CEH Footprinting, Reconnaissance and Scanning Practice Question
Which TWO of the following describe the state of a port when Nmap reports it as 'filtered'? (Select 2)
⚠ Common exam trap
Many candidates confuse 'filtered' with 'closed' or 'open', not realizing that 'filtered' specifically indicates a firewall or filter is interfering with the probe, rather than the port's actual listening state.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A firewall is dropping the probe packets
Nmap reports a port as 'filtered' when it cannot determine whether the port is open or closed because probe packets are being dropped or blocked. Option C is correct because a firewall dropping probe packets (e.g., via a DROP rule in iptables or an ACL) prevents Nmap from receiving any response, leading to a 'filtered' state. Option D is correct because a firewall responding with ICMP unreachable errors (e.g., ICMP Type 3 Code 13, 'Communication Administratively Prohibited') also causes Nmap to classify the port as 'filtered', as the response indicates a filtering device is interfering.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The port is closed and the target sends a RST packet
Why it's wrong here
When a target port is closed, the operating system's TCP/IP stack typically responds to a SYN probe with a TCP RST/ACK packet. This explicit reset response signals to the scanner that the port is definitively not listening for connections. Nmap interprets this RST as an "unfiltered" or "closed" state, indicating that no firewall is actively blocking the probe, rather than a "filtered" state where the probe's fate is unknown.
- ✗
The port is open and a service is listening
Why it's wrong here
If a port is open and a service is actively listening, a standard TCP SYN probe will elicit a SYN/ACK response from the target. This three-way handshake initiation confirms the port's availability and readiness to accept connections. Nmap unequivocally identifies such a response as an "open" state, as the probe successfully reached the service and received a positive acknowledgment, indicating no filtering.
- ✓
A firewall is dropping the probe packets
Why this is correct
When a firewall is configured to silently drop probe packets, the scanning host receives no response whatsoever within the timeout period. This lack of any acknowledgment, reset, or error message prevents Nmap from determining the true state of the port behind the firewall. Consequently, Nmap marks the port as "filtered," indicating that the probes are being intercepted or discarded by an intermediary device, making the port's actual open or closed status ambiguous.
- ✓
A firewall is responding with ICMP unreachable errors
Why this is correct
If a firewall or an intermediate router explicitly rejects a probe by sending an ICMP Destination Unreachable message, such as "Port Unreachable" (Type 3, Code 3) or "Host Unreachable," Nmap interprets this as an indication of filtering. While a response is received, it signifies that the probe did not reach the intended application layer on the target host, but was instead blocked or rejected by a network device. This prevents Nmap from confirming if the port is truly open or closed, leading to a "filtered" state.
- ✗
The port is open but no service is responding
Why it's wrong here
If a port is technically open at the operating system level but no application service is bound to it or responding, the behavior can vary. Often, the OS might still send a RST packet if it receives a SYN for a port without a listener, leading to a "closed" state. However, if the OS simply drops the packet without responding, Nmap would mark it as "filtered" due to a timeout, or potentially "open|filtered" if other scan types yield ambiguous results, but not definitively "filtered" as the sole reason for the port being filtered.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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