Scan Types | Scanning

As a CEH, you need to be familiar with the following scan types and uses:
  • SYN A SYN or stealth scan is also called a half-open scan because it doesn't complete the TCP three-way handshake. (The TCP/IP three-way handshake will be covered in the next section.) A hacker sends a SYN packet to the target; if a SYN/ACK frame is received back, then it's assumed the target would complete the connect and the port is listening. If an RST is received back from the target, then it's assumed the port isn't active or is closed. The advantage of the SYN stealth scan is that fewer IDS systems log this as an attack or connection attempt.
  • XMAS XMAS scans send a packet with the FINURG, and PSH flags set. If the port is open, there is no response; but if the port is closed, the target responds with a RST/ACK packet. XMAS scans work only on target systems that follow the RFC 793 implementation of TCP/IP and don't work against any version of Windows.
  • FIN A FIN scan is similar to an XMAS scan but sends a packet with just the FIN flag set. FIN scans receive the same response and have the same limitations as XMAS scans.
  • NULL A NULL scan is also similar to XMAS and FIN in its limitations and response, but it just sends a packet with no flags set.
  • IDLE An IDLE scan uses a spoofed IP address to send a SYN packet to a target. Depending on the response, the port can be determined to be open or closed. IDLE scans determine port scan response by monitoring IP header sequence numbers.

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