What is Nmap?

Nmap, or Network Mapper,  is an open source Linux command line tool for network exploration and security auditing. With Nmap, server administrators can quickly reveal hosts and services, search for security issues, and scan for open ports.

The NMAP tool can audit and discover local and remote open ports, as well as network information and hosts.

Here are some of the most useful Nmap commands in Linux with examples.

nmap command examples for linux and network admins

1. Nmap Command to Scan For Open Ports

When scanning hosts, Nmap commands can use server names, IPV4 addresses or IPV6 addresses. A basic Nmap command will produce information about the given host.

nmap subdomain.server.com​

Without flags, as written above, Nmap reveals open services and ports on the given host or hosts.

nmap 192.168.0.1

Nmap can reveal open services and ports by IP address as well as by domain name.

nmap -F 192.168.0.1

If you need to perform a scan quickly, you can use the “-F” flag. The “-F” flag will list ports on the nmap-services files. Because the -F “Fast Scan” flag does not scan as many ports, it isn’t as thorough.

2. Scan Multiple Hosts

Nmap can scan multiple locations at once rather than scanning a single host at a time. This is useful for more extensive network infrastructures. There are several ways to scan numerous locations at once, depending on how many locations you need to examine.

nmap 192.168.0.1 192.168.0.2 192.168.0.3

Add multiple domains or multiple IP addresses in a row to scan multiple hosts at the same time.

nmap 192.168.0.* 

Use the * wildcard to scan an entire subnet at once.

nmap 192.168.0.1,2,3

Separate different address endings with commas rather than typing out the entire IP address.

nmap 192.168.0.1-4

Use a hyphen to scan a range of IP addresses.

3. Excluding Hosts from Search

When scanning a network, you may want to select an entire group (such as a whole subnet) while excluding a single host.

nmap 192.168.0.* --exclude 192.168.0.2

You can exclude certain hosts from your search using the –exclude flag.

nmap 192.168.0.* --excludefile /file.txt

You can also exclude a list of hosts from your search using the –exclude flag and linking to a specific file. This is the easiest way to exclude multiple hosts from your search.

4. Scan To Find Out OS Information

In addition to general information, Nmap can also provide operating system detection, script scanning, traceroute, and version detection. It’s important to note that Nmap will do its best to identify things like operating systems and versions, but it may not always be entirely accurate.

nmap -A 192.168.0.1

Addin the -A flag on your Nmap command, you can discover the operating system information of the hosts that are mapped. The -A flag can be used in combination with other Nmap commands.

nmap -O 192.168.0.1

Using the -O flag on your Nmap command will reveal further operating system information of the mapped hosts. The -O flag enables OS detection. Additional tags include –osscan-limit and -osscan-guess.

The “–osscan-limit” command will only guess easy operating system targets. The “-osscan-guess” command will be more aggressive about guessing operating systems. Again, operating systems are detected based on certain hallmarks: it isn’t a certainty that the information is accurate.

5. Scan To Detect Firewall Settings

Detecting firewall settings can be useful during penetration testing and vulnerability scans. Several functions can be used to detect firewall settings across the given hosts, but the -sA flag is the most common.

nmap -sA 192.168.0.1

Using the sA flag will let you know whether a firewall is active on the host. This uses an ACK scan to receive the information.

6. Find Information About Service Versions

At times, you may need to detect service and version information from open ports. This is useful for troubleshooting, scanning for vulnerabilities, or locating services that need to be updated.

nmap -sV 192.168.0.1

This will give you the necessary information regarding the services across the given host.

You can use –version-intensity “level” from 0 to 9 to determine the intensity level of this search. You can also use –version-trace to show more detailed information of the scan if the scan does not come out with the results that you would ordinarily expect.

7. Scan for Ports

Port scanning is one of the basic utilities that Nmap offers and consequently there are a few ways that this command can be customized.

nmap -p 443 192.168.0.1

With the “-p” flag followed by a port, you can scan for information regarding a specific port on a host.

nmap -p T:8888,443 192.168.0.1

By adding a type of port before the port itself, you can scan for information regarding a specific type of connection.

nmap -p 80,443 192.168.0.1

You can scan for multiple ports with the -p flag by separating them with a comma.

nmap -p 80-443 192.168.0.1

You can also scan for multiple ports with the -p flag by marking a range with the hyphen. To scan ports in order rather than randomly, add the flag “-r” to the command.

