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Port Scanning

Port Scanning is one of the most popular reconnaissance techniques attackers use to discover services they can break into. All machines connected to a Local Area Network (LAN) or Internet run many services that listen at well-known and not so well known ports. A port scan helps the attacker find which ports are available (i.e., what service might be listing to a port). Essentially, a port scan consists of sending a message to each port, one at a time. The kind of response received indicates whether the port is used and can therefore be probed further for weakness. The various techniques used in a port scan are summarized below.

Fragmented packet Port Scan: The scanner splits the TCP header into several IP fragments. This bypasses some packet filter firewalls because they cannot see a complete TCP header that can match their filter rules. Some packet filters and firewalls do queue all IP fragments, but many networks cannot afford the performance loss caused by the queuing.

SYN scan: This technique is also called half-open scanning, because a TCP connection is not completed. A SYN packet is sent (as if we are going to open a connection), and the target host responds with a SYN+ACK, this indicates the port is listening, and an RST indicates a non- listener. The server process is never informed by the TCP layer because the connection did not complete. FIN scan: The typical TCP scan attempts to open connections (at least part way). Another technique sends erroneous packets at a port, expecting that open listening ports will send back different error messages than closed ports. The scanner sends a FIN packet, which should close a connection that is open. Closed ports reply to a FIN packet with a RST. Open ports, on the other hand, ignore the packet in question. This is required TCP behavior.

If no service is listening at the target port, the operating system will generate an error message. If a service is listening, the operating system will silently drop the incoming packet. Therefore, silence indicates the presence of a service at the port. However, since packets can be dropped accidentally on the wire or blocked by firewalls, this isn't a very effective scan. Other techniques that have been used consist of XMAS scans where all flags in the TCP packet are set, or NULL scans where none of the bits are set. However, different operating systems respond differently to these scans, and it becomes important to identify the OS and even its version and patch level.

Bounce Scan: The ability to hide their tracks is important to attackers. Therefore, attackers scour the Internet looking for systems they can bounce their attacks through. FTP bounce scanning takes advantage of a vulnerability of the FTP protocol itself. It requires support for proxy ftp connections. This bouncing through an FTP server hides where the attacker comes from. This technique is similar to IP spoofing in that it hides where the attacker comes from. For example, badwebsitexyz.com establishes a control connection to the FTP server-PI (protocol interpreter) of say, badspiderbites.com, then requests that the server-PI initiate an active server-DTP (data transfer process) to send a file anywhere on the Internet.