Port scanning is a technique used in computer networking to discover open ports on a target device.
It’s like knocking on the doors of a house to see which are open & which are closed.
Ports are the virtual endpoints that allow different types of network services & applications to communicate over a network.
It’s a way for someone like a security expert to check which “doors” (or ports) on a network are accessible.
Every computer has a bunch of ports, like gates, and each one is responsible for different types of communication.
Some ports are open and ready to talk to others – while some are closed and won’t respond.
When someone does a port scan, they send small messages to these ports to see if they get a response.
If they do, it means the port is open, and there might be a way to communicate with the computer or network through that port.
This can be used for both legitimate purposes (like checking if your own network is secure) and potentially malicious ones (like trying to find weak points to break into a system).
Importance of Port Scanning
Port scanning plays an important role in computer networking for several reasons. Some of them are:
Port scanning is an essential tool for network admins to discover devices on their network and the services they are running.
It helps in maintaining an accurate device inventory and their configurations, which is important for network management.
It can help pinpoint the error when network issues arise, such as connectivity problems. Admins can identify misconfigured services or firewalls that may be blocking traffic by checking the status of specific ports.
One of the primary uses of port scanning is in network security assessments. It can be used to identify potential vulnerabilities in network devices.
For example – An open port running an outdated service could be an entry point for attackers.
It is used to test and validate firewall configurations. Network admins can make sure that firewalls are properly filtering traffic & protecting the network by scanning from both internal and external perspectives.
IDS and IPS solutions often use port scanning techniques to identify malicious network activity. If an IDS detects an unusual number of connection attempts on specific ports, it may trigger an alert.
Also helps in creating network maps, which are essential for understanding the network topology.
Port scanning is also used for monitoring services to make sure they are running correctly. Automated scans can regularly check the status of critical ports & trigger alerts if any issues are detected.
Ethical hackers generally use port scanning as part of penetration testing to assess the security of a network. It helps them in finding potential weaknesses that malicious attackers could exploit.
How Port Scanning Works?
Here’s a step-by-step explanation of how this port scanning works
#1. Selecting the Target
The first step is selecting the target device or IP address that you want to scan. This could be a single device or an entire network.
#2. Choosing the Scanning Tool
Port scanning is generally conducted using specialized tools known as port scanners. The user may configure the scanning process with various scanning parameters if needed.
#3. Sending Probe Packets
The port scanner starts by sending a series of probe packets to the target. Each probe packet is directed at a specific port on the target device.
These packets are used to check the status of the target ports and are designed to appear like actual network traffic.
#4. Analyzing Responses
The target device processes the incoming probe packets & responds according to the state of the port being probed. There are four common responses.
Open: The target device responds positively to the probe packet if the port is open and a service is actively listening to it. This indicates that the port is accessible.
Closed: If you get a response “port is closed,” that means no service is currently active on the target machine.
Filtered: The probe does not receive a response if the port is filtered. This could be due to a firewall or other security measures blocking the probe.
Unfiltered: In some cases, the probe may not receive a response that makes it unclear whether the port is open or closed.
#5. Recording Results
The port scanner records the responses received from the target device for each probed port. It keeps track of which ports are open, closed, and filtered.
#6. Generating a Report
After scanning each of the designated ports, the tool compiles the results into a report. It gives complete info about the open ports and their corresponding services, which can be used for further analysis.
#7. Interpreting the Result
The final step is to analyze the results of the port scan to assess the security posture of the target device. This can identify potential vulnerabilities & misconfigurations that require attention.
Types of Port Scanning
Here are some common types of port scanning techniques.
#1. TCP Connect Scan
This is the most basic form of port scanning. It tries to establish a full TCP connection with each port to determine whether it is open or closed.
The port is considered as open if a connection is made successfully. This scan is relatively noisy and easy to detect.
#2. TCP SYN Scan (Half-Open Scan)
The scanner sends a TCP SYN packet to the target port in this method. A SYN-ACK message is returned if the port is open.
The scanner sends an RST (reset) message to break the connection rather than completing the three-way handshake.
Compared to a full connect scan, this approach is more stealthy.
#3. TCP FIN Scan
A TCP FIN packet is transmitted to the target port by the scanner.
It replies with an RST packet if the port is closed. The FIN packet is normally ignored if the port is open. This scan can be effective in some cases but may not work against all systems.
#4. TCP XMAS Tree Scan
It delivers a TCP packet with the FIN, URG, and PSH flags set just as the FIN scan.
It should respond with an RST packet if the port is closed. The packet is ignored if the port is open.
#5. TCP Null Scan
The scanner sends a TCP packet with no flags specified during a null scan.
Open ports ignore the data being sent, and closed ports frequently respond with a RST response.
Similar to the XMAS Tree scan, this approach is stealthy but not always successful.
#6. UDP Scan
Scanning UDP ports is more difficult because UDP is connectionless.
The scanner sends a UDP packet to the target port and waits for a response.
It might reply with an ICMP Port Unreachable message if the port is closed. A response at the application level is possible if it is open.
UDP scans can be slower due to the lack of a connection-oriented protocol.
#7. ACK Scan
An ACK (acknowledgment) packet is sent to the target port during this scan. The response depends on whether the port is filtered or unfiltered.
