Network Mapper (Nmap) is an open-source network analysis and security auditing tool written in C, C++, Python, and Lua. It is designed to scan networks and identify which hosts are available on the network using raw packets, and services and applications, including the name and version, where possible.
- Host discovery
- Port scanning
- Service enumeration and detection
- OS detection
- Scriptable interaction with the target service (Nmap Scripting Engine)
The syntax for Nmap is fairly simple and looks like this:
nmap <scan types> <options> <target>Nmap offers many different scanning techniques, making different types of connections and using differently structured packets to send. Here we can see all the scanning techniques Nmap offers:
nmap --help
<SNIP>
SCAN TECHNIQUES:
-sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans
-sU: UDP Scan
-sN/sF/sX: TCP Null, FIN, and Xmas scans
--scanflags <flags>: Customize TCP scan flags
-sI <zombie host[:probeport]>: Idle scan
-sY/sZ: SCTP INIT/COOKIE-ECHO scans
-sO: IP protocol scan
-b <FTP relay host>: FTP bounce scan
<SNIP>sudo nmap 10.129.2.0/24 -sn -oA tnet | grep for | cut -d" " -f5| Scanning Options | Description |
|---|---|
10.129.2.0/24 | Target network range. |
-sn | Disables port scanning. |
-oA tnet | Stores the results in all formats starting with the name ‘tnet’. |
This scanning method works only if the firewalls of the hosts allow it.
List could look something like this:
cat hosts.lst
10.129.2.4
10.129.2.10
10.129.2.11
10.129.2.18
10.129.2.19
10.129.2.20
10.129.2.28If we use the same scanning technique on the predefined list, the command will look like this:
sudo nmap -sn -oA tnet -iL hosts.lst | grep for | cut -d" " -f5| Scanning Options | Description |
|---|---|
-sn | Disables port scanning. |
-oA tnet | Stores the results in all formats starting with the name ‘tnet’. |
-iL | Performs defined scans against targets in provided ‘hosts.lst’ list. |
In this example, we see that only 3 of 7 hosts are active. Remember, this may mean that the other hosts ignore the default ICMP echo requests because of their firewall configurations. Since Nmap does not receive a response, it marks those hosts as inactive.
It can also happen that we only need to scan a small part of a network. An alternative to the method we used last time is to specify multiple IP addresses.
sudo nmap -sn -oA tnet 10.129.2.18 10.129.2.19 10.129.2.20| grep for | cut -d" " -f5If these IP addresses are next to each other, we can also define the range in the respective octet.
sudo nmap -sn -oA tnet 10.129.2.18-20| grep for | cut -d" " -f5Before we scan a single host for open ports and its services, we first have to determine if it is alive or not.
sudo nmap 10.129.2.18 -sn -oA host | Scanning Options | Description |
|---|---|
10.129.2.18 | Performs defined scans against the target. |
-sn | Disables port scanning. |
-oA host | Stores the results in all formats starting with the name ‘host’.Stores the results in all formats starting with the name ‘host’. |
If we disable port scan (-sn), Nmap automatically ping scan with ICMP Echo Requests (-PE). Once such a request is sent, we usually expect an ICMP reply if the pinging host is alive. The more interesting fact is that our previous scans did not do that because before Nmap could send an ICMP echo request, it would send an ARP ping resulting in an ARP reply. We can confirm this with the ”--packet-trace” option. To ensure that ICMP echo requests are sent, we also define the option (-PE) for this.
sudo nmap 10.129.2.18 -sn -oA host -PE --packet-trace| Scanning Options | Description |
|---|---|
10.129.2.18 | Performs defined scans against the target. |
-sn | Disables port scanning. |
-oA host | Stores the results in all formats starting with the name ‘host’.Stores the results in all formats starting with the name ‘host’. |
-PE | Performs the ping scan by using ‘ICMP Echo requests’ against the target. |
--packet-trace | Shows all packets sent and received |
Another way to determine why Nmap has our target marked as ”alive” is with the ”--reason” option.
sudo nmap 10.129.2.18 -sn -oA host -PE --reason | Scanning Options | Description |
|---|---|
10.129.2.18 | Performs defined scans against the target. |
-sn | Disables port scanning. |
-oA host | Stores the results in all formats starting with the name ‘host’.Stores the results in all formats starting with the name ‘host’. |
-PE | Performs the ping scan by using ‘ICMP Echo requests’ against the target. |
--reason | Displays the reason for specific result. |
We see here that Nmap does indeed detect whether the host is alive or not through the ARP request and ARP reply alone. To disable ARP requests and scan our target with the desired ICMP echo requests, we can disable ARP pings by setting the ”--disable-arp-ping” option. Then we can scan our target again and look at the packets sent and received.
