How to check port opened in linux

How can I see what ports are open on my machine?

I would like to see what ports are open on my machine, e.g. what ports my machine is listening on. E.g. port 80 if I have installed a web server, and so on. Is there any command for this?

10 Answers 10

If the netstat command is not available, install it with:

sudo apt install net-tools 

-l already filters for listening. grep LISTEN won’t help beyond hiding 2 lines of header information.

-t : tcp, -l : listening socket, -p : show pid and program name, -n : print 127.0.0.1:80 instead of localhost:http . Reference: linux.die.net/man/8/netstat

The expanded command is sudo netstat —tcp —listening —programs —numeric . There’s no need to use grep unless you want to eliminate column headers.

nmap (install)

Nmap («Network Mapper») is a free and open source utility for network exploration or security auditing.

Use nmap 192.168.1.33 for internal PC or nmap external IP address .

More information man nmap .

Zenmap is the official GUI frontend.

Remember that there is a difference between nmap localhost and nmap 192.168.0.3 (or what ever you machine IP is)

I think netstat is a better answer to this. netstat will list what the system is listening on directly, and without using an additional application or doing unnecessary calls over localhost or thought the network.

This is stupid. If you have access to the computer, just use netstat -ln . You’ll instantly see all the open ports.

nmap localhost didn’t find services that were bound only to localhost. For example, I run influxd with bind-address:localhost:8086 . That didn’t show up in sudo nmap localhost , but did show up in sudo netstat -tulpn .

Other good ways to find out what ports are listenting and what your firewall rules are:

To list open ports use the netstat command.

 $ sudo netstat -tulpn | grep LISTEN tcp 0 0 127.0.0.1:53 0.0.0.0:* LISTEN 5452/dnsmasq tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN 1037/cupsd tcp6 0 0 ::1:631 . * LISTEN 1037/cupsd 

In the above example three services are bound to the loopback address.

IPv4 services bound to the loopback address «127.0.0.1» are only available on the local machine. The equivalent loopback address for IPv6 is «::1». The IPv4 address «0.0.0.0» means «any IP address», which would mean that other machines could potentially connect to any of the locally configured network interfaces on the specific port.

Another method is to use the lsof command:

 $ sudo lsof -nP -i | grep LISTEN cupsd 1037 root 9u IPv6 11276 0t0 TCP [::1]:631 (LISTEN) cupsd 1037 root 10u IPv4 11277 0t0 TCP 127.0.0.1:631 (LISTEN) dnsmasq 5452 nobody 5u IPv4 212707 0t0 TCP 127.0.0.1:53 (LISTEN) 

For more details see man netstat or man lsof .

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How to check if a certain port is open and unused?

Could you please help and tell how can i find out if port 80 is open and unused so that I can start installation.

For what it’s worth, /etc/services is completely static. Grepping it can tell you if a port is officially designated by IANA or some such, but does not tell whether or not it’s in local use.

9 Answers 9

sudo netstat -anp | grep ':80 ' 

That should give you pid & name of the process that holds port 80

This can be achieved using the nc command as follows:

It will return TRUE if the port is already in use, or FALSE is it (i.e, available not listening currently).

I don’t recommend lsof or netstat method as it first try to scan all running PIDs to get all bounded ports:

# time lsof -i:8888 real 0m1.194s user 0m0.137s sys 0m1.056s``` # time nc -z 127.0.0.1 8888 real 0m0.014s user 0m0.011s sys 0m0.004s 

Here 8888 is an unused port. The nc command is ~85 times faster in the above example.

Eg 1:

$ nc -z 127.0.0.1 80 && echo "IN USE" || echo "FREE" IN USE $ nc -z 127.0.0.1 81 && echo "IN USE" || echo "FREE" FREE 

Eg 2:

If you are trying with a remote IP, it is better to add a timeout to auto-exit if it is not accepting connection for the specified time.

Its Google’s IP which is not used, so it will timeout after trying for 2 seconds.

This also works greatly when running inside the Docker image that uses host network. Inside the image, lsof incorrectly reports the port is not in use when it actually is.

The traditional version of nc does not include the -z option. See the differences between traditional and openbsd.

netstat -tln | tail -n +3 | awk '< print $4 >' 

This one displays bind addresses of TCP listening endpoints. All other endpoints are free; Also if on Unix and you are not root, then you can’t bind to a ‘privileged’ port number (port number lower than 1024).

Explained in more details:

• netstat -tln — all listening tcp ports
• tail -n +3 — cut off the header of netstat command
• awk ‘< print $4 >‘ — print the fourth column that consists of [ip]:[port]

For the general case you still need to care to cut out all irrelevant interfaces; a listening address 0.0.0.0 is listening on all network cards, if there is an IP address then that’s the specific IP of the network card/network interface.

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How to Check (Scan) for Open Ports in Linux

In this tutorial, we will discuss many methods for determining which ports on your Linux system are available to the outside world.

Introduction

One of the first things to check when troubleshooting network connectivity issues or establishing a firewall is whether ports are truly open on your machine.

In this tutorial, we will discuss many methods for determining which ports on your Linux system are available to the outside world.

What is Open Port?

A program that listens on a network port is known as a listening port. You may retrieve a list of your system’s listening ports by using tools like ss , netstat , or lsof to query the network stack. Using a firewall, each listening port can be opened or blocked (filtered).

A network port that admits incoming packets from faraway destinations is known as an open port.

For instance, if your web server listens on ports 80 and 443 and those ports are open on your firewall, anyone (excluding blocked ips) can use his browser to view websites housed on your web server. Both ports 80 and 443 are open in this scenario.