It might react with a RST packet or nothing at all if it is filtered. If unfiltered, it may ignore the packet. This scan comes in handy to identify stateful firewalls.
#8. Idle Scan (Zombie Scan)
This scan is a more advanced method that uses a “zombie” host—an unknowing intermediary to conduct the scan.
The scanner can figure out the target port’s status by carefully examining the IPID (Identification field) sequence numbers in the answers from the zombie host.
Idle scanning is highly stealthy but complex to execute.
Important Note: Unauthorized port scanning can be considered an intrusion. Always make sure you have the necessary permissions & legal clearance before conducting port scans.
Best Port Scanners
Port scanner tools automate & simplify many manual tasks related to network reconnaissance.
#1. Nmap (Network Mapper)
One of the most powerful open-source network scanning tools on the market right now is Nmap.
It comes with a scripting engine that allows users to write & run custom scripts to perform various tasks during the scanning process.
A TCP Port Scanner is a free & user-friendly network utility designed to analyze the status of TCP/IP ports on a target device.
One of the excellent features of a TCP Port Scanner is its speed. It is capable of scanning networks at a rate of up to 10,000 ports/second.
It mainly uses the SYN scanning method that sends a TCP SYN packet to the target port & analyzes the response.
Users can also save the results of their scans into a text file.
Netcat is a powerful networking tool used for reading & writing data across network connections using the TCP/IP protocol and is commonly used for network debugging.
It can establish both outbound & inbound network connections using TCP or UDP protocols.
It also supports tunneling, which allows you to create specialized connections, such as converting UDP to TCP, and provides flexibility in configuring network parameters like interfaces and listening ports.
Netcat offers advanced usage options like buffered send mode that sends data at specified intervals, and hexdump, which can display transmitted & received data in hexadecimal format.
It also includes an optional RFC854 telnet codes parser and responder, which can be helpful for emulating telnet sessions.
#4. Advanced Port Scanner
Advanced Port Scanner is another powerful networking tool that can quickly check which devices are connected to a network & which of them have open ports.
It can easily figure out what kind of programs or software are running on those open ports and also helps in easily accessing the resources behind them if it finds any ports open.
For example, it can help you open websites (HTTP/HTTPS), download files (FTP), or access shared folders on other computers.
It even lets you control other computers from a distance (remote controlling).
You can think of it as being able to operate a computer in another location, almost as if you were sitting in front of it. This can be handy for tasks like troubleshooting or managing multiple computers.
How Cyber Attackers Use Port Scanning?
Attackers use port scanning as a reconnaissance technique to gather information about potential targets and vulnerabilities. Here’s how they use it.
Identifying Vulnerable Services
Attackers scan a range of IP addresses or specific targets to find open ports and services.
They may target well-known ports associated with common services (e.g., port 80 for HTTP) to identify potential vulnerabilities in web servers or other services.
Mapping the Network
Port scanning helps attackers map the network’s layout, identifying devices and their roles. This information helps in planning further attacks.
Attackers analyze the responses they receive from open ports to identify the specific software and version running on the target. This helps them to design attack vectors for the vulnerabilities in that software.
Attackers may perform port scans to find open ports that bypass firewall rules, which potentially allows them to gain unauthorized access to a network/device.
Defenses Against Port Scanning
Consider the following measures to defend against port scanning and protect your network.
Implement robust firewall rules to restrict incoming and outgoing traffic. Only open necessary ports for essential services and regularly review and update firewall configurations.
IDS and IPS system
Deploy Intrusion detection and prevention solutions to monitor network traffic for suspicious activities, including port scanning attempts, and take manual actions to block or alert on such activities.
Regularly scan your own network for open ports and services. Close any unused ports to reduce the attack surface.
Continuously monitor network traffic for unusual or repetitive connection attempts that could indicate a port scanning attack.
Implement rate-limiting rules in firewalls/routers to restrict the number of connection attempts from a single IP address, which makes it more difficult for attackers to perform extensive scans.
Deploy honeypots or decoy systems to divert attacker attention and gather information about their tactics without risking your actual network.
Keep all software up-to-date with security patches to minimize the likelihood of vulnerabilities that attackers might exploit.
Port Scanning vs. Network Scanning
Here’s a clear comparison between port scanning and network scanning:
Focus: Identifying open ports and services on a specific device or IP address.
Goal: To find entry points for communication or potential vulnerabilities on a single target.
Method: Sends data packets to individual ports to determine their status (open, closed, filtered).
Scope: Limited to a single device or IP address.
Use Cases:Network security, troubleshooting, service discovery, and penetration testing.
Focus: Mapping and discovering devices, subnets, and network topology across an entire network.
Goal: To create an inventory of network assets, including devices and their characteristics.
Method: Scan multiple IP addresses and devices to identify their presence/attributes.
Scope: Covers an entire network, including multiple devices, subnets, and network segments.
Use Cases: Network management, security assessment, inventory management, and vulnerability assessment.
Port scanning is all about identifying open ports and services on a specific device, while network scanning involves discovering and mapping network layouts across a broader network infrastructure. Both techniques serve different purposes in network administration.
Port Scanning is a valuable asset in maintaining the health and security of computer networks.
I hope you found this article helpful in learning about port scanning and the importance of it.
You may also be interested in learning about the best Network Access Control software for conducting the security assessment.