Linux check open ports on remote ip

How to Check Remote Ports are Reachable Using ‘nc’ Command

A port is a logical entity which acts as a endpoint of communication associated with an application or process on an Linux operating system. It is useful to know which ports are open and running services on a target machine before using them.

We can easily list open ports in Linux on a local machine using the netstat or several other Linux commands such NMAP.

In this guide, we will show you how to determine if ports on a remote host are reachable/open using simple netcat (in short nc) command.

netcat (or nc in short) is a powerful and easy-to-use utility that can be employed for just about anything in Linux in relation to TCP, UDP, or UNIX-domain sockets.

We can use it to: open TCP connections, listen on arbitrary TCP and UDP ports, send UDP packets, do port scanning under both IPv4 and IPv6 and beyond.

Using netcat, you can check if a single or multiple or a range of open ports as follows. The command below will help us see if the port 22 is open on the host 192.168.56.10:

In the command above, the flag:

1. -z – sets nc to simply scan for listening daemons, without actually sending any data to them.
2. -v – enables verbose mode.

The next command will check if ports 80, 22 and 21 are open on the remote host 192.168.5.10 (we can use the hostname as well):
nc -zv 192.168.56.10 80 22 21

It is also possible to specify a range of ports to be scanned:’

For more examples and usage of netcat command, read through our articles as follows.

That’s all. In this article, we explained how to check if ports on a remote host are reachable/open using simple netcat commands. Make use of the comment section below to write back to us concerning about this tip.

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Check open ports in Linux | Test firewall rules

Table of Contents

Lately I was going through some articles from different search engine to check open ports in Linux. I was quiet surprised with the results as most of these articles in the search results talks about using ss, netstat etc to check if a port is open in Linux.

You have to understand the difference and your requirement.

Do you want to check if a port is open on the destination server or between source and destination server?

Do you want to check if a port is in LISTENING state on the destination server.

As it doesn’t necessarily mean that if a port is not listening then it is not open. A port will be in LISTEN state only when it is in use by some process or else a port can be open and free but since it is not in USE it will not be listed with netstat, lsof, ss commands etc.

So I hope you are clear with your requirement.

In this article I will share the different ways to check open ports or if a port is open on the destination server in Linux.

Method-1: Check open ports using nmap

nmap is an open source tool for network exploration and security auditing. Let’s verify if nmap can successfully give us list of open ports on a Linux server:

Currently I have not added any firewall rules on my destination server:

Now let us check open ports between 35520-35522 on this server using some different client machine with nmap:

We have used -PN to perform TCP SYN/ACK check on the provided list of ports but the output claims that all of these ports are closed. But this is not correct as there is no firewall on server-2 and this setup in in my local LAN so no other firewalls in between these servers.

Now here are the list of listening ports on server-2 :

So if we try to scan these listening ports using nmap:

All of these ports are marked as OPEN. So unless your ports are in use by some process, they will not be marked as OPEN.

Let’s perform some more tests. I will DROP all the incoming requests and only allow certain ports on server-2 using iptables:

List the applied rules

So I have explicitly allowed port 35520 , now let’s perform nmap scan for this port and some others:

Now port 35520 is marked as CLOSED as it is explicitly allowed in iptables but currently is not in use while others are marked as FILTERED as they are blocked in firewall.

Understanding different states in nmap

Though the current version of NMAP is capable of performing many tasks, it initially started out as a port scanner. NMAP has certain ways to detect whether the port on the target system is open or closed. NMAP detects the status of the target port using predefined states as follows:

• Open: The Open state indicates that an application on the target system is actively listening for connections/packets on that port.
• Closed: The Closed state indicates there isn’t any application listening on that port. However, the port state could change to Open in the future.
• Filtered: The Filtered state indicates that either a firewall, a filter, or some kind of network hurdle is blocking the port and hence NMAP isn’t able to determine whether it is open or closed.
• Unfiltered: The Unfiltered state indicates that ports are responding to NMAP probes; however, it isn’t possible to determine whether they are open or closed.
• Open/Filtered: The Open/Filtered state indicates that the port is either filtered or open; however, NMAP isn’t precisely able to determine the state.
• Closed/Filtered: The Closed/Filtered state indicates that the port is either filtered or closed; however, NMAP isn’t precisely able to determine the state.

Method-2: Check list of open ports in Linux using hping3

Another wonderful tool to perform network scan is hping3 in Linux. You may download hping3 using EPEL repo.

We will retain the iptables rule which we applied in the previous example where we had blocked all the ports except 22 and 35520 and perform network scan using hping3:

So based on the above hint, hping3 got a RESET request with ACK on port 35520 which means the port may be in OPEN state while the other ports in the provided range are not responding so they are closed.

If any of the port is in LISTENING state then hping3 will return SYN and ACK flag as shown below:

Method-3: Test firewall rules

Now you may using the tools which I explained above to check open ports but if you wish to test firewall rules then I would recommend using netcat or nc tool. nc can be used to open any port and mark it as listening and then on the client server you can again use nc to send some dummy data to test firewall rule.

Let us retain the above applied iptables rule, so on our server port 35520 is allowed in the firewall. To test this firewall rule, we will enable port 35520 using nc:

So our server is now listening on port 35520, now let’s try to connect to this port using any other client node:

As you can see, nc was able to connect to server-2 using port 35520. Now if you try to send any string from client to server:

Monitor the console on server-1 and you will receive this string:

Similarly let’s try to test firewall rule for any other port which is blocked (we have blocked all the ports except 35520 and 22):

Since port 35521 port is blocked in the firewall, the same will not be accessible from server-1 :

Summary

In this article I shared different methods to perform network scanning to check open ports and test firewall rules in Linux. There are many other tools available which can be used for this purpose. But again I don’t rely on telnet, ss or netstat commands for this purpose as they may not give you accurate data based on your requirement. nmap, hping3 are very vast tools with alot of different options and features which are not covered in this article. I would suggest to go through their man page and explore different options.

