Checking port connection linux

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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Check if port is open or closed on a Linux server?

It’s not quite clear what you’re asking. What do you mean by «open»? Do you mean some server is listening on that port? Or do you mean it’s allowed by the system firewall? Or what?

nc -w5 -z -v , you should get something like Connection to 127.0.0.1 9000 port [tcp/*] succeeded! , otherwise port is closed.

A topic that contains an answer also for kernel level services and programs serverfault.com/questions/1078483/…

8 Answers 8

You can check if a process listens on a TCP or UDP port with netstat -tuplen .

To check whether some ports are accessible from the outside (this is probably what you want) you can use a port scanner like Nmap from another system. Running Nmap on the same host you want to check is quite useless for your purpose.

GNU netstat knows the parameters -t , -u , -p , -l , -e , and -n . Thanks to the options parser it can be expressed as -tuplen . linux.die.net/man/8/netstat

Also, the telnet command usually does only supports TCP, so you’re out of luck if the service you want to check runs on another protocol.

According to article: computingforgeeks.com/netstat-vs-ss-usage-guide-linux netstat is deprecated, and ss is it’s replacement, so you can do ss -an , ss -tuplen or for tcp listening sockets ss -ntlp .

Quickest way to test if a TCP port is open (including any hardware firewalls you may have), is to type, from a remote computer (e.g. your desktop):

Which will try to open a connection to port 80 on that server. If you get a time out or deny, the port is not open 🙂

OK, in summary, you have a server that you can log into. You want to see if something is listening on some port. As root, run:

this will show a listing of processes listening on TCP and UDP ports. You can scan (or grep) it for the process you’re interest in,and/or the port numbers you expect to see.

If the process you expect isn’t there, you should start up that process and check netstat again. If the process is there, but it’s listening on a interface and port that you did not expect, then there’s a configuration issue (e.g., it could be listening, but only on the loopback interface, so you would see 127.0.0.1:3306 and no other lines for port 3306, in the case of the default configuration for MySQL).

If the process is up, and it’s listening on the port you expect, you can try running a «telnet» to that port from your Macbook in your office/home, e.g.,

 telnet xxxxxxxxxxxx.co.uk 443 

That will test if (assuming standard ports) that there’s a web server configured for SSL. Note that this test using telnet is only going to work if the process is listening on a TCP port. If it’s a UDP port, you may as well try with whatever client you were going to use to connect to it. (I see that you used port 224. This is masqdialer, and I have no idea what that is).

If the service is there, but you can’t get to it externally, then there’s a firewall blocking you. In that case, run:

This will show all the firewall rules as defined on your system. You can post that, but, generally, if you’re not allowing everything on the INPUT chain, you probably will need to explicitly allow traffic on the port in question:

 iptables -I INPUT -p tcp --dport 224 -j ACCEPT 

or something along those lines. Do not run your firewall commands blindly based on what some stranger has told you on the Internet. Consider what you’re doing.

If your firewall on the box is allowing the traffic you want, then your hosting company may be running a firewall (e.g., they’re only allowing SSH (22/tcp), HTTP (80/tcp) and HTTPS (443/tcp) and denying all other incoming traffic). In this case, you will need to open a helpdesk ticket with them to resolve this issue, though I suppose there might be something in your cPanel that may allow it.

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How to Test Port [TCP/UDP] Connectivity from a Linux Server

Here is a short post to check port [TCP/UDP] connectivity from a Linux server. A TCP/IP network connection may be either blocked, dropped, open, or filtered. These actions are generally controlled by the IPtables firewall the system uses and is independent of any process or program that may be listening on a network port.

Telnet and nc are common tools used to test port connectivity from Linux server. Telnet can be used to test tcp port connections, where as nc can be used to test both tcp/udp ports connectivity. Make sure telnet and nc tools are installed on the Linux server you are trying to test connectivity.

# yum install nc # yum install telnet

Testing TCP port connectivity with telnet

Lets see how we can use telnet command to test the TCP port connectivity. The syntax to use the telnet command is as follows:

# telnet [hostname/IP address] [port number]

Example of successful connection:

# telnet 192.168.12.10 22 Trying 192.168.12.10. Connected to 192.168.12.10. Escape character is '^]'. SSH-2.0-OpenSSH_6.6.1 Protocol mismatch. Connection closed by foreign host.

Example of unsuccessful connection:

# telnet 192.168.12.10 22 Trying 192.168.12.10. telnet: connect to address 192.168.12.10: No route to host

Using nc command to test TCP port connectivity

The syntax to use nc command for testing TCP post connectivity is as follows:

# nc -z -v [hostname/IP address] [port number]

Example of successful connection:

# nc -z -v 192.168.10.12 22 Connection to 192.118.20.95 22 port [tcp/ssh] succeeded!

Example of unsuccessful connection:

# nc -z -v 192.168.10.12 22 nc: connect to 192.118.20.95 port 22 (tcp) failed: No route to host

Testing UDP port connectivity wit nc command

The syntax to test UDP port connectivity with nc command is as follows:

# nc -z -v -u [hostname/IP address] [port number]

Example of successful connection:

# nc -z -v -u 192.168.10.12 123 Connection to 192.118.20.95 123 port [udp/ntp] succeeded!

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