During your career as a cybersecurity professional, penetration tester, or ethical hacker, you'll often encounter organizations—be they small, medium, or large enterprises—that have some sort of firewall appliance or software on their network infrastructure.
Firewalls can prevent network scans and create a challenge for us as penetration testers. The following are various operators that can be used in Nmap to evade firewalls:
Nmap's firewall evasion operators
Additionally, we can send custom probes with specific flags to a target and analyze the responses.
In the following sections, we'll take a look at how to determine whether a stateful firewall is present on a network.