A hacker named Jack is trying to compromise a bank's computer system. He needs to know the operating system of that computer to launch further attacks. What process would help him?

Banner grabbing connects to common service ports such as 21 (FTP), 22 (SSH), or 80 (HTTP) and captures the text banner the service returns upon connection, which frequently includes the OS name, version, and application details. This gives an attacker like Jack reliable OS identification without sending aggressive or unusual probes. It is one of the simplest and most widely used techniques in the information gathering phase of an attack.

IDLE/IPID scanning is a stealth port discovery method using a zombie host to infer open ports, not a technique for identifying the target operating system.

SSDP scanning targets UPnP-enabled devices for network service discovery and does not reliably disclose the underlying OS.

UDP scanning probes for open UDP ports but does not retrieve banner information that identifies the operating system.