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NMAP Trivia ANSWERS: Mastering Network Mapping and Scanning

Three weeks ago we published the NMAP Trivia challenge. Thanks to all ISC readers that submitted their responses! A special mention goes to the winning entry from Jason DePriest, an extensive and elaborated submission, available here. Congratulations! The prize (technical book) is on his way! ;)

Jon Kibler provided an in-progress nmap idea for a new features, a scan proxy engine equivalent to the FTP bounce scan to scan through HTTP or SOCKS.

Now... it is time for the answers:

1. What are the default target ports used by the current nmap version (4.76)? How can you change the target ports list? What (nmap) options can be used to speed up scans by reducing the number of target ports and still check (potentially) the most relevant ones? How can you force nmap to check all target ports?

Fyodor performed a thorough port scan research this last summer to identify the most common ports available on the Internet [1]. The current nmap version scans by default the 1000 most popular ports. The popularity of each port is coded inside the nmap-services configuration file (by default under /usr/local/share/nmap).

unknown 4/tcp   0.000477
rje     5/udp   0.000593        # Remote Job Entry
unknown 6/tcp   0.000502
echo    7/tcp   0.004855
echo    7/udp   0.024679
unknown 8/tcp   0.000013

Nmap provides an option for quick scans, "-F". It scans the 100 most popular ports, reducing the default load in one order of magnitude. Additionally, you can decide how many popular ports you want to scan through the "--top-ports N" option, where "N" is the top number of ports.

# ./nmap -F

Starting Nmap 4.76 ( ) at 2009-01-21 10:44 GMT
Interesting ports on (
Not shown: 95 filtered ports
22/tcp  open   ssh
25/tcp  closed smtp
53/tcp  open   domain
80/tcp  open   http
113/tcp closed auth

Nmap done: 1 IP address (1 host up) scanned in 4.04 seconds

# ./nmap --top-ports 5

Starting Nmap 4.76 ( ) at 2009-01-21 10:44 GMT
Interesting ports on (
21/tcp  filtered ftp
22/tcp  open     ssh
23/tcp  filtered telnet
80/tcp  open     http
443/tcp filtered https

Nmap done: 1 IP address (1 host up) scanned in 8.56 seconds

Finally, nmap allows you to define the specific set of ports to scan through the "-p" option, as in "-pT:22,80,443,U:53,69,514". All ports, including port 0, can be scanned by providing the "-p0-" option, meaning from 0 till the end of the range, that is, port 65535. You need to specify if they are TCP or UDP ports, or both ("-sSU").

# nmap -p0-


2. How can you force nmap to scan a specific list of 200 target ports, only relevant to you?

If you don't want to scan the most popular ports, you can tell nmap what particular list of ports to scan by specifying them with the "-p" option, one by one or in ranges, like in "-p 20-23,25,80,443". Because this can be too tedious for long lists of ports, the recommended way is to copy and edit the "nmap-services" file and create a custom version containing your list of interesting ports. The new custom file can be referenced using the "--servicedb" (for individual files) or "--datadir" (for the configuration files directory) options, as in:

# nmap --datadir ./myconfig

If your custom file contains more than 200 target services, then you can use the "--top-ports 200" option again. The specific file and directory search order followed by nmap is detailed on page 370 of the  nmap book:

3. What is the default port used by nmap for UDP ping discovery (-PU)? Why? If you don't know it from the top of your head ;), how can you easily identify this port without using other tools (such as a sniffer) or inspecting nmap's source code?

By default, nmap sends an empty UDP packet to port UDP/31338 for the UDP ping discovery method ("-PU"). The reason is that there is a high chance this random high port is closed. This is the preferred state expected by nmap trying to elicit an ICMP port unreachable packet in return and, as a result, identify the existence of a new host. The port number is defined in nmap.h, specifically in the DEFAULT_UDP_PROBE_PORT_SPEC constant. Did you notice it is 31337 plus 1, the elite port (31337 in haxor speech) plus one.

Currently, nmap provides the "--packet-trace" option to gather detailed information about the network traffic and individual packets sent and received during its operations. Effectively, this option acts as a built in sniffer, very useful to get details about what nmap is doing on the backstage.

# nmap -PU --packet-trace

Starting Nmap 4.76 ( ) at 2009-01-21 10:58 GMT
SENT (0.6580s) UDP > ttl=58 id=45958 iplen=28
SENT (1.6560s) UDP > ttl=59 id=46599 iplen=28
Note: Host seems down. If it is really up, but blocking our ping probes, try -PN
Nmap done: 1 IP address (0 hosts up) scanned in 2.68 seconds

4. When nmap is run, sometimes it is difficult to know what is going on the backstage. What two (nmap) options allow you to gather detailed but not overwhelming information about nmap's port scanning operations? What other extra (nmap) options are available for ultra detailed information?

