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6 ways to Check a remote port is open in Linux

Checking remote port status is a common task for Linux admin. Now we collect 6 different ways for this task. We don’t need to install any package if we use the following two python commands. We need to install the package if we choose nc, nmap,telnet.

Methods to check if a remote port is open in Linux

The following commands can be used to check if a port is open on the remote server in Linux.

• Use nc command nc -zvw10 192.168.0.1 22
• Use nmap command nmap 192.168.0.1 -p 22
• Use telnet command telnet 192.168.0.1 22
• Use python telnet module
• Use python socket module
• Use curl command

Use nc command to check the remote port is open in Linux

$ nc [-options] [HostName or IP] [PortNumber]

• z: zero-I/O mode which is used for scanning
• v: for verbose output
• w10: timeout wait 10 seconds

The “nc” command stands for “netcat”. The “nc” command is a very versatile command that can be used for a variety of purposes, including network administration and data transmission.

For example, the “nc” command can be used to create a simple TCP connection between two computers. The “nc” command can be used to connect to a remote server on a given port and send/receive data.

For example, if you want to connect to a remote server on port xx, you would use the following command: nc -zv port

In this example, “” is the IP address or hostname of the remote server, and “” is the port that you want to connect to.

I needed to see if the port 22 (SSH) on a remote machine was open, so I opened a terminal and ran the following command:

The -v option enabled verbose output, and the -z option instructed nc to only scan for open ports, without actually establishing a connection.

The output showed me the results of the port scan:

Connection to hostname.com 22 port [tcp/ssh] succeeded!

This told me that the port 22 was open and that I could connect to the remote machine using SSH.

In another scenario, if the port was not open, the output would look something like this:

nc: connect to hostname.com port 22 (tcp) failed: Connection refused

You can also use the “nc” command to open a port in Linux. To do this, you would use the following command: nc -l -p 1234

In this example, “-l” is used to listen for a connection on port 1234

Use nmap to check the remote port is open in Linux

$ nmap [-options] [HostName or IP] [-p] [PortNumber]

The “nmap” command is a command-line tool used for network exploration and security auditing. The “nmap” command can be used to scan for open ports on a remote server, as well as to identify the operating system and services running on that server.

For example, if you want to scan for open ports on a remote server, you would use the following command:

In this example, “” is the IP address or hostname of the remote server, and “” is the port that you want to scan.

Use telnet to check the remote port is open in Linux

$ telnet [HostName or IP] [PortNumber]

The telnet command is a command-line tool used for network communication. The telnet command can be used to connect to a remote server on a given port.

For example, if you want to connect to a remote server on port, you would use the following command: telnet port

In this example, “” is the IP address or hostname of the remote server, and “” is the port that you want to connect to.

Use python telnet to check remote port is open in Linux

python -c «import telnetlib; tel=telnetlib.Telnet(‘192.168.0.1′,’22’,10); print tel; tel.close()»

If you are using Python3, using the following command:

python3 -c «import telnetlib; tel=telnetlib.Telnet(‘10.248.169.140′,’5432’,10); print(tel); tel.close()»

Telnetlib is a module in Python that allows you to communicate with remote servers using the Telnet protocol. The Telnet protocol is a text-based protocol used for communicating with remote servers.

To use the Telnetlib module, you first need to import it into your Python program: import telnetlib

Next, you need to create an instance of the Telnet object: telnet = telnetlib.Telnet()

The Telnet object has a number of methods that allow you to send and receive data. For example, the send() method allows you to send text data to the remote server, and the recv() method allows you to receive text data from the remote server.

Use python socket to check remote port is open in Linux

Python -c «import socket; s = socket.socket(); s.settimeout(10); s.connect((‘192.168.0.1’, 22)); «

The “socket” module is a module in Python that allows you to create and use sockets. A socket is a communication channel that allows two processes to connect and send/receive data.

The “socket” module has a number of functions that allow you to do a variety of things, including creating sockets, binding sockets to addresses, and sending/receiving data.

In order to use the “socket” module, you first need to import it into your Python program. You can do this by using the following command: import socket

Once you have imported the “socket” module, you can then use its functions to create sockets and communicate with other processes.

Use curl to check remote port is open in Linux

We have another solution for this with the curl command. curl -v telnet://192.168.0.1:22

The “curl” command is a tool used for transferring data with URL syntax. The “curl” command can be used to send data to a remote server, or it can be used to download data from a remote server.

If you want to download data from a remote server, you can use the following command: curl port -o filename.txt

In this example, “” is the IP address or hostname of the remote server, and “” is the port that you want to download data from.

The “curl” command can also be used to check whether a port is open or not. To do this, you would use the following command:

In this example, “” is the IP address or hostname of the remote server, and “” is the port that you want to check.

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Checking Open and Listening Ports on Linux Using netstat and ss

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One step in securing a Linux computer system is identifying which ports are active. Your system’s active ports give you information about which outside applications may be connected to your system. You can also discover if you are unintentionally exposing an application or service to the internet, like a MySQL database. There are several Linux tools that help you discover which ports are in use and identify both ends of active communications. This guide introduces three common tools you can use with links to guides that dive deeper into each tool.

What is a Port in Computer Networking?

Service names and port numbers are used to distinguish between different services that run over transport protocols. Common transport protocols are TCP, UDP, DCCP, and SCTP. These protocols enable communication between applications by establishing a connection and ensuring data is transmitted successfully. Well-known port assignments, such as HTTP at port 80 over TCP and UDP, are listed at the IANA Service Name and Transport Protocol Port Number Registry. These port assignments help distinguish different types of network traffic across the same connection.

How to Check Which Linux Ports Are in Use?

