Are you confused about using Nmap to scan Open Ports?

This guide will help you.

Nmap builds on previous network auditing tools to provide quick, detailed scans of network traffic.
It works by using IP packets to identify the hosts and IPs active on a network and then analyze these packets to provide information on each host and IP, as well as the operating systems they are running.
A single IP address can have several services running, such as a web server, an application server, etc. In order for each of them to communicate, they listen and communicate on a specific port.
Hence, when we make a connection to a server, we connect to both the IP address and a port.
For example, when we connect to https://ibmimedia.com, we connect to the ibmimedia.com server on port 443, the default port for secure web traffic.
Here at Ibmi Media, as part of our Server Management Services, we regularly help our Customers to perform Nmap related queries.
In this context, we shall look into how to use Nmap to scan Open Ports.

How To Use Nmap to Scan Open Ports?

Here, let us discuss ports in detail.
World's leading port security network scanner, Nmap hosted security tool can help us determine how well the firewall and security configuration is working.
"nmap" scans IPv4 addresses by default. However, if we specify the proper option, it can also scan IPv6 addresses.

How to Identify Common Ports ?

Ports are specified by a number ranging from 1 to 65535.
We cannot register ports between 49152 and 65535 and are suggested for private use.
Since there is a vast number of ports available, we don't have to bother the majority of the services that tend to bind to specific ports.
However, there are some ports that are worth knowing due to their ubiquity.
The following are a few:

20: FTP data

21: FTP control port

22: SSH

23: Telnet

25: SMTP

43: WHOIS protocol

53: DNS services

67: DHCP server port

68: DHCP client port

80: HTTP – Unencrypted Web traffic

110: POP3 mail port

113: Ident authentication services on IRC networks

143: IMAP mail port

161: SNMP

194: IRC

389: LDAP port

443: HTTPS – Secure web traffic

587: SMTP – message submission port

631: CUPS printing daemon port

666: DOOM – This legacy game actually has its own special port

To configure a specific application we need to find the appropriate ports.
We should make sure, both the client and server are configured to use a non-standard port.
To get a list of a few common ports, look at the /etc/services file:

$ less /etc/services

The output will have a list of common ports and their associated services:
Output

. . .

tcpmux 1/tcp # TCP port service multiplexer

echo 7/tcp

echo 7/udp

discard 9/tcp sink null

discard 9/udp sink null

systat 11/tcp users

daytime 13/tcp

daytime 13/udp

netstat 15/tcp

qotd 17/tcp quote

msp 18/tcp # message send protocol

. . .

How to check Open Ports ?

Generally, we have a number of tools to scan for open ports.
netstat is the default for most Linux distributions.
To easily discover the services running, we can issue:

$ sudo netstat -plunt

We will receive a result like the following:
Output

Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name

tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 785/sshd

tcp6 0 0 :::22 :::* LISTEN 785/sshd

Our output shows the port and listening socket associates with the service and lists both UDP and TCP protocols.

How to use Nmap ?

Part of securing a network involves infiltrating the network and discovering weaknesses in the same way an attacker might.
Out of all of the available tools for this, the most powerful is the Nmap tool.

To install Nmap on an Ubuntu or Debian machine, we run:

$ sudo apt-get update

$ sudo apt-get install nmap

The installation gives us an improved port mapping file. A more extensive association between ports and services can be seen at:

$ less /usr/share/nmap/nmap-services

Output

. . .

tcpmux 1/tcp 0.001995 # TCP Port Service Multiplexer [rfc-1078]

tcpmux 1/udp 0.001236 # TCP Port Service Multiplexer

compressnet 2/tcp 0.000013 # Management Utility

compressnet 2/udp 0.001845 # Management Utility

compressnet 3/tcp 0.001242 # Compression Process

compressnet 3/udp 0.001532 # Compression Process

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

echo 7/sctp 0.000000

. . .

How to Scan Ports with Nmap ?

Nmap can reveal a lot of information about a host. For this reason, our Support Experts recommend testing it on your own servers or after notifying the owners.
The Nmap creators provide a test server located at scanme.nmap.org. This or your own servers are good targets for practicing Nmap.
Moving ahead let us see a few common operations we can perform with Nmap.

Scan for the host operating system:

$ sudo nmap -O scanme.nmap.org

We assume the host is online and skip the network discovery portion.

This helps if we get the reply, “Note: Host seems down” in other tests.
Add this to the other options:

$ sudo nmap -PN scanme.nmap.org

Scan without performing a reverse DNS lookup on the IP address specified.

This will speed up the results in most cases:

$ sudo nmap -n scanme.nmap.org

Scan a specific port instead of all common ports:

$ sudo nmap -p 80 scanme.nmap.org

To scan for TCP connections, Nmap can perform a 3-way handshake, with the targeted port. Execute it like this:

$ sudo nmap -sT scanme.nmap.org

Similarly, to scan for UDP connections, we type:

$ sudo nmap -sU scanme.nmap.org

We can scan for every TCP and UDP open port using:

$ sudo nmap -n -PN -sT -sU -p- scanme.nmap.org

A TCP “SYN” scan exploits the way that TCP establishes a connection.
To perform an SYN scan, we execute:

$ sudo nmap -sS scanme.nmap.org

A more stealthy approach is to send invalid TCP headers.
This will work on non-Windows-based servers.

We can use the “-sF”, “-sX”, or “-sN” flags.

All of which will produce the response we look for:

$ sudo nmap -PN -p 80 -sN scanme.nmap.org

To check the version of a service is running on the host, we try this command.

It tries to determine the service and version by testing different responses from the server:

$ sudo nmap -PN -p 80 -sV scanme.nmap.org

Finally, we can use Nmap to scan multiple machines.

In addition, to specify a range of IP addresses with “-” or “/24” to scan a number of hosts at once, we run:

$ sudo nmap -PN xxx.xxx.xxx.xxx-yyy

Or scan a network range for available services with a command like this:

$ sudo nmap -sP xxx.xxx.xxx.xxx-yyy

This should help us explore the networking vulnerabilities.

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