You can also use the command “–top-ports” followed by a number to find the most common ports, up to that amount.

8. Complete a Scan in Stealth Mode

If it is necessary to complete a stealthy scan, use the Nmap command:

nmap -sS 192.168.0.1

Using the “-sS” flag will initiate a stealth scan with TCP SYN. The “-sS” flag can be used in conjunction with other types of Nmap commands. However, this type of scan is slower and may not be as aggressive as other options.

9. Identify Hostnames

There are a few ways you can implement host discovery through Nmap. The most common of which is through -sL. For example:

nmap -sL 192.168.0.1

The “-sL” flag will find the hostnames for the given host, completing a DNS query for each one. Additionally, the “-n” command can be used to skip DNS resolution, while the “-R” command can be used to always resolve DNS. The “-Pn” flag will skip host discovery entirely, instead of treating hosts as though they are online regardless.

10. Scan From a File

If you have a long list of addresses that you need to scan, you can import a file directly through the command line.

nmap -iL /file.txt

This will produce a scan for the given IP addresses. In addition to scanning those IP addresses, you can also add other commands and flags. This is useful if there is a set of hosts that you often need to reference.

11. Get More Information with Verbose

Verbose output generally gives you far more information regarding a command. Sometimes this output is unnecessary. However, if you’re debugging a particularly tricky situation or you want more information, you can set the given command to verbose mode.

nmap -v 192.168.0.1

The “-v” flag will provide additional information about a completed scan. It can be added to most commands to give more information. Without the “-v” flag, Nmap will generally return only the critical information available.

12. Scan IPv6 Addresses

IPv6 is becoming more commonplace, and Nmap supports it just as it supports domains and older IP addresses. IPv6 works with any of the available Nmap commands. But, a flag is required to tell Nmap that an IPv6 address is being referenced.

nmap -6 ::ffff:c0a8:1

Use the -6 command with other flags and commands to perform more complicated Nmap functions with IPv6.

13. Scan to See Which Servers Are Active

One of the most simple abilities for Nmap is the ability to ping active machines. The “-sP” command locates machines, make sure that machines are responding, or identifies unexpected machines across a network.

nmap -sP 192.168.0.0/24

The “-sP” command will produce a list of which machines are active and available.

14. Find Host Interfaces, Routes, and Packets

It may become necessary to find host interfaces, print interfaces, and routes to debug.

To do this, use the iflist command:

nmap --iflist

The “–iflist” command will produce a list of the relevant interfaces and routes.

nmap --packet-trace

Similarly, “–packet-trace” will show packets sent and received, providing similar value for debugging.

15. Aggressive Scans and Timings

Sometimes you may need to scan more aggressively or want to run a quick scan. You can control this through the use of the timing mechanisms. In NMAP, timing controls both the speed and the depth of the scan.

nmap -T5 192.168.0.1

An aggressive scan is going to be faster, but it also could be more disruptive and also inaccurate. There are other options such as the T1, T2, T3, and T4 scans. For most scans, T3 and T4 timings will be sufficient.

16. Get Some Help

If you have any questions about Nmap or any of the given commands, you can use a tag to get context-based information.

nmap -h

The -h tag will show the help screen for Nmap commands, including giving information regarding the available flags.

17. Create Decoys While Scanningp

Nmap can also be used to create decoys, which are intended to fool firewalls. While decoys can be used for nefarious purposes, it’s generally used to debug.

nmap -D 192.168.0.1,192.168.0.2,...

When using the -D command, you can follow the command with a list of decoy addresses. These decoy addresses will also show as though they are scanning the network, to obfuscate the scan that is ecoysactually being done.

Similarly, it’s possible to use commands such as “–spoof-mac” to spoof an Nmap MAC address, as well as the command “-S” to spoof a source address.

Key Takeaways

With the right Nmap commands, you can quickly find out information about ports, routes, and firewalls.

Nmap has several settings and flags for a system administrator to explore. In addition to being able to run in a cloaked mode, initiate decoys, and aggressively and quickly scan for potential vulnerabilities.