The first of the options has been mentioned and used on the previous question, "--packet-trace". It allows to get a tcpdump-like output about packets sent and received. Additionally, nmap provides the "--reason" option to display the reason why a port has been clasiffied on an specific state: open, closed, filtered, etc.

# nmap -F -sSU --reason

Starting Nmap 4.76 ( ) at 2009-01-21 11:00 GMT
Interesting ports on (
Not shown: 99 open|filtered ports, 96 filtered ports
Reason: 194 no-responses and 1 admin-prohibited
22/tcp  open   ssh     syn-ack
25/tcp  closed smtp    reset
53/tcp  open   domain  syn-ack
80/tcp  open   http    syn-ack
113/tcp closed auth    reset

Nmap done: 1 IP address (1 host up) scanned in 7.95 seconds

# nmap -F -sU --reason

Starting Nmap 4.76 ( ) at 2009-01-21 11:02 GMT
Interesting ports on (
Not shown: 99 open|filtered ports
Reason: 99 no-responses
520/udp filtered route   admin-prohibited from

Nmap done: 1 IP address (1 host up) scanned in 15.90 seconds

For those interested on gathering as much information as possible about nmap's operations, the "-v" verbosity option, or the "-dN" debugging option are available. These options specify nmap to be verbose (multiple verbosity levels are allowed), or the nmap debug level for troubleshooting purposes, where N can have a value between 1 and 9. Be careful when you use it! Try it and be ready for a Matrix-like output 8-)

# nmap -p80 -sS -v

Starting Nmap 4.76 ( ) at 2009-01-21 11:07 GMT
Initiating Ping Scan at 11:07
Scanning [2 ports]
Completed Ping Scan at 11:07, 0.24s elapsed (1 total hosts)
Initiating Parallel DNS resolution of 1 host. at 11:07
Completed Parallel DNS resolution of 1 host. at 11:07, 0.24s elapsed
Initiating SYN Stealth Scan at 11:07
Scanning ( [1 port]
Discovered open port 80/tcp on
Completed SYN Stealth Scan at 11:07, 0.26s elapsed (1 total ports)
Host ( appears to be up ... good.
Interesting ports on (
80/tcp open  http

Read data files from: .
Nmap done: 1 IP address (1 host up) scanned in 6.13 seconds
           Raw packets sent: 3 (112B) | Rcvd: 2 (72B)

# nmap -p80 -sS -d1

Starting Nmap 4.76 ( ) at 2009-01-21 11:08 GMT
--------------- Timing report ---------------
Initiating Ping Scan at 11:08
Scanning [2 ports]
Nmap done: 1 IP address (1 host up) scanned in 0.74 seconds
           Raw packets sent: 3 (112B) | Rcvd: 2 (72B)

Try it by your own! ;)

5. What are the preferred (nmap) options to run a stealthy TCP port scan? Particularly, try to avoid detection from someone running a sniffer near the person running nmap and focus on the extra actions performed by the tool (assuming the packets required to complete the port scan are not detected)?

Most current network IDS can detect the default packets generated by nmap when port scanning a target. We are assuming here these cannot be detected, so a stealthier scan can be launched by using the "-n" option (not used in any of the Nmap Trivia examples), that is, disable all reverse DNS resolution at the nmap level. Most Unix-based security tools provide this same option for the same purpose.

# nmap -F -n

However, this way you lose the sometimes valuable DNS information. You can use the "--dns-servers" option to indicate the DNS recursive servers to use as DNS proxies when analyzing the target IP address.
More stealthier details on answer number 12.

6. Why port number 49152 is relevant to nmap?

Port 49152 is the first of the ephemeral ports for dynamic usage based on IANA. However, the port assignment depends on the implementation of your tools or operating system. See
- The Well Known Ports are those from 0 through 1023
- The Registered Ports are those from 1024 through 49151
- The Dynamic and/or Private Ports are those from 49152 through 65535

7. What is the only nmap TCP scan type that classifies the target ports as "unfiltered"? Why? What additional nmap scan type can be used to discern if those ports (previously identified as "unfiltered") are in an open or closed state?

The only nmap scan type that can show a port in the "unfiltered" state is the TCP ACK scan, "-sA" option. The reason is because this scan cannot differentiate between an open and closed port, as a target hosts (if unfiltered) will always reply with a RST packet. This is the standard behaviour for a closed port, and is also standar for an open port for which there is not a previously established connection to map the ACK packet to. Therefore, nmap's ACK scan cannot be considered a port scan, as it cannot differentiate between port states, but a host discovery scan.

