver, ensuring that data is delivered and received in the correct order without loss.

On the other hand, UDP is connectionless and unreliable. It does not establish a connection before sending data and does not guarantee delivery or order of packets. UDP is often used for applications where speed is more critical than reliability.

Using these protocols, various techniques can be employed to conduct port scans effectively.

Types of Port Scans

Port scanning involves sending packets to destination port numbers using various techniques. Some of these techniques include:

• Ping scans: Also known as ICMP (Internet Control Message Protocol) requests, ping scans are the simplest form of port scanning. They send multiple ICMP requests to different servers to elicit a response. Ping scans are useful for troubleshooting network issues but can be blocked or disabled by firewalls.
• Vanilla scan: This basic technique attempts to connect to all 65,536 ports simultaneously by sending a SYN (Synchronize) flag or connect request. When a SYN-ACK (Synchronize-Acknowledgment) response is received, indicating an open port, the scanner responds with an ACK (Acknowledgment) flag. While accurate, this scan is easily detectable as it results in a full connection being logged by firewalls.
• SYN scan: Also known as a half-open scan, this technique sends a SYN flag to the target port and waits for a SYN-ACK response. If a response is received, the scanner does not complete the TCP connection, thus avoiding logging. This method provides quick insight into open ports and is commonly used by hackers to identify vulnerabilities.
• XMAS and FIN scans: XMAS scans set specific flags within packets to resemble a blinking Christmas tree in packet analyzers like Wireshark. These scans reveal information about firewall behavior and port states. FIN scans involve sending a FIN (Finish) flag to a port and analyzing the system’s response to gauge activity levels and firewall configurations.
• FTP bounce scan: This technique involves using an FTP server to bounce packets, allowing the sender to mask their location.
• Sweep scan: A preliminary scanning method that sends traffic to ports across multiple computers on a network to identify active systems. While it doesn’t reveal specific port activity, it indicates whether systems are in use.

What are the Different Port Scanning Techniques?

Various techniques exist for port scanning, each chosen based on specific objectives. It’s worth noting that cybercriminals select scanning techniques based on their attack goals and strategies.

Here are some techniques and their functionalities:

• Ping scans: These are the most basic port scans, known as ping scans. In networking, a ping verifies whether a network data packet can reach an IP address without errors. Ping scans use ICMP (Internet Control Message Protocol) requests, sending a series of automated ICMP requests to different servers to elicit responses. IT administrators may employ this technique for troubleshooting, and it can be countered by disabling ICMP responses using a firewall, preventing attackers from discovering the network via pings.
• Half-open or SYN scans: This technique, also called a SYN scan, allows attackers to ascertain port status without establishing a full connection. It involves sending only a SYN message and not completing the connection, leaving the target port in a pending state. This quick and stealthy method aims to identify potential open ports on target devices.
• XMAS scans: XMAS scans are even more discreet and evade detection by firewalls. Typically, FIN (Finish) packets are sent to terminate a connection after a successful TCP 3-way handshake and data transfer. These FIN packets often go unnoticed by firewalls focused on SYN packets. In contrast, XMAS scans send packets with all flags set (including FIN), expecting no response if the port is open. A RST (Reset) response indicates a closed port. XMAS scans rarely appear in monitoring logs and provide insights into a network’s security measures and firewall configuration.

What Type of Port Scan Results can you Get From Port Scanning?

Port scan results categorize the status of the network or server into three main categories: open, closed, or filtered.

• Open ports: These indicate that the target server or network actively accepts connections or datagrams and responds with a packet indicating it is listening. It also signifies that the scanned service (usually TCP or UDP) is operational. Discovering open ports is typically the primary objective of port scanning and signifies a potential avenue for cybercriminals seeking to exploit vulnerabilities. IT administrators face the challenge of securing open ports with firewalls to prevent unauthorized access while ensuring legitimate users can still connect.
• Closed ports: Closed ports indicate that the server or network received the request, but there is no service listening on that port. While closed ports are accessible, they do not offer any active service. IT administrators should monitor closed ports as they may become open in the future, potentially posing security risks. Consideration should be given to blocking closed ports with a firewall, converting them into “filtered” ports.
• Filtered ports: These indicate that a request packet was sent, but the host did not respond and is not listening. This typically occurs when a firewall filters out or blocks the request packet. The lack of response prevents attackers from obtaining further information. Filtered ports often generate error messages such as “destination unreachable” or “communication prohibited.”

