Introduction to Nmap and Port scanning

This post reviews usage of Nmap and demonstrates various use cases using Nmap command line options. This post uses practical scenarios covered in two TryHackMe rooms.

Room one

Room two

In another update, we also covered the answers for the below rooms:

Room answers with full writeup can be found here.

Intro to Port Scanning with Nmap

When it comes to hacking, knowledge is power. The more knowledge you have about a target system or network, the more options you have available. This makes it imperative that proper enumeration is carried out before any exploitation attempts are made.

Say we have been given an IP (or multiple IP addresses) to perform a security audit on. Before we do anything else, we need to get an idea of the “landscape” we are attacking. What this means is that we need to establish which services are running on the targets. For example, perhaps one of them is running a webserver, and another is acting as a Windows Active Directory Domain Controller. The first stage in establishing this “map” of the landscape is something called port scanning. When a computer runs a network service, it opens a networking construct called a “port” to receive the connection. Ports are necessary for making multiple network requests or having multiple services available. For example, when you load several webpages at once in a web browser, the program must have some way of determining which tab is loading which web page. This is done by establishing connections to the remote webservers using different ports on your local machine. Equally, if you want a server to be able to run more than one service (for example, perhaps you want your webserver to run both HTTP and HTTPS versions of the site), then you need some way to direct the traffic to the appropriate service. Once again, ports are the solution to this. Network connections are made between two ports — an open port listening on the server and a randomly selected port on your own computer. For example, when you connect to a web page, your computer may open port 49534 to connect to the server’s port 443.

Every computer has a total of 65535 available ports; however, many of these are registered as standard ports. For example, a HTTP Webservice can nearly always be found on port 80 of the server. A HTTPS Webservice can be found on port 443. Windows NETBIOS can be found on port 139 and SMB can be found on port 445. It is important to note; however, that especially in a CTF setting, it is not unheard of for even these standard ports to be altered, making it even more imperative that we perform appropriate enumeration on the target.

When port scanning with Nmap, there are three basic scan types. These are:

TCP Connect Scans (-sT)
SYN “Half-open” Scans (-sS)
UDP Scans (-sU)
TCP ACK Scan: As the name implies, an ACK scan will send a TCP packet with the ACK flag set. Use the -sA option to choose this scan.
Window Scan: Another similar scan is the TCP window scan. The TCP window scan is almost the same as the ACK scan; however, it examines the TCP Window field of the RST packets returned. On specific systems, this can reveal that the port is open. You can select this scan type with the option -sW
Custom Scan: If you want to experiment with a new TCP flag combination beyond the built-in TCP scan types, you can do so using --scanflags. For instance, if you want to set SYN, RST, and FIN simultaneously, you can do so using --scanflags RSTSYNFIN.

Additionally there are several less common port scan types, some of which we will also cover (albeit in less detail). These are:

TCP Null Scans (-sN) : The null scan does not set any flag; all six flag bits are set to zero. You can choose this scan using the -sN option. A TCP packet with no flags set will not trigger any response when it reaches an open port, as shown in the figure below. Therefore, from Nmap’s perspective, a lack of reply in a null scan indicates that either the port is open or a firewall is blocking the packet.
TCP FIN Scans (-sF): The FIN scan sends a TCP packet with the FIN flag set. You can choose this scan type using the -sF option. Similarly, no response will be sent if the TCP port is open. Again, Nmap cannot be sure if the port is open or if a firewall is blocking the traffic related to this TCP port.
TCP Xmas Scans (-sX):
The Xmas scan gets its name after Christmas tree lights. An Xmas scan sets the FIN, PSH, and URG flags simultaneously. You can select Xmas scan with the option -sX. Like the Null scan and FIN scan, if an RST packet is received, it means that the port is closed. Otherwise, it will be reported as open|filtered.

Most of these (with the exception of UDP scans) are used for very similar purposes, however, the way that they work differs between each scan. This means that, whilst one of the first three scans are likely to be your go-to in most situations, it’s worth bearing in mind that other scan types exist.

Nmap Scripting Engine

The Nmap Scripting Engine (NSE) is an incredibly powerful addition to Nmap, extending its functionality quite considerably. NSE Scripts are written in the Lua programming language, and can be used to do a variety of things: from scanning for vulnerabilities, to automating exploits for them. The NSE is particularly useful for reconnaisance, however, it is well worth bearing in mind how extensive the script library is.

There are many categories available. Some useful categories include:

safe:- Won’t affect the target
intrusive:- Not safe: likely to affect the target
vuln:- Scan for vulnerabilities
exploit:- Attempt to exploit a vulnerability
auth:- Attempt to bypass authentication for running services (e.g. Log into an FTP server anonymously)
brute:- Attempt to bruteforce credentials for running services
discovery:- Attempt to query running services for further information about the network (e.g. query an SNMP server).

A more exhaustive list can be found here.

Host Discovery with Nmap

We can ping every IP address on a target network and see who would respond to our ping (ICMP Type 8/Echo) requests with a ping reply (ICMP Type 0). Simple, isn’t it? Although this would be the most straightforward approach, it is not always reliable. Many firewalls block ICMP echo; new versions of MS Windows are configured with a host firewall that blocks ICMP echo requests by default. Remember that an ARP query will precede the ICMP request if your target is on the same subnet.

To use ICMP echo request to discover live hosts, add the option -PE. (Remember to add -sn if you don’t want to follow that with a port scan.)

TCP SYN Ping

We can send a packet with the SYN (Synchronize) flag set to a TCP port, 80 by default, and wait for a response. An open port should reply with a SYN/ACK (Acknowledge); a closed port would result in an RST (Reset). In this case, we only check whether we will get any response to infer whether the host is up. The specific state of the port is not significant here.

