Identifying Open Ports

In the vast landscape of network scanning, identifying open ports is a fundamental technique employed in offensive cybersecurity. Open ports act as gateways for establishing communication with networked services, making them potential entry points for exploitation. Understanding and utilizing both techniques and tools effectively is crucial in this context.

Techniques

TCP SYN Scanning

TCP SYN scanning, commonly known as "half-open" scanning, involves sending a SYN packet to the target port. If the port is open, the target responds with a SYN-ACK packet, indicating a willingness to establish a connection. At this point, the scanner sends an RST packet to disrupt the connection before it is fully established, thereby avoiding logging on some systems.

Advantages of TCP SYN scanning include its ability to be quicker and more discreet compared to full connection scans. It is capable of bypassing certain firewalls and intrusion detection systems (IDS) due to its minimal packet exchange. However, network defenses that monitor half-open connections can still potentially detect this type of scan.

TCP Connect Scanning

TCP Connect scanning establishes a full connection to the target port by completing the TCP three-way handshake (SYN, SYN-ACK, ACK). This method is useful in scenarios where raw packet manipulation is restricted, such as when performing scans through certain firewalls or using unprivileged network user permissions.

While more detectable due to the full three-way handshake, TCP Connect scanning enables a reliable confirmation of the port's status and is logged by the target system, making it easier to analyze results. It is critical in cases where SYN scanning is not supported or allowed.

UDP Scanning

UDP Scanning identifies open UDP ports by sending a UDP packet to each target port and analyzing the response. Unlike TCP, UDP does not have error recovery or acknowledgment mechanisms, making its scanning slower and often more difficult.

The primary challenge with UDP scanning is the lack of response from open ports, which requires indirect techniques, such as sending application-layer payloads to elicit identifiable responses. Closed ports often respond with ICMP Port Unreachable messages, which can be used to deduce that the port is closed. Differentiating open, closed, and filtered states through continued observation and probing is essential.

ACK Scanning

ACK scanning is used predominantly to map firewall rulesets rather than directly identifying open or closed ports. By sending an ACK packet to a target, the scanner can determine if the port is filtered or unfiltered based on the response or lack thereof.

In scenarios where stateful firewalls are implemented, ACK scans are particularly effective for distinguishing between open and closed states by observing if the packets pass through, get blocked, or are dropped. Crafting specific packet headers allows the scanner to probe both TCP states and firewall configurations.

FIN, Xmas, and Null Scans

FIN, Xmas, and Null scans exploit variations of TCP packet flags to detect open ports by eliciting responses inconsistent with standard TCP behavior, as per RFC guidelines. These scans can be stealthed and potentially avoid detection by simplistic IDS/IPS systems.

• FIN Scan: Sends a packet with the FIN flag.
• Xmas Scan: Sends a packet with FIN, URG, and PSH flags set.
• Null Scan: Sends a packet with no flags set.

These scans are based on the expectation that a compliant TCP implementation will send no response if the port is open or an RST if closed. Their subtlety makes them useful for bypassing filters that react to more obvious scan types. However, their effectiveness varies with the target operating system's TCP/IP stack implementation.

Idle Scan

Idle scanning uses a third-party, or "zombie" host, to conduct a blind TCP port scan. This method leverages IP address spoofing by sending SYN packets with the zombie's address as the source and analyzing its IP ID sequence to infer the target's port status based on the observed changes.

The idle scan is extremely stealthy as it hides the source of the scan from the target. The primary limitation is finding a suitable idle host with predictable IP ID increments. These scans are beneficial in scenarios requiring extreme stealth and offer anonymity to the attacker.

Tools

Nmap

Nmap (Network Mapper) is a robust and versatile tool widely preferred for its scanning flexibility and extensive feature set. It supports various scanning techniques from simple SYN scans to complex scriptable probes using its Nmap Scripting Engine (NSE).

Example command for SYN scan:

nmap -sS [target]

By offering options to adjust scan timing, parallel execution, and port ranges, Nmap adapts well to both small-scale and extensive scanning tasks. Its ability to customize output formats ensures that results are both actionable and interpretable.

Masscan

Masscan claims to be the fastest port scanner, thanks to its asynchronous operation capabilities, which allow it to conduct scans at a much higher rate than traditional scanners like Nmap.

Example command for a high-speed scan:

masscan [target subnet] -p1-65535 --rate=10000

Masscan is ideal for rapidly scanning large networks but requires careful configuration to avoid alarming network defenses due to its speed. Attention to detail in rate limiting and output handling is crucial to prevent disruptions and achieve actionable results.

Unicornscan

Unicornscan is designed for information gathering and correlating large-scale scanning data. Its asynchronous and distributed scanning methodologies provide substantial flexibility in handling complex network environments.

Its ability to perform distributed scans and correlate results makes it particularly useful for mapping vast and segmented networks. Unicornscan’s advantages lie in its ability to manage packet parallelism effectively and provide detailed forensic data correlations post-scan.

Zmap

Zmap, another speed-oriented scanner, is optimized for conducting internet-wide scans. It uses IP randomization to enhance stealth and can scan the entire IPv4 address space remarkably quickly.

Example command for internet-wide scan:

zmap -p 80 -o results.csv

Zmap’s configuration for tuning performance and avoiding detection by employing random IP selection emphasizes the need for ethical considerations and proper configuration to prevent unnecessary network disruptions.

Hping

Hping allows for crafting custom TCP/IP packets, making it excellent for advanced probing and evading firewall rules while scanning ports.

Example command for a SYN scan using Hping:

hping3 -S -p [port] [target]

Hping is useful for simulating various types of network attacks and conducting fine-tuned analysis on how networks and firewalls respond to non-standard packet configurations. It offers a unique layer of customization that complements structured scans from other tools.

Understanding these techniques and tools to identify open ports is a critical component of any penetration testing endeavor, facilitating a strategic approach that aligns with specific operational goals and targets network security measures.