2 Comments on “What is the proper response for a NULL scan if the port is closed?”

1. Tyler says:

   August 19, 2015 at 7:57 pm

   RST does make sense as a response by any machine when a null scan is sent to it but shouldn’t it be accompanied with an ACK as well?

2. Dinghus says:

   September 9, 2015 at 7:38 pm

   An attacker uses a TCP NULL scan to determine if ports are closed on the target machine. This scan type is accomplished by sending TCP segments with no flags in the packet header, generating packets that are illegal based on RFC 793. The RFC 793 expected behavior is that any TCP segment with an out-of-state Flag sent to an open port is discarded, whereas segments with out-of-state flags sent to closed ports should be handled with a RST in response. This behavior should allow an attacker to scan for closed ports by sending certain types of rule-breaking packets (out of sync or disallowed by the TCB) and detect closed ports via RST packets. he major advantage of this scan type is its ability to scan through stateless firewall or ACL filters. Such filters are configured to block access to ports usually by preventing SYN packets, thus stopping any attempt to ‘build’ a connection. NULL packets, like out-of-state FIN or ACK packets, tend to pass through such devices undetected. Many operating systems, however, do not implement RFC 793 exactly and for this reason NULL scans do not work as expected against these devices. Some operating systems, like Microsoft Windows, send a RST packet in response to any out-of-sync (or malformed) TCP segments received by a listening socket (rather than dropping the packet via RFC 793), thus preventing an attacker from distinguishing between open and closed ports.

   1. Speed: TCP NULL scanning is fast compared to other types of scans
   2. Stealth: TCP NULL scanning was once stealthy, but is now easily detected by IDS/IPS systems
   3. Open Port: Detects an open port via no response to the segment
   4. Closed Port: Detects that a closed via a RST received in response to the FIN
   5. Filtered Port: Cannot distinguish between a filtered port and an open port
   6. Unfiltered Port: Cannot distinguish between an unfiltered port and a non-stateful filtered port

   NULL scans are limited by the range of platforms against which they work. Additionally, because open ports are inferred via no responses being generated, one cannot distinguish an open port from a filtered port without further analysis. For instance, NULL scanning a system protected by a stateful firewall may indicate all ports being open. Because of their obvious rule-breaking nature, NULL scans are flagged by almost all intrusion prevention or intrusion detection systems.