More strategies about host discovery can be found at:
https://nmap.org/book/host-discovery-strategies.html
sudo nmap 10.129.2.28 --top-ports=10 -sS SYN scan
-p <port> define port to scan
--top-ports=10 scan top 10 ports
-p- all port
-F top 100 ports
sudo nmap 10.129.2.28 -p 21 --packet-trace -Pn -n --disable-arp-ping--packet-trace Shows all packets sent and received.
-n Disables DNS resolution.
--disable-arp-ping Disables ARP ping.
The Nmap TCP Connect Scan (-sT) uses the TCP three-way handshake to determine if a specific port on a target host is open or closed. The scan sends an SYN packet to the target port and waits for a response. It is considered open if the target port responds with an SYN-ACK packet and closed if it responds with an RST packet.
sudo nmap 10.129.2.28 -p 443 --packet-trace --disable-arp-ping -Pn -n --reason -sTLet us look at an example where the firewall drops the TCP packets we send for the port scan. Therefore we scan the TCP port 139, which was already shown as filtered. To be able to track how our sent packets are handled, we deactivate the ICMP echo requests (-Pn), DNS resolution (-n), and ARP ping scan (--disable-arp-ping) again.
sudo nmap 10.129.2.28 -p 139 --packet-trace -n --disable-arp-ping -Pn
Starting Nmap 7.80 ( https://nmap.org ) at 2020-06-15 15:45 CEST
SENT (0.0381s) TCP 10.10.14.2:60277 > 10.129.2.28:139 S ttl=47 id=14523 iplen=44 seq=4175236769 win=1024 <mss 1460>
SENT (1.0411s) TCP 10.10.14.2:60278 > 10.129.2.28:139 S ttl=45 id=7372 iplen=44 seq=4175171232 win=1024 <mss 1460>
Nmap scan report for 10.129.2.28
Host is up.
PORT STATE SERVICE
139/tcp filtered netbios-ssn
MAC Address: DE:AD:00:00:BE:EF (Intel Corporate)
Nmap done: 1 IP address (1 host up) scanned in 2.06 secondsWe see in the last scan that Nmap sent two TCP packets with the SYN flag. By the duration (2.06s) of the scan, we can recognize that it took much longer than the previous ones (~0.05s). The case is different if the firewall rejects the packets. For this, we look at TCP port 445, which is handled accordingly by such a rule of the firewall.
sudo nmap 10.129.2.28 -p 445 --packet-trace -n --disable-arp-ping -Pn
Starting Nmap 7.80 ( https://nmap.org ) at 2020-06-15 15:55 CEST
SENT (0.0388s) TCP 10.129.2.28:52472 > 10.129.2.28:445 S ttl=49 id=21763 iplen=44 seq=1418633433 win=1024 <mss 1460>
RCVD (0.0487s) ICMP [10.129.2.28 > 10.129.2.28 Port 445 unreachable (type=3/code=3) ] IP [ttl=64 id=20998 iplen=72 ]
Nmap scan report for 10.129.2.28
Host is up (0.0099s latency).
PORT STATE SERVICE
445/tcp filtered microsoft-ds
MAC Address: DE:AD:00:00:BE:EF (Intel Corporate)
Nmap done: 1 IP address (1 host up) scanned in 0.05 secondsAs a response, we receive an ICMP reply with type 3 and error code 3, which indicates that the desired host is unreachable. Nevertheless, if we know that the host is alive, we can strongly assume that the firewall on this port is rejecting the packets, and we will have to take a closer look at this port later.