The TCP Window scan, "-sW" option, is similar to the TCP ACK scan, but it can differentiate between open and closed ports is some scenarios.

8. When (and it what nmap version) the default state for a non-responsive UDP port was changed on nmap (from "open" to "open|filtered")? Why?

The default state for a non-responsive UDP port was changed (from "open" to "open|filtered") on nmap version v3.70 in 2004. The reason was accurancy, as extensive use of filtering devices by that time made filtered UDP ports always appear as open in previous nmap versions.

9. What is the default scan type used by nmap when none is specified, as in "nmap -T4"? Is this always the default scan method? If not, what other scan method does nmap default to, under what conditions, and why?

The current nmap version performs a TCP SYN scan ("-sS" option) by default when no scan type is specified. However, this is only the default behavior when nmap is launched as a privileged user (eg. root in Linux). The TCP connect scan, "-sT" option (connect() syscall), is used by default with non-privileged users as these cannot send raw packets (used by the SYN scan) or if there are IPv6 targets.

# ./nmap -PN -p80,81 --packet-trace

Starting Nmap 4.76 ( ) at 2009-01-21 11:22 GMT
SENT (0.3730s) TCP > S ttl=50 \
    id=8102 iplen=44  seq=1698869517 win=3072 <mss 1460>
SENT (0.3740s) TCP > S ttl=43 \
    id=48226 iplen=44  seq=1698869517 win=4096 <mss 1460>
RCVD (0.6120s) TCP > SA ttl=48 \
    id=0 iplen=44  seq=2849983456 win=5840 ack=1698869518 <mss 1452>
RCVD (1.9570s) TCP > SA ttl=48 \
    id=0 iplen=44  seq=2805666242 win=5840 ack=2103880733 <mss 1452>
SENT (2.5730s) TCP > S ttl=55 \
    id=14744 iplen=44  seq=1698935052 win=4096 <mss 1460>
Interesting ports on (
80/tcp open     http
81/tcp filtered hosts2-ns

Nmap done: 1 IP address (1 host up) scanned in 3.79 seconds

$ ./nmap -PN -p80,81 --packet-trace

Starting Nmap 4.76 ( ) at 2009-01-21 11:25 GMT
CONN (0.1290s) TCP localhost > => Operation now in progress
CONN (0.1290s) TCP localhost > => Operation now in progress
CONN (2.3510s) TCP localhost > => Operation now in progress
Interesting ports on (
80/tcp open     http
81/tcp filtered hosts2-ns

Nmap done: 1 IP address (1 host up) scanned in 3.57 seconds


10. What nmap features (can make or) make use of nmap's raw packet capabilities? What nmap features rely on the OS TCP/IP stack instead?

Nmap makes use of the raw packet capabilities by default, "--send-eth" option, as demonstrated in the previous question for some features, such as TCP and UDP port scans launched by privileged users (except for the connect scan and the FTP bounce scan), or fragmentation probes. Other features like the Nmap Scripting Engine and version detection relay on the OS TCP/IP stack.

11. Nmap's performance has been sometimes criticized versus other network scanners. What (nmap) options can you use to convert nmap into a faster, stateless scanner for high performance but less accurate results?

If the congestion controls and packet loss detection algorithms are omitted, a scanner will run faster. Nmap can achieve a similar behaviour as stateless scanners, no code to track and retransmit probes, using the following options:

# ./nmap --min-rate 1000 --max-retries 0 ...

These indicate nmap to send at least 1000 packets per second (if your system or wire can) and disable retransmission of timed-out probes. However, take into account the impact this might have in the accurancy of the results.

# ./nmap -PN -n --min-rate 1000 --max-retries 0 -F

Starting Nmap 4.76 ( ) at 2009-01-21 12:08 GMT
Warning: Giving up on port early because retransmission cap hit.
Interesting ports on
Not shown: 95 filtered ports
22/tcp  open   ssh
25/tcp  closed smtp
53/tcp  open   domain
80/tcp  open   http
113/tcp closed auth

Nmap done: 1 IP address (1 host up) scanned in 1.06 seconds

12. What relevant nmap feature does not allow an attacker to use the decoy functionality (-D) and might reveal his real IP address?

Apart from the previously mentioned "-n" option to run stealthier scans and avoid IDS detection, there are other related options, such as "--data-length", to change the default empty packet used for some probes, "--ttl" to modify the TTL on the sent packets, timing options ("-T"), "--randomize-hosts" to change the order the target hosts are scanned, or "-D" to launch a decoy scan (simulate the scan is coming from multiple hosts).

