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?

When a client connects to a service such as FTP, SSH, HTTP, or Telnet, many servers respond with a banner that includes the software name, version, and often OS details. An attacker can capture these banners using tools like Netcat or Telnet to infer the underlying OS without sending crafted packets. This passive, service-level fingerprinting is specifically called banner grabbing and is a standard step in active reconnaissance.

IDLE/IPID scanning is a stealthy port scanning technique that uses a zombie host to obscure the attacker's identity and does not directly reveal OS information.

SSDP scanning targets Universal Plug and Play devices on a local network and is used for device discovery, not OS identification.

UDP scanning identifies open UDP ports and available services but does not inherently provide OS identification information.