Some system administrators sometimes forget to filter the UDP ports in addition to the TCP ones. Since UDP is a stateless protocol and does not require a three-way handshake like TCP. We do not receive any acknowledgment. Consequently, the timeout is much longer, making the whole UDP scan (-sU) much slower than the TCP scan (-sS).
sudo nmap 10.129.2.28 -F -sUAnother disadvantage of this is that we often do not get a response back because Nmap sends empty datagrams to the scanned UDP ports, and we do not receive any response. So we cannot determine if the UDP packet has arrived at all or not. If the UDP port is open, we only get a response if the application is configured to do so.
sudo nmap 10.129.2.28 -sU -Pn -n --disable-arp-ping --packet-trace -p 137 --reason
Starting Nmap 7.80 ( https://nmap.org ) at 2020-06-15 16:15 CEST
SENT (0.0367s) UDP 10.10.14.2:55478 > 10.129.2.28:137 ttl=57 id=9122 iplen=78
RCVD (0.0398s) UDP 10.129.2.28:137 > 10.10.14.2:55478 ttl=64 id=13222 iplen=257
Nmap scan report for 10.129.2.28
Host is up, received user-set (0.0031s latency).
PORT STATE SERVICE REASON
137/udp open netbios-ns udp-response ttl 64
MAC Address: DE:AD:00:00:BE:EF (Intel Corporate)
Nmap done: 1 IP address (1 host up) scanned in 0.04 secondsIf we get an ICMP response with error code 3 (port unreachable), we know that the port is indeed closed.
sudo nmap 10.129.2.28 -sU -Pn -n --disable-arp-ping --packet-trace -p 100 --reason
Starting Nmap 7.80 ( https://nmap.org ) at 2020-06-15 16:25 CEST
SENT (0.0445s) UDP 10.10.14.2:63825 > 10.129.2.28:100 ttl=57 id=29925 iplen=28
RCVD (0.1498s) ICMP [10.129.2.28 > 10.10.14.2 Port unreachable (type=3/code=3) ] IP [ttl=64 id=11903 iplen=56 ]
Nmap scan report for 10.129.2.28
Host is up, received user-set (0.11s latency).
PORT STATE SERVICE REASON
100/udp closed unknown port-unreach ttl 64
MAC Address: DE:AD:00:00:BE:EF (Intel Corporate)
Nmap done: 1 IP address (1 host up) scanned in 0.15 secondsFor all other ICMP responses, the scanned ports are marked as (open|filtered).
sudo nmap 10.129.2.28 -sU -Pn -n --disable-arp-ping --packet-trace -p 138 --reason
Starting Nmap 7.80 ( https://nmap.org ) at 2020-06-15 16:32 CEST
SENT (0.0380s) UDP 10.10.14.2:52341 > 10.129.2.28:138 ttl=50 id=65159 iplen=28
SENT (1.0392s) UDP 10.10.14.2:52342 > 10.129.2.28:138 ttl=40 id=24444 iplen=28
Nmap scan report for 10.129.2.28
Host is up, received user-set.
PORT STATE SERVICE REASON
138/udp open|filtered netbios-dgm no-response
MAC Address: DE:AD:00:00:BE:EF (Intel Corporate)
Nmap done: 1 IP address (1 host up) scanned in 2.06 secondsAnother handy method for scanning ports is the -sV option which is used to get additional available information from the open ports. This method can identify versions, service names, and details about our target.
