Tcp port in use linux

How to Find Out List of All Open Ports in Linux

In this article, we will briefly talk about ports in computer networking and move to how you can list all open ports in Linux.

In computer networking, and more definitely in software terms, a port is a logical entity which acts as a endpoint of communication to identify a given application or process on an Linux operating system. It is a 16-bit number (0 to 65535) which differentiates one application from another on end systems.

The two most popular Internet transport protocols, Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) and other less known protocols use port numbers for communication sessions (source and destination port numbers in conjunction with the source and destination IP addresses).

In addition, a combination of an IP address, port and protocol such as TCP/UDP is known as a socket, and every service must have a unique socket.

Below are the different categories of ports:

1. 0-1023 – the Well Known Ports, also referred to as System Ports.
2. 1024-49151 – the Registered Ports, also known as User Ports.
3. 49152-65535 – the Dynamic Ports, also referred to as the Private Ports.

You can view a list of different applications and port/protocol combination in /etc/services file in Linux using cat command:

$ cat /etc/services OR $ cat /etc/services | less

# /etc/services: # $Id: services,v 1.48 2009/11/11 14:32:31 ovasik Exp $ # # Network services, Internet style # IANA services version: last updated 2009-11-10 # # Note that it is presently the policy of IANA to assign a single well-known # port number for both TCP and UDP; hence, most entries here have two entries # even if the protocol doesn't support UDP operations. # Updated from RFC 1700, ``Assigned Numbers'' (October 1994). Not all ports # are included, only the more common ones. # # The latest IANA port assignments can be gotten from # http://www.iana.org/assignments/port-numbers # The Well Known Ports are those from 0 through 1023. # The Registered Ports are those from 1024 through 49151 # The Dynamic and/or Private Ports are those from 49152 through 65535 # # Each line describes one service, and is of the form: # # service-name port/protocol [aliases . ] [# comment] tcpmux 1/tcp # TCP port service multiplexer tcpmux 1/udp # TCP port service multiplexer rje 5/tcp # Remote Job Entry rje 5/udp # Remote Job Entry echo 7/tcp echo 7/udp discard 9/tcp sink null discard 9/udp sink null systat 11/tcp users systat 11/udp users daytime 13/tcp daytime 13/udp qotd 17/tcp quote qotd 17/udp quote msp 18/tcp # message send protocol msp 18/udp # message send protocol chargen 19/tcp ttytst source chargen 19/udp ttytst source ftp-data 20/tcp ftp-data 20/udp # 21 is registered to ftp, but also used by fsp ftp 21/tcp ftp 21/udp fsp fspd ssh 22/tcp # The Secure Shell (SSH) Protocol ssh 22/udp # The Secure Shell (SSH) Protocol telnet 23/tcp telnet 23/udp

To list all open ports or currently running ports including TCP and UDP in Linux, we will use netstat, is a powerful tool for monitoring network connections and statistics.

$ netstat -lntu Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN tcp 0 0 0.0.0.0:3306 0.0.0.0:* LISTEN tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN tcp 0 0 . 22 . * LISTEN tcp 0 0 . 80 . * LISTEN tcp 0 0 . 25 . * LISTEN udp 0 0 0.0.0.0:68 0.0.0.0:*

1. -l – prints only listening sockets
2. -n – shows port number
3. -t – enables listing of tcp ports
4. -u – enables listing of udp ports

You can also use ss command, a well known useful utility for examining sockets in a Linux system. Run the command below to list all your open TCP and UCP ports:

$ ss -lntu Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port udp UNCONN 0 0 *:68 *:* tcp LISTEN 0 128 . 22 . * tcp LISTEN 0 128 *:22 *:* tcp LISTEN 0 50 *:3306 *:* tcp LISTEN 0 128 . 80 ::* tcp LISTEN 0 100 . 25 . * tcp LISTEN 0 100 *:25

Make it a point to read through the man pages of the commands above for more usage information.

In summary, understanding the concept of ports in computer networking is very vital for system and network administrators. You can as well go through this netstat guide with simple, precise and well explained examples.

Last but not least, get in touch with us by sharing other methods for listing open ports in Linux or asking a question via the response form below.

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How to Check If a Port Is in Use in Linux

If you are from a computer science background or even a little bit familiar with networking, then you may have heard of the TCP/IP stack. The TCP/IC stack comprises of five different layers, namely, the Physical Layer, Data Link Layer, Network Layer, Transport Layer, and Application Layer. Every layer of the TCP/IP stack has a different means of communication, and all communication within the Transport Layer is done via port numbers.

A port number is used to uniquely identify a device alongside the IP address. Inter-process communication is common when using computer systems. To facilitate this communication, operating systems keep certain ports open, depending upon the entity with which the user wishes to communicate. So, at any single instance, multiple ports can be open in your system.

When we say that a port is in use, we are essentially referring to a port that is open, or, in other words, a port that is in the listening state (ready to accept connections). There are multiple ways of determining the ports that are open in an operating system. This article shows you four possible methods to use to check whether a port is in use in Linux.

Note: All the methods demonstrated in this article have been executed in Linux Mint 20.

To determine whether a port is in use in Linux Mint 20, any of the following four methods can be used.

Method 1: Using the lsof Command

The lsof command can be used to list all the ports in use in your system in the following manner:

First, launch the Linux Mint 20 terminal by clicking on its shortcut icon. The terminal is shown in the image below:

Next, you will have to install the lsof command if you have never used it before. To do so, execute the following command in the terminal:

Upon the successful installation of the command, you will see the following output in the terminal:

Once this command has been installed, it can be used for querying any ports that are in use in Linux. To check your system for open ports, execute the following command in your terminal:

In the output of this command, the ports listed in the “LISTEN” state are the ones that are in use, as shown in the image below:

Method 2: Using the ss Command

The ss command can be used to determine any open TCP and UDP ports in your system in the following manner:

To query both the TCP and UDP ports that are in use, execute the following command in the terminal:

In the output of this command, the ports (both TCP and UDP) that are in use have the “LISTEN” state, whereas all the other ports show the “UNCONN” state.

Method 3: Using the netstat Command

The netstat command can also be used to determine any open TCP and UDP ports in your system in the following manner:

To query for the TCP and UDP ports that are in use, run the following command in the terminal:

If you try to run this command without the “sudo” keyword, you will not be able to access all the ports. If you are logged in with the root user account, then you may skip this keyword.

When you run this command, you will be able to see that all ports in use are in the “LISTEN” state, whereas the states of all other ports are unavailable, as shown in the image below:

Method 4: Using the nmap Command

The nmap command is yet another utility that can be used to determine the TCP and UDP ports that are in use in the following manner:

If the nmap utility is not yet installed on your Linux Mint 20 system, as it does not come installed by default, you may have to manually install it. To do so, execute the following command:

Once you have successfully installed the nmap utility on your Linux Mint 20 system, your terminal will return you the control back so that you can execute the next command, as shown in the image below:

After installing this utility, query for both the TCP and UDP ports that are in use in your system by running the following command in the terminal:

Once you have executed this command, the state of all ports that are in use will be “open,” as shown in the output in the image below:

Conclusion

This article showed you four different methods for checking whether a port is in use in your Linux system. All of these methods were tested with Linux Mint 20, however, you can also run the commands shown in these methods with any other distribution of Linux, with slight variations. Each of the commands used in these methods takes only a few seconds to execute. So, you have the time to try any of the four methods to see which one works best for you.

About the author

Aqsa Yasin

I am a self-motivated information technology professional with a passion for writing. I am a technical writer and love to write for all Linux flavors and Windows.

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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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