Port Scanning vs Network Scanning

Network scanning involves identifying active hosts on a network and associating them with their respective IP addresses. This step is essential before conducting a port scan.

Network scanning, also referred to as host discovery, serves as the initial phase for hackers preparing an attack. They utilize two primary protocols: Address Resolution Protocol (ARP) scans and various ICMP scans. An ARP scan links IP addresses to media access control (MAC) addresses, helping identify active hosts within a local-area network (LAN). This method requires the attacker to be connected to the internal network.

Outside the LAN, various ICMP packets, such as address mark, echo, and timestamp requests, can be utilized to conduct network scans. The discovery of hosts relies on receiving replies from the targeted hosts. A lack of response indicates either no host at the target address or that the request was blocked by a firewall or packet filter.

Once the network scan is completed and a list of available hosts is compiled, a port checker or port scanner can be used to identify the status of specific ports. Ports are typically categorized as open, closed, or filtered based on the scan results.

How to Prevent Port Scan Attacks?

Port scanning is a common technique used by cybercriminals to identify vulnerable servers, assess organizations’ security posture, evaluate firewall effectiveness, and uncover potential network or server vulnerabilities. Certain TCP methods also allow attackers to obfuscate their origins.

Cybercriminals conduct network scans to observe port behaviors, gaining insights into the security measures and deployed systems of targeted businesses.

Effective defense against port scan attacks relies on robust, up-to-date threat intelligence aligned with evolving threats. Businesses should deploy strong security software, port scanning tools, and security alerts to monitor ports and block malicious actors from accessing their network.

Additional defensive measures include:

• Strong firewall implementation: Firewalls prevent unauthorized access to a business’s private network by controlling port visibility and detecting ongoing port scans to mitigate threats.
• TCP wrappers: These tools offer administrators the ability to allow or deny server access based on IP addresses and domain names, enhancing network security.
• Identification of network vulnerabilities: Businesses can utilize port checkers or port scanners to assess open ports and identify potential weaknesses or unnecessary exposures. Regular system checks are essential to report and address vulnerabilities that could be exploited by attackers.

FAQ’s

What is port scanning, and why is it used?

Port scanning is a technique used to determine which ports on a network are open and capable of sending or receiving data. It is employed by cybercriminals to assess organizations’ security levels, detect vulnerabilities, and evaluate firewall effectiveness.

What are the different types of port scanning techniques?

Port scanning techniques include ping scans, SYN scans, XMAS scans, FTP bounce scans, and sweep scans. Each technique offers unique functionalities, allowing attackers to gather specific information about targeted networks.

How can businesses prevent port scan attacks?

Businesses can prevent port scan attacks by deploying robust security measures such as firewalls, TCP wrappers, and security software. Regular network vulnerability assessments using port scanning tools are also essential for identifying and addressing potential weaknesses.

What is the difference between port scanning and network scanning?

Port scanning focuses on identifying open ports on a network, while network scanning involves identifying active hosts and mapping them to their IP addresses. Network scanning serves as the initial phase for attackers preparing an attack, while port scanning is used to gather detailed information about specific ports.

What are the possible outcomes of a port scan?

Port scan results categorize ports into three main categories: open, closed, or filtered. Open ports indicate active services, closed ports indicate no active service, and filtered ports indicate that the host did not respond, possibly due to firewall filtering.

What are some commonly used port scanning tools?

Commonly used port scanning tools include IP scanning tools like Nmap and Netcat. These tools offer comprehensive features for conducting port scans, identifying vulnerabilities, and assessing network security levels.

How do cybercriminals utilize port scanning techniques?

Cybercriminals use port scanning techniques to identify potential attack avenues, assess network security policies, and identify vulnerabilities. Port scanning serves as the initial step for hackers before launching an attack, allowing them to gather crucial information about targeted networks.

Why are port numbers important in port scanning?

Port numbers are crucial for identifying specific services or applications running on a network. They help cybercriminals determine the nature of open ports and potential vulnerabilities, enabling them to tailor their attack strategies accordingly.

Conclusion

Port scanning is pivotal in both defending against cyber threats and enabling cyberattacks. It aids in evaluating network security, pinpointing vulnerabilities, and gauging firewall efficacy. Businesses must employ proactive measures, including regular vulnerability assessments and robust security software, to thwart unauthorized access and mitigate risks. By staying vigilant, organizations can safeguard their networks against malicious port scan attacks and protect sensitive data from exploitation.