sudo nmap 10.129.2.28 -Pn -n --disable-arp-ping --packet-trace -p 445 --reason -sV
Starting Nmap 7.80 ( https://nmap.org ) at 2022-11-04 11:10 GMT
SENT (0.3426s) TCP 10.10.14.2:44641 > 10.129.2.28:445 S ttl=55 id=43401 iplen=44 seq=3589068008 win=1024 <mss 1460>
RCVD (0.3556s) TCP 10.129.2.28:445 > 10.10.14.2:44641 SA ttl=63 id=0 iplen=44 seq=2881527699 win=29200 <mss 1337>
NSOCK INFO [0.4980s] nsock_iod_new2(): nsock_iod_new (IOD #1)
NSOCK INFO [0.4980s] nsock_connect_tcp(): TCP connection requested to 10.129.2.28:445 (IOD #1) EID 8
NSOCK INFO [0.5130s] nsock_trace_handler_callback(): Callback: CONNECT SUCCESS for EID 8 [10.129.2.28:445]
Service scan sending probe NULL to 10.129.2.28:445 (tcp)
NSOCK INFO [0.5130s] nsock_read(): Read request from IOD #1 [10.129.2.28:445] (timeout: 6000ms) EID 18
NSOCK INFO [6.5190s] nsock_trace_handler_callback(): Callback: READ TIMEOUT for EID 18 [10.129.2.28:445]
Service scan sending probe SMBProgNeg to 10.129.2.28:445 (tcp)
NSOCK INFO [6.5190s] nsock_write(): Write request for 168 bytes to IOD #1 EID 27 [10.129.2.28:445]
NSOCK INFO [6.5190s] nsock_read(): Read request from IOD #1 [10.129.2.28:445] (timeout: 5000ms) EID 34
NSOCK INFO [6.5190s] nsock_trace_handler_callback(): Callback: WRITE SUCCESS for EID 27 [10.129.2.28:445]
NSOCK INFO [6.5320s] nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 34 [10.129.2.28:445] (135 bytes)
Service scan match (Probe SMBProgNeg matched with SMBProgNeg line 13836): 10.129.2.28:445 is netbios-ssn. Version: |Samba smbd|3.X - 4.X|workgroup: WORKGROUP|
NSOCK INFO [6.5320s] nsock_iod_delete(): nsock_iod_delete (IOD #1)
Nmap scan report for 10.129.2.28
Host is up, received user-set (0.013s latency).
PORT STATE SERVICE REASON VERSION
445/tcp open netbios-ssn syn-ack ttl 63 Samba smbd 3.X - 4.X (workgroup: WORKGROUP)
Service Info: Host: Ubuntu
Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 6.55 secondssudo nmap 10.129.2.28 -Pn -n --disable-arp-ping --packet-trace -p 445 --reason -sV
Starting Nmap 7.80 ( https://nmap.org ) at 2022-11-04 11:10 GMT
SENT (0.3426s) TCP 10.10.14.2:44641 > 10.129.2.28:445 S ttl=55 id=43401 iplen=44 seq=3589068008 win=1024 <mss 1460>
RCVD (0.3556s) TCP 10.129.2.28:445 > 10.10.14.2:44641 SA ttl=63 id=0 iplen=44 seq=2881527699 win=29200 <mss 1337>
NSOCK INFO [0.4980s] nsock_iod_new2(): nsock_iod_new (IOD #1)
NSOCK INFO [0.4980s] nsock_connect_tcp(): TCP connection requested to 10.129.2.28:445 (IOD #1) EID 8
NSOCK INFO [0.5130s] nsock_trace_handler_callback(): Callback: CONNECT SUCCESS for EID 8 [10.129.2.28:445]
Service scan sending probe NULL to 10.129.2.28:445 (tcp)
NSOCK INFO [0.5130s] nsock_read(): Read request from IOD #1 [10.129.2.28:445] (timeout: 6000ms) EID 18
NSOCK INFO [6.5190s] nsock_trace_handler_callback(): Callback: READ TIMEOUT for EID 18 [10.129.2.28:445]
Service scan sending probe SMBProgNeg to 10.129.2.28:445 (tcp)
NSOCK INFO [6.5190s] nsock_write(): Write request for 168 bytes to IOD #1 EID 27 [10.129.2.28:445]
NSOCK INFO [6.5190s] nsock_read(): Read request from IOD #1 [10.129.2.28:445] (timeout: 5000ms) EID 34
NSOCK INFO [6.5190s] nsock_trace_handler_callback(): Callback: WRITE SUCCESS for EID 27 [10.129.2.28:445]
NSOCK INFO [6.5320s] nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 34 [10.129.2.28:445] (135 bytes)
Service scan match (Probe SMBProgNeg matched with SMBProgNeg line 13836): 10.129.2.28:445 is netbios-ssn. Version: |Samba smbd|3.X - 4.X|workgroup: WORKGROUP|
NSOCK INFO [6.5320s] nsock_iod_delete(): nsock_iod_delete (IOD #1)
Nmap scan report for 10.129.2.28
Host is up, received user-set (0.013s latency).
PORT STATE SERVICE REASON VERSION
445/tcp open netbios-ssn syn-ack ttl 63 Samba smbd 3.X - 4.X (workgroup: WORKGROUP)
Service Info: Host: Ubuntu
Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 6.55 secondsNmap can save the results in 3 different formats.