Open ports can be a security issue since attackers can use them to exploit vulnerabilities or carry out other types of attacks. All other ports should be closed and just the ports required for your application’s operation should be exposed.

Check Open Ports with nmap

Nmap is a network scanning program capable of scanning both single hosts and big networks. It’s mostly used for penetration testing and security assessments.

When it comes to port scanning, nmap should be your first choice if it is available. Nmap can determine the Mac address, OS type, kernel versions, and much more in addition to port scanning.

Which ports are listening for TCP connections from the network can be determined by using the following command from the console:

The -sT option instructs nmap to scan for TCP ports, whereas the -p- option instructs it to scan for all 65535 ports. If the -p- option is not specified, nmap will only scan the 1000 most popular ports.

Output Starting Nmap 7.60 ( https://nmap.org ) at 2019-07-09 23:10 CEST Nmap scan report for 10.10.8.8 Host is up (0.0012s latency). Not shown: 998 closed ports PORT STATE SERVICE 22/tcp open ssh 80/tcp open http MAC Address: 08:00:27:05:49:23 (Oracle VirtualBox virtual NIC) Nmap done: 1 IP address (1 host up) scanned in 0.41 seconds 

Only ports 22 , 80 , and 8069 are open on the target system, according to the output.

Instead of -sT , use -sU to scan for UDP ports:

Visit the nmap man page for more information and to learn about all of this tool’s other useful features.

Check Open Ports with netcat

Netcat (or nc ) is a command-line utility that uses the TCP or UDP protocols to read and write data across network connections.

Netcat can scan a single port or a range of ports.

To search for open TCP ports on a distant system with IP address 10.10.8.8 in the range 20-80 , for example, execute the command:

The -z option instructs nc to scan only for open ports and not transfer any data, whereas the -v option provides more detailed information.

This is what the output will look like:

Output The -z option instructs nc to scan only for open ports and not transfer any data, whereas the -v option provides more detailed information. This is what the final product will look like: 

If you just want the lines with open ports written on the screen, use the grep command to filter the results.

nc -z -v 10.10.8.8 20-80 2>&1 | grep succeeded 

Output Connection to 10.10.8.8 22 port [tcp/ssh] succeeded! Connection to 10.10.8.8 80 port [tcp/http] succeeded! 

Pass the -u argument to the nc command to scan for UDP ports:

nc -z -v -u 10.10.8.8 20-80 2>&1 | grep succeeded 

Check Open Ports using Bash Pseudo Device

The Bash shell /dev/tcp/.. or /dev/udp/.. pseudo-device can also be used to determine whether a port is open or closed.

Bash will open a TCP or UDP connection to the specified host on the specified port when a command is run on a /dev/$PROTOCOL/$HOST/$IP pseudo-device.

The if..else statement below will check if port 443 on kernel.org is open:

if timeout 5 bash -c '/dev/null' then echo "Port is open" else echo "Port is closed" fi 

What is the purpose of the code above?

Because the default timeout when connecting to a port via a pseudo-device is so long, we’re utilizing the timeout command to kill the test command after 5 seconds. The test command will return true if the connection to kernel.org port 443 is established.

Use the for loop to check for a port range:

for PORT in ; do timeout 1 bash -c "/dev/null" && echo "port $PORT is open" done 

You will get an output like below:

Output port 22 is open port 80 is open 

Conclusion

We’ve taught you how to scan for open ports with a variety of tools. You can also check for open ports using other utilities and methods, such as the Python socket module, curl , telnet , or wget .

If you have any queries, please leave a comment below and we’ll be happy to respond to them.

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How to Check for Open Ports in Linux

When you are troubleshooting networking issues in Linux or are looking for ways to improve the security of your Linux machine, you will need to know if and which ports are open. In this article we will look at different ways to list or display open ports in Linux.

What is a Port?

A port is a 16-bit number (0 to 65535) to help identify a given application or process on a Linux (Unix) operating system. Port differentiates one application from another on a Linux system.

Below are the different categories of ports:

• 0 – 1023 – Referred to as Well Known Ports
• 1024 – 49151 –Referred to as Registered Ports
• 49152 – 65535 – Referred to as Dynamic Ports

Using the following command, a list of applications and ports is displayed on your terminal:

TCP: TCP stands for Transmission Control Protocol. It is the most commonly-used protocol on the Internet. TCP is not just one-way communication, rather it sends packets back to acknowledge it’s received your packets.

UDP: Also known as User Datagram Protocol. It is an alternative communications protocol to TCP. The UDP protocol works similar to TCP. However, it ignores all error-checking stuff. UDP is necessary when speed is desirable and error correction is not needed.

SOCKETS: Socket allows communication from two different processes on the same or different machines.

Let’s look at different ways to list an open port in Linux.

1. Netstat

In this method we will use the command netstat -atu to check for open ports in Linux.

We used the -a , -t and -u flags for netstat.

You can also add the -p flag to show related PID of the process or program name.

To display only UDP ports, you can use the following command:

Also, you can use the following command to search for TCP ports:

2. lsof

Instead of using netstat, we can use the lsof command to display open ports in Linux:

The following command can also help to display open sockets:

Also, you can use the command below to list all TCP connections:

Moreover, you can use the following command for UDP connections:

3. Network Mapped Command

In this method we will use nmap to detect the open port on your system. We can use the following command to show tcp port connections:

Finally, to show udp port connections, we can use the following command:

Conclusion

When it comes to the security of your Linux PC, the first thing to do is to close all unnecessary ports to prevent external access. With the methods listed above, you will be able to easily check for open ports on a Linux system and determine which ports should be closed or remain open.

Michael wears many hat in the opensource industry. He is based in Accra, Ghana. He revels in anything Linux and Devops.

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