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This scan type is also known as "half-open scanning", because it never actually opens a full TCP connection. The port scanner generates a SYN packet. If the target port is open, it will respond with a SYN-ACK packet. The scanner host responds with an RST packet, closing the connection before the handshake is completed. [3]
The term half-open connection can also be used to describe an embryonic connection, i.e. a TCP connection that is in the process of being established. TCP has a three state system for opening a connection. First, the originating endpoint (A) sends a SYN packet to the destination (B). A is now in an embryonic state (specifically, SYN_SENT), and ...
An idle scan is a TCP port scan method for determining what services are open on a target computer [1] without leaving traces pointing back at oneself. This is accomplished by using packet spoofing to impersonate another computer (called a " zombie ") so that the target believes it's being accessed by the zombie.
The TCP checksum is a weak check by modern standards and is normally paired with a CRC integrity check at layer 2, below both TCP and IP, such as is used in PPP or the Ethernet frame. However, introduction of errors in packets between CRC-protected hops is common and the 16-bit TCP checksum catches most of these.
The machine listens for Address Resolution Protocol requests that go unanswered (indicating unused addresses), then replies to those requests, receives the initial SYN packet of the scanner and sends a SYN/ACK in response. It does not open a socket or prepare a connection, in fact it can forget all about the connection after sending the SYN/ACK.
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SYN scan has the advantage that the individual services never actually receive a connection while some services can be crashed with a connect scan. However, the RST during the handshake can cause problems for some network stacks, particularly simple devices like printers.
If you've been having trouble with any of the connections or words in Wednesday's puzzle, you're not alone and these hints should definitely help you out. Plus, I'll reveal the answers further ...