- Normal output (
-oN) with the.nmapfile extension - Grepable output (
-oG) with the.gnmapfile extension - XML output (
-oX) with the.xmlfile
We can also specify the option (-oA) to save the results in all formats. The command could look like this:
sudo nmap 10.129.2.28 -p- -oA target-oA target => Saves the results in all formats, starting the name of each file with ‘target’.
With the XML output, we can easily create HTML reports that are easy to read, even for non-technical people.
To convert the stored results from XML format to HTML, we can use the tool xsltproc.
xsltproc target.xml -o target.htmlA full port scan takes quite a long time. To view the scan status, we can press the [Space Bar] during the scan, which will cause Nmap to show us the scan status.
sudo nmap 10.129.2.28 -p- -sVAnother option (--stats-every=5s) that we can use is defining how periods of time the status should be shown. Here we can specify the number of seconds (s) or minutes (m), after which we want to get the status.
sudo nmap 10.129.2.28 -p- -sV --stats-every=5s--stats-every=5s => Shows the progress of the scan every 5 seconds.
We can also increase the verbosity level (-v / -vv), which will show us the open ports directly when Nmap detects them.
sudo nmap 10.129.2.28 -p- -sV -v -v Increases the verbosity of the scan, which displays more detailed information.
Once the scan is complete, we will see all TCP ports with the corresponding service and their versions that are active on the system.
sudo nmap 10.129.2.28 -p- -sVPrimarily, Nmap looks at the banners of the scanned ports and prints them out. If it cannot identify versions through the banners, Nmap attempts to identify them through a signature-based matching system, but this significantly increases the scan’s duration. One disadvantage to Nmap’s presented results is that the automatic scan can miss some information because sometimes Nmap does not know how to handle it. Let us look at an example of this.
sudo nmap 10.129.2.28 -p- -sV -Pn -n --disable-arp-ping --packet-trace-p- => Scans all ports.
-sV => Performs service version detection on specified ports.
-Pn => Disables ICMP Echo requests.
-n => Disables DNS resolution.
--disable-arp-ping => Disables ARP ping.
--packet-trace => Shows all packets sent and received.
However, it can happen that some services do not immediately provide such information. It is also possible to remove or manipulate the banners from the respective services. If we manually connect to the SMTP server using nc, grab the banner, and intercept the network traffic using tcpdump, we can see what Nmap did not show us.
Tcpdump
sudo tcpdump -i eth0 host 10.10.14.2 and 10.129.2.28Nc
nc -nv 10.129.2.28 25Tcpdump - Intercepted Traffic
18:28:07.128564 IP 10.10.14.2.59618 > 10.129.2.28.smtp: Flags [S], seq 1798872233, win 65535, options [mss 1460,nop,wscale 6,nop,nop,TS val 331260178 ecr 0,sackOK,eol], length 0
18:28:07.255151 IP 10.129.2.28.smtp > 10.10.14.2.59618: Flags [S.], seq 1130574379, ack 1798872234, win 65160, options [mss 1460,sackOK,TS val 1800383922 ecr 331260178,nop,wscale 7], length 0
18:28:07.255281 IP 10.10.14.2.59618 > 10.129.2.28.smtp: Flags [.], ack 1, win 2058, options [nop,nop,TS val 331260304 ecr 1800383922], length 0
18:28:07.319306 IP 10.129.2.28.smtp > 10.10.14.2.59618: Flags [P.], seq 1:36, ack 1, win 510, options [nop,nop,TS val 1800383985 ecr 331260304], length 35: SMTP: 220 inlane ESMTP Postfix (Ubuntu)Nmap Scripting Engine (NSE) is another handy feature of Nmap. It provides us with the possibility to create scripts in Lua for interaction with certain services. There are a total of 14 categories into which these scripts can be divided:
| Category | Description |
|---|---|
auth | Determination of authentication credentials. |
brodcast | Scripts, which are used for host discovery by broadcasting and the discovered hosts, can be automatically added to the remaining scans. |
brute | Executes scripts that try to log in to the respective service by brute-forcing with credentials. |
default | Default scripts executed by using the -sC option. |
discovery | Evaluation of accessible services. |
dos | These scripts are used to check services for denial of service vulnerabilities and are used less as it harms the services. |
exploit | This category of scripts tries to exploit known vulnerabilities for the scanned port. |
external | Scripts that use external services for further processing. |
fuzzer | This uses scripts to identify vulnerabilities and unexpected packet handling by sending different fields, which can take much time. |
intrusive | Intrusive scripts that could negatively affect the target system. |
malware | Checks if some malware infects the target system. |
safe | Defensive scripts that do not perform intrusive and destructive access. |
version | Extension for service detection. |
vuln | Identification of specific vulnerabilities. |
sudo nmap <target> -sCsudo nmap <target> --script <category>sudo nmap <target> --script <script-name>,<script-name>,...sudo nmap 10.129.2.28 -p 25 --script banner,smtp-commands--script banner,smtp-commands => Uses specified NSE scripts.