Decoys are used in the ping discovery, port scanning, and remote OS detection phases. However, this feature does not apply when DNS queries or service version detection ("-sV" or "-A") are used, being the source IP address disclosed.

13. What are the (nmap) options you can use to identify all the steps followed by nmap to fingerprint and identify the Web server version running on

# ./nmap -sSV -p80 --version-trace

Starting Nmap 4.76 ( ) at 2009-01-21 12:17 GMT
SCRIPT ENGINE: Initiating script scanning.
SCRIPT ENGINE: Script scanning (
SCRIPT ENGINE: Initialized 4 rules
SCRIPT ENGINE: Matching rules.
SCRIPT ENGINE: Running scripts.
SCRIPT ENGINE: Script scanning completed.
Scanned at 2009-01-21 12:17:57 GMT for 8s
Interesting ports on (
80/tcp open  http    Apache httpd 2.2.2 ((Fedora))
Final times for host: srtt: 238764 rttvar: 179294  to: 955940

Read from .: nmap-rpc nmap-service-probes nmap-services.
Service detection performed. Please report any incorrect results at .
Nmap done: 1 IP address (1 host up) scanned in 8.17 seconds

The "-sSV" option allows you to focus on a TCP scan type (SYN scan in this case, "-sS"), and fingerprint the service ("-sV"). In order to just target the web server (supposing HTTP (TCP/80) is the target port, and not HTTPS (TCP/443)), the "-p80" option must be used.

The "--version-trace" option is similar to the "--packet-trace" option, but instead of dumping the network traffic, it dumps all the actions or steps performed by nmap during the execution of the service fingerprinting modules. Additionally, other debug options ("-dN") can be added to gather further details.

14. As an attacker, what port number would you select to hide a listening service backdoor trying to avoid an accurate detection by nmap's default aggressive fingerprinting tests? Would it be TCP or UDP? Why? What additional (nmap) options do you need to specify as a defender to fingerprint the hidden service backdoor?

If a port in the range of TCP/9100-9107 is selected for a backdoor, due to the fact these are common ports for printer services, nmap won`t fingerprint the service. These ports are excluded by default on the service fingerprinting tests ("-sV") or aggressive scan options ("-A") trying to save the planet, trees and forests specifically, by not making printers dump dozens of pages full of nmap probes and garbage as a result of the stimulous received from the scan.

If you want to enable service fingerprinting on all ports, there are two options. The "--allports" option can be specified, as in "nmap -A --allports", or the nmap-service-probes file can be modified to enable these ports by removing the "Exclude" directive.

15. What is the language used to write NSE scripts, and what two other famous open-source security tools/projects currently use the same language?

Nmap uses the LUA ( programming language. LUA (pronounced LOO-ah) means "Moon" in Portuguese, or "Luna" in Spanish ;) Other famous open-source security tools, like Wireshark and Snort use LUA to extend their capabilities.

16. What Linux/Windows command can you use to identify the list of NSE scripts that belong to the "discovery" category and will execute when this set of scripts is selected with the "--script discovery" nmap option?

By default, NSE scripts are available under the "scripts" directory (however, nmap searched in other locations too: --datadir, $NAMPDIR, etc), with the ".nse" file extension. All NSE scripts belong to one or more categories, define inside the script, and indexed by the scripts/script.db database (if updated through the "--script-updatedb" option).

Therefore a couple of options to search for discovery scripts in Linux are:

# grep discovery scripts/*.nse
scripts/ASN.nse:categories = {"discovery", "external"}
scripts/HTTP_open_proxy.nse:categories = {"default", "discovery", "external", "intrusive"}
scripts/HTTPtrace.nse:categories = {"discovery"}

# grep discovery scripts/script.db
Entry{ category = "discovery", filename = "HTTPtrace.nse" }
Entry{ category = "discovery", filename = "rpcinfo.nse" }
Entry{ category = "discovery", filename = "SMTPcommands.nse" }

You can perform a similar search in Windows using the built-in search capabilities (searching by "A word or phrase in the file" to look inside the directory) or the find or findstr commands (to search within a file or set of files).

17. How can you know the specific arguments accepted by a specific NSE script, such as those accepted by the whois.nse script?

In order to identify the arguments that can be passed through the "--script-args" option to a NSE script, eg. whois.nse, check the documentation or code within the script file. If it is properly documented, search by "-- @args" to go to the arguments documentation section.

Finally, a couple of extra questions for the real nmap-lovers:

  1. How can you get in real-time the open ports discoverd by nmap before the final report is displayed?
  2. What happens when you run nmap in verbose mode on September 1?

That's all folks! Happy nmap discovery and scanning!

Raul Siles

Raul Siles

152 Posts
Jan 21st 2009

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