UDP Port Scanning

Determining if a UDP port is active is more difficult because the scanner cannot rely on a SYN/ACK response; it often needs to provoke a reply from the listening service instead.

With many potential open ports and services responding only to specific data types, scanning all ports in a reasonable timeframe can be time-consuming and challenging.

Consider the following exchange where Eve tries to check if a UDP port is open:

Eve must use the correct protocol and ensure packets reach their destination without loss; otherwise, she may not detect an open port. Consequently, UDP scanning can be quite time-consuming when aiming to scan all ports.

Useful Nmap Scan Types and Options

While there are many scanners available, this section focuses on maximizing the potential of Nmap.

You can instruct Nmap to scan the most common ports using the –top-ports argument.

The scanner can attempt to identify the versions of the applications listening on a port, a process known as service scanning, which can be activated using the -sV flag.

Nmap includes a variety of built-in scripts aimed at targeting specific services and interacting with them. These scripts can perform various tasks, such as extracting information from the service or attempting to exploit it. You can activate the script scanner using the -sC flag, which allows only safe checks, ensuring that no denial of service or exploitation attempts are made.

Operating system detection can be performed by the scanner, enabling it to identify the running OS by analyzing various parameters to estimate the likelihood of the OS in use. This feature can be activated using the -O argument.

Nmap’s aggressive mode activates multiple flags simultaneously, allowing the scanner to perform version and OS detection, enable the script scanner, and scan the top 1,000 most common ports. This mode can be activated using the -A option.

Nmap can also scan IPv6 using all of the aforementioned flags by including the -6 option.