sudo nmap 10.129.2.28 -p 80 -ANow let us move on to HTTP port 80 and see what information and vulnerabilities we can find using the vuln category from NSE.
sudo nmap 10.129.2.28 -p 80 -sV --script vulnScanning performance plays a significant role when we need to scan an extensive network or are dealing with low network bandwidth. We can use various options to tell Nmap how fast (-T <0-5>), with which frequency (--min-parallelism <number>), which timeouts (--max-rtt-timeout <time>) the test packets should have, how many packets should be sent simultaneously (--min-rate <number>), and with the number of retries (--max-retries <number>) for the scanned ports the targets should be scanned.
When Nmap sends a packet, it takes some time (Round-Trip-Time - RTT) to receive a response from the scanned port. Generally, Nmap starts with a high timeout (--min-RTT-timeout) of 100ms.
sudo nmap 10.129.2.0/24 -Fsudo nmap 10.129.2.0/24 -F --initial-rtt-timeout 50ms --max-rtt-timeout 100ms-F => Scans top 100 ports.
--initial-rtt-timeout 50ms => Sets the specified time value as initial RTT timeout.
--max-rtt-timeout 100ms => Sets the specified time value as maximum RTT timeout.
Another way to increase the scans’ speed is to specify the retry rate of the sent packets (--max-retries). The default value for the retry rate is 10, so if Nmap does not receive a response for a port, it will not send any more packets to the port and will be skipped.
sudo nmap 10.129.2.0/24 -F | grep "/tcp" | wc -lsudo nmap 10.129.2.0/24 -F --max-retries 0 | grep "/tcp" | wc -l--max-retries 0 => Sets the number of retries that will be performed during the scan.
During a white-box penetration test, we may get whitelisted for the security systems to check the systems in the network for vulnerabilities and not only test the protection measures. If we know the network bandwidth, we can work with the rate of packets sent, which significantly speeds up our scans with Nmap. When setting the minimum rate (--min-rate <number>) for sending packets, we tell Nmap to simultaneously send the specified number of packets. It will attempt to maintain the rate accordingly.
sudo nmap 10.129.2.0/24 -F -oN tnet.defaultsudo nmap 10.129.2.0/24 -F -oN tnet.minrate300 --min-rate 300-oN tnet.minrate 300 => Saves the results in normal formats, starting the specified file name.
--min-rate 300 => Sets the minimum number of packets to be sent per second.
cat tnet.default | grep "/tcp" | wc -lcat tnet.minrate300 | grep "/tcp" | wc -lBecause such settings cannot always be optimized manually, as in a black-box penetration test, Nmap offers six different timing templates (-T <0-5>) for us to use. These values (0-5) determine the aggressiveness of our scans.
-T 0/-T paranoid-T 1/-T sneaky-T 2/-T polite-T 3/-T normal-T 4/-T aggressive-T 5/-T insane
sudo nmap 10.129.2.0/24 -F -oN tnet.default sudo nmap 10.129.2.0/24 -F -oN tnet.T5 -T 5-oN tnet.T5 => Saves the results in normal formats, starting the specified file name.
-T 5 => Specifies the insane timing template.
cat tnet.default | grep "/tcp" | wc -lcat tnet.T5 | grep "/tcp" | wc -lNmap has tricks to sneak past firewalls and IDS / IPS. For example, it can send packets in tiny pieces or pretend to be someone else.
We already know that when a port is shown as filtered, it can have several reasons. In most cases, firewalls have certain rules set to handle specific connections. The packets can either be dropped, or rejected. The dropped packets are ignored, and no response is returned from the host.
