Introduction to Nmap and Portscanning

Introduction to Nmap and Portscanning

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

Offensive Security Certified Professional Study Notes and Guide

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)

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)
• TCP FIN Scans (-sF)
• TCP Xmas Scans (-sX)

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.

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.

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.

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.

Room Answers

Room answers can be found here.