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1 thought on “Check open ports in Linux | Test firewall rules”

Hi I’m Very interesting to all your tutoes. And i think they can help many people to have a better understanting even me as a beginner in linux.

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How to check if a port is open on remote Linux system

When installing or configuring an application in the Linux system, the associated port should also be open which allows the application for external access. If the application port is not open, it will make the program throw errors and hence malfunction.

For instance, when you configure the Apache Web server on Linux, you must open ports 80 and 443 that listens to incoming connections for Apache on the firewall, and that allows users to access websites hosted on your web server through the browser.

Please note: You also need to open these network ports in the hardware firewall against your server IP, if you already managing it.

In this article, we will show you, how to check if a network port is opened or closed on a remote Linux system using the below three methods.

• nc: Netcat is a simple Unix utility which reads and writes data across network connections, using TCP or UDP protocol.
• nmap: Nmap (“Network Mapper”) is an open source tool for network exploration and security auditing. It was designed to rapidly scan large networks.
• telnet: The telnet command is used for interactive communication with another host using the TELNET protocol.

All of these commands allow you to scan against one server at a time, but if you want to scan open ports on multiple servers, please check out the following post that contains the shell script:

1) Check open ports with netcat

nc stands for netcat. Netcat is a simple and powerful tool which reads and writes data across network connections, using TCP or UDP protocol.

It allows user to scan a single port or a port range.

Common Syntax for nc (netcat):

In this example, we will check whether port 22 is open or not on the remote Linux system.

If it’s a success then you will be getting the following output:

Details:

• nc: It’s a command.
• z: zero-I/O mode (used for scanning).
• v: For verbose.
• w3: timeout wait seconds
• 192.168.1.8: Destination system IP.
• 22: Port number needs to be verified.

If it fails then you will be getting the following output:

2) Check open ports using nmap command

nmap (Network Mapper) is a powerful open source network scanning tool that is designed to rapidly scan large networks, but can also be used to scan a single host.

It is specifically used for security auditing and penetration testing, but it can also be used for port scanning, host scanning, and network inventory.

Common Syntax for nmap:

If it’s a success then you will be getting the following output:

If it fails then you will be getting the following output:

3) Check open ports with telnet

The telnet command is used for interactive communication with another host using the TELNET protocol. It doesn’t provides built-in security so traffic between a TELNET client and TELNET server is not encrypted.

Common Syntax for telnet:

If it’s a success then you will be getting the following output:

If it fails then you will be getting the following output:

We found only the above three methods. If you are aware of any other ways, please let us know by sharing your findings in the comments section below.

Wrapping Up

We have shown you several commands to find out whether a particular port is open on a remote Linux system.

If you have any questions or feedback, feel free to comment below.

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🏷 3 способа проверить, открыт ли порт на удаленной системе Linux

В этой статье мы покажем вам, как проверить, какие порты открыты в удаленной системе Linux, используя три метода.

Это можно сделать с помощью следующих команд Linux.

• nc: Netcat – простая утилита Unix, которая считывает и записывает данные через сетевые соединения, используя протокол TCP или UDP.
• nmap: Nmap («Network Mapper») – это инструмент с открытым исходным кодом для исследования сети и аудита безопасности. Он был разработан для быстрого сканирования больших сетей.
• telnet: команда telnet используется для интерактивного взаимодействия с другим хостом по протоколу TELNET.

Как проверить, открыт ли порт на удаленной системе Linux с помощью команды nc (netcat)?

nc означает netcat.

Netcat – это простая утилита Unix, которая читает и записывает данные через сетевые соединения, используя протокол TCP или UDP.

Она разработана, чтобы быть надежным «внутренним» инструментом, который может использоваться напрямую или легко управляться другими программами и скриптами.

В то же время это многофункциональный инструмент для отладки и исследования сети, поскольку он может создавать практически любые типы соединений, которые вам понадобятся, и имеет несколько интересных встроенных возможностей.

Netcat имеет три основных режима работы.

Это режим подключения, режим прослушивания и туннельный режим.

Общий синтаксис для nc (netcat):

В этом примере мы собираемся проверить, открыт ли порт 22 в удаленной системе Linux.

В случае успеха вы получите следующий результат.

Если порт не доступен, вы получите следующий вывод.

Как проверить, открыт ли порт на удаленной системе Linux с помощью команды nmap?

Nmap («Network Mapper») – это инструмент с открытым исходным кодом для исследования сети и аудита безопасности.

Он был разработан для быстрого сканирования больших сетей, хотя он отлично работает на отдельных хостах.

Хотя Nmap обычно используется для аудита безопасности, многие системные и сетевые администраторы считают его полезным для рутинных задач, таких как инвентаризация сети, управление расписаниями обновления служб и мониторинг времени работы хоста или службы.

Общий синтаксис для nmap:

В случае успеха вы получите следующий результат.

Если это не удастся, вы получите следующий вывод.

См. еще про Nmap:

Как проверить, открыт ли порт на удаленной системе Linux с помощью команды telnet?

Команда telnet используется для интерактивного взаимодействия с другим хостом по протоколу TELNET.

Общий синтаксис для telnet:

В случае успеха вы получите следующий результат.

Если это не удастся, вы получите следующий вывод.

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