If you want Nmap to use TCP SYN ping, you can do so via the option -PS followed by the port number, range, list, or a combination of them. For example, -PS21 will target port 21, while -PS21-25 will target ports 21, 22, 23, 24, and 25. Finally -PS80,443,8080 will target the three ports 80, 443, and 8080.

Privileged users (root and sudoers) can send TCP SYN packets and don’t need to complete the TCP 3-way handshake even if the port is open, as shown in the figure below. Unprivileged users have no choice but to complete the 3-way handshake if the port is open.

TCP ACK Ping

As you have guessed, this sends a packet with an ACK flag set. You must be running Nmap as a privileged user to be able to accomplish this. If you try it as an unprivileged user, Nmap will attempt a 3-way handshake.

By default, port 80 is used. The syntax is similar to TCP SYN ping. -PA should be followed by a port number, range, list, or a combination of them. For example, consider -PA21, -PA21-25 and -PA80,443,8080. If no port is specified, port 80 will be used.

UDP Ping

Finally, we can use UDP to discover if the host is online. Contrary to TCP SYN ping, sending a UDP packet to an open port is not expected to lead to any reply. However, if we send a UDP packet to a closed UDP port, we expect to get an ICMP port unreachable packet; this indicates that the target system is up and available.

Enumeration with Nmap

e mentioned the different techniques we can use for scanning in Task 1. Before we explain each in detail and put it into use against a live target, we need to specify the targets we want to scan. Generally speaking, you can provide a list, a range, or a subnet. Examples of target specification are:

list: MACHINE_IP scanme.nmap.org example.com will scan 3 IP addresses.
range: 10.11.12.15-20 will scan 6 IP addresses: 10.11.12.15, 10.11.12.16,… and 10.11.12.20.
subnet: MACHINE_IP/30 will scan 4 IP addresses.

You can also provide a file as input for your list of targets, nmap -iL listofhosts.txt

Firewall Evasion with Nmap

There are a variety of other switches which Nmap considers useful for firewall evasion. We will not go through these in detail, however, they can be found here.

You can also check out Firewall and IDS Evasion with NMAP | Practical Scenario post.

The following switches are of particular note:

-f:- Used to fragment the packets (i.e. split them into smaller pieces) making it less likely that the packets will be detected by a firewall or IDS.
An alternative to -f, but providing more control over the size of the packets: --mtu <number>, accepts a maximum transmission unit size to use for the packets sent. This must be a multiple of 8.
--scan-delay <time>ms:- used to add a delay between packets sent. This is very useful if the network is unstable, but also for evading any time-based firewall/IDS triggers which may be in place.
--badsum:- this is used to generate in invalid checksum for packets. Any real TCP/IP stack would drop this packet, however, firewalls may potentially respond automatically, without bothering to check the checksum of the packet. As such, this switch can be used to determine the presence of a firewall/IDS.

Spoofed Scan with Nmap

In brief, scanning with a spoofed IP address is three steps:

Attacker sends a packet with a spoofed source IP address to the target machine.
Target machine replies to the spoofed IP address as the destination.
Attacker captures the replies to figure out open ports.

In general, you expect to specify the network interface using -e and to explicitly disable ping scan -Pn. Therefore, instead of nmap -S SPOOFEDIP MACHINEIP, you will need to issue nmap -e NETINTERFACE -Pn -S SPOOFEDIP MACHINE_IP to tell Nmap explicitly which network interface to use and not to expect to receive a ping reply. It is worth repeating that this scan will be useless if the attacker system cannot monitor the network for responses.

When you are on the same subnet as the target machine, you would be able to spoof your MAC address as well. You can specify the source MAC address using --spoof-mac SPOOFED_MAC. This address spoofing is only possible if the attacker and the target machine are on the same Ethernet (802.3) network or same WiFi (802.11).

Spoofing only works in a minimal number of cases where certain conditions are met. Therefore, the attacker might resort to using decoys to make it more challenging to be pinpointed. The concept is simple, make the scan appear to be coming from many IP addresses so that the attacker’s IP address would be lost among them. As we see in the figure below, the scan of the target machine will appear to be coming from 3 different sources, and consequently, the replies will go the decoys as well.

Fragemented Scan with Nmap

Nmap provides the option -f to fragment packets. Once chosen, the IP data will be divided into 8 bytes or less. Adding another -f (-f -f or -ff) will split the data into 16 byte-fragments instead of 8. You can change the default value by using the --mtu; however, you should always choose a multiple of 8.

To properly understand fragmentation, we need to look at the IP header in the figure below. It might look complicated at first, but we notice that we know most of its fields. In particular, notice the source address taking 32 bits (4 bytes) on the fourth row, while the destination address is taking another 4 bytes on the fifth row. The data that we will fragment across multiple packets is highlighted in red.

Description of port scanning output with Nmap

Open: indicates that a service is listening on the specified port.
Closed: indicates that no service is listening on the specified port, although the port is accessible. By accessible, we mean that it is reachable and is not blocked by a firewall or other security appliances/programs.
Filtered: means that Nmap cannot determine if the port is open or closed because the port is not accessible. This state is usually due to a firewall preventing Nmap from reaching that port. Nmap’s packets may be blocked from reaching the port; alternatively, the responses are blocked from reaching Nmap’s host.
Unfiltered: means that Nmap cannot determine if the port is open or closed, although the port is accessible. This state is encountered when using an ACK scan -sA.
Open|Filtered: This means that Nmap cannot determine whether the port is open or filtered.
Closed|Filtered: This means that Nmap cannot decide whether a port is closed or filtered.