Three tools to help you check ports in use on a Linux system are:

• netstat: This tool shows your server’s network status.
• ss: You can view socket statistics with the ss tool. For example, ss allows you to monitor TCP, UDP, and UNIX sockets.
• lsof: This Linux utility lists open files. Since everything on a Linux system can be considered a file, lsof provides a lot of information on your entire system.

While all three tools help you learn how to check if a port is open in Linux, each program has its own advantages and disadvantages. See the following examples to identify which tool is the best fit for your purpose.

Using netstat

The netstat tool is great for inspecting the following areas of your Linux system:

• Unix sockets and network connections
• Routing tables
• Network interfaces
• Network protocols
• Multicast group membership

Running netstat without any options displays all open sockets and network connections. While this checks if a port is open in Linux, it can generate a lot of output. You can control the output using netstat’s command-line options. For example, to view the PID and program name for a system’s listening TCP connections, run netstat with the following command-line options:

The output resembles the following:

Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name tcp 0 0 0.0.0.0:http-alt 0.0.0.0:* LISTEN 381070/monitorix-ht tcp 0 0 localhost:domain 0.0.0.0:* LISTEN 553/systemd-resolve tcp 0 0 0.0.0.0:ssh 0.0.0.0:* LISTEN 2145/sshd: /usr/sbi tcp 0 0 localhost:33060 0.0.0.0:* LISTEN 9638/mysqld tcp 0 0 localhost:mysql 0.0.0.0:* LISTEN 9638/mysqld tcp6 0 0 [::]:http [::]:* LISTEN 10997/apache2 tcp6 0 0 [::]:ssh [::]:* LISTEN 2145/sshd: /usr/sbi

To learn how to install netstat, interpret its output, and view common command line options, see our Inspecting Network Information with netstat guide.

Using ss

Another way to have Linux check ports is via the ss tool. ss was created to improve upon netstat and provides more functionality. It allows you to monitor TCP, UDP, and UNIX sockets. A socket enables programs to communicate with each other across a network and is comprised of an IP address and a port number.

Running the ss with no options displays TCP, UDP, and UNIX sockets. Similar to netstat, this unrestricted list can get quite big on busy machines, so it is useful to restrict the ss command’s output by using command-line options. For example, to view all listening and non-listening TCP sockets issue the following command:

The output resembles the following:

State Recv-Q Send-Q Local Address:Port Peer Address:Port Process LISTEN 0 4096 0.0.0.0:http-alt 0.0.0.0:* LISTEN 0 4096 127.0.0.53%lo:domain 0.0.0.0:* LISTEN 0 128 0.0.0.0:ssh 0.0.0.0:* LISTEN 0 70 127.0.0.1:33060 0.0.0.0:* LISTEN 0 151 127.0.0.1:mysql 0.0.0.0:* ESTAB 0 0 192.0.2.0:ssh 192.0.2.1:51617 TIME-WAIT 0 0 192.0.2.0:ssh 192.0.2.2:60630 TIME-WAIT 0 0 192.0.2.0:ssh 192.0.2.3:51312 TIME-WAIT 0 0 127.0.0.1:http-alt 127.0.0.1:52456 TIME-WAIT 0 0 192.0.2.0:ssh 192.0.2.4:44364 ESTAB 0 0 192.0.2.0:ssh 192.0.2.5:51718 LISTEN 0 511 *:http *:* LISTEN 0 128 [::]:ssh [::]:*

Using just the -l parameter tells ss to list all Linux’s listening ports, which are omitted by default, making it easier to check for listening ports in Linux.

To take a deeper dive into the ss tool, read our Learning to Use the ss Tool to its Full Potential guide. This guide provides commands specific to each protocol, commands to view general statistics about a system’s current connections, and ways to filter your output.

Using lsof

Since everything on a Linux system can be considered a file, the lsof tool can report on many aspects of a system, including open network interfaces and network connections. By default, it will list open ports in Linux. The lsof tool is preinstalled on many Linux distributions, so you may consider using it before a tool you need to install.

While one of the most frequent uses of lsof is determining which program listens to a given TCP port, one unique feature of the lsof tool is repeat mode*. This mode allows you to run the lsof command continuously on a timed interval. When inspecting your system to find information about which ports are in use, lsof can return information about which user and processes are using a specific port. For example, when working with a local development environment you may want to find which localhost ports are currently in use. Use the following command to retrieve this information:

The output returns a similar response:

COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME Adobe\x20 932 jdoe 14u IPv4 0x3dab8c45775e6b5b 0t0 TCP localhost:15292 (LISTEN) Code\x20H 38254 jdoe 81u IPv4 0x3dab8c45922118fb 0t0 TCP localhost:49336 (LISTEN) VBoxHeadl 49798 jdoe 15u IPv4 0x3dab8c45a01fcf1b 0t0 TCP localhost:rockwell-csp2 (LISTEN) Google 55001 jdoe 37u IPv4 0x3dab8c457579acbb 0t0 TCP localhost:51706->localhost:bmc_patroldb (ESTABLISHED) hugo 57981 jdoe 8041u IPv4 0x3dab8c45a423853b 0t0 TCP localhost:bmc_patroldb (LISTEN) hugo 57981 jdoe 8042u IPv4 0x3dab8c45a3a8e2db 0t0 TCP localhost:bmc_patroldb->localhost:51706 (ESTABLISHED)

lsof is a powerful diagnostic tool capable of a significant number of ways that you can combine its command line options to troubleshoot various issues. To learn more about the lsof command read our How to List Open Files with lsof guide. This guide provides information about command-line options, the anatomy of the lsof output, and filtering your output with regular expressions.

This page was originally published on Thursday, February 25, 2021.

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