This is different for rejected packets that are returned with an RST flag. These packets contain different types of ICMP error codes or contain nothing at all.
SYN-Scan
sudo nmap 10.129.2.28 -p 21,22,25 -sS -Pn -n --disable-arp-ping --packet-traceACK-Scan
sudo nmap 10.129.2.28 -p 21,22,25 -sA -Pn -n --disable-arp-ping --packet-trace-sS Performs SYN scan on specified ports.
-sA Performs ACK scan on specified ports.
-Pn Disables ICMP Echo requests.
-n Disables DNS resolution.
--disable-arp-ping Disables ARP ping.
--packet-trace Shows all packets sent and received.
sudo nmap 10.129.2.28 -n -Pn -p445 -OScan by Using Different Source IP
sudo nmap 10.129.2.28 -n -Pn -p 445 -O -S 10.129.2.200 -e tun0Unlike firewalls and their rules, the detection of IDS/IPS systems is much more difficult because these are passive traffic monitoring systems
IDS systems examine all connections between hosts. If the IDS finds packets containing the defined contents or specifications, the administrator is notified and takes appropriate action in the worst case.
IDS systems alone are usually there to help administrators detect potential attacks on their network. They can then decide how to handle such connections. We can trigger certain security measures from an administrator, for example, by aggressively scanning a single port and its service. Based on whether specific security measures are taken, we can detect if the network has some monitoring applications or not.
One method to determine whether such IPS system is present in the target network is to scan from a single host (VPS). If at any time this host is blocked and has no access to the target network, we know that the administrator has taken some security measures. Accordingly, we can continue our penetration test with another VPS.
Decoy scanning method (-D) is the right choice. With this method, Nmap generates various random IP addresses inserted into the IP header to disguise the origin of the packet sent.
RND value for specific number of IP address separated by a colon (:).
Scan by Using Decoys
sudo nmap 10.129.2.28 -p 80 -sS -Pn -n --disable-arp-ping --packet-trace -D RND:5-sS Performs SYN scan on specified ports.
-Pn Disables ICMP Echo requests.
-n Disables DNS resolution.
--disable-arp-ping Disables ARP ping.
--packet-trace Shows all packets sent and received.
-D RND:5 Generates five random IP addresses that indicates the source IP the connection comes from.
The spoofed packets are often filtered out by ISPs and routers, even though they come from the same network range. Therefore, we can also specify our VPS servers’ IP addresses and use them in combination with “IP ID” manipulation in the IP headers to scan the target.
Another scenario would be that only individual subnets would not have access to the server’s specific services. So we can also manually specify the source IP address (-S) to test if we get better results with this one. Decoys can be used for SYN, ACK, ICMP scans, and OS detection scans.
sudo nmap 10.129.2.28 -n -Pn -p 445 -O -S 10.129.2.200 -e tun0By default, Nmap performs a reverse DNS resolution unless otherwise specified to find more important information about our target. These DNS queries are also passed in most cases because the given web server is supposed to be found and visited. The DNS queries are made over the UDP port 53. The TCP port 53 was previously only used for the so-called ”Zone transfers” between the DNS servers or data transfer larger than 512 bytes. More and more, this is changing due to IPv6 and DNSSEC expansions. These changes cause many DNS requests to be made via TCP port 53.
However, Nmap still gives us a way to specify DNS servers ourselves (--dns-server <ns>,<ns>). This method could be fundamental to us if we are in a demilitarized zone (DMZ). The company’s DNS servers are usually more trusted than those from the Internet. So, for example, we could use them to interact with the hosts of the internal network. As another example, we can use TCP port 53 as a source port (--source-port) for our scans. If the administrator uses the firewall to control this port and does not filter IDS/IPS properly, our TCP packets will be trusted and passed through.
SYN-Scan of a Filtered Port
sudo nmap 10.129.2.28 -p50000 -sS -Pn -n --disable-arp-ping --packet-traceSYN-Scan From DNS Port
sudo nmap 10.129.2.28 -p50000 -sS -Pn -n --disable-arp-ping --packet-trace --source-port 53Now that we have found out that the firewall accepts TCP port 53, it is very likely that IDS/IPS filters might also be configured much weaker than others. We can test this by trying to connect to this port by using Netcat.
Connect To The Filtered Port
ncat -nv --source-port 53 10.129.2.28 50000