Linux name port in

How to check if port is in use on Linux or Unix

H ow do I determine if a port is in use under Linux or Unix-like system? How can I verify which ports are listening on Linux server? How do I check if port is in use on Linux operating system using the CLI?

It is important you verify which ports are listening on the server’s network interfaces. You need to pay attention to open ports to detect an intrusion. Apart from an intrusion, for troubleshooting purposes, it may be necessary to check if a port is already in use by a different application on your servers. For example, you may install Apache and Nginx server on the same system. So it is necessary to know if Apache or Nginx is using TCP port # 80/443. This quick tutorial provides steps to use the netstat, nmap and lsof command to check the ports in use and view the application that is utilizing the port.

Tutorial details
Difficulty level	Easy
Root privileges	Yes
Requirements	lsof, ss, and netstat on Linux
Est. reading time	3 minutes

How to check if port is in use in

To check the listening ports and applications on Linux:

1. Open a terminal application i.e. shell prompt.
2. Run any one of the following command on Linux to see open ports:
   sudo lsof -i -P -n | grep LISTEN
   sudo netstat -tulpn | grep LISTEN
   sudo ss -tulpn | grep LISTEN
   sudo lsof -i:22 ## see a specific port such as 22 ##
   sudo nmap -sTU -O IP-address-Here
3. For the latest version of Linux use the ss command. For example, ss -tulw

Let us see commands and its output in details.

Option #1: lsof command

The syntax is:
$ sudo lsof -i -P -n
$ sudo lsof -i -P -n | grep LISTEN
$ doas lsof -i -P -n | grep LISTEN ### [OpenBSD] ###
Sample outputs:

Fig.01: Check the listening ports and applications with lsof command

Option #2: netstat command

You can check the listening ports and applications with netstat as follows.

Linux netstat syntax

Run netstat command along with grep command to filter out port in LISTEN state:
$ netstat -tulpn | grep LISTEN
The netstat command deprecated for some time on Linux. Therefore, you need to use the ss command as follows:
sudo ss -tulw
sudo ss -tulwn
sudo ss -tulwn | grep LISTEN

Where, ss command options are as follows:

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• -t : Show only TCP sockets on Linux
• -u : Display only UDP sockets on Linux
• -l : Show listening sockets. For example, TCP port 22 is opened by SSHD server.
• -p : List process name that opened sockets
• -n : Don’t resolve service names i.e. don’t use DNS

FreeBSD/MacOS X netstat syntax

$ netstat -anp tcp | grep LISTEN
$ netstat -anp udp | grep LISTEN

OpenBSD netstat syntax

$ netstat -na -f inet | grep LISTEN
$ netstat -nat | grep LISTEN

Option #3: nmap command

The syntax is:
$ sudo nmap -sT -O localhost
$ sudo nmap -sU -O 192.168.2.13 ##[ list open UDP ports ]##
$ sudo nmap -sT -O 192.168.2.13 ##[ list open TCP ports ]##
Sample outputs:

Fig.02: Determines which ports are listening for TCP connections using nmap

A note about Windows users

You can check port usage from Windows operating system using following command:
netstat -bano | more
netstat -bano | grep LISTENING
netstat -bano | findstr /R /C:»[LISTEING]»

Conclusion

This page explained command to determining if a port is in use on Linux or Unix-like server. For more information see the nmap command and lsof command page online here

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How do I check if a port is in use on Linux?

I am a new Linux system user. I need to find out which process is listening on a port on Linux using the command line. How do you find out which process is listening on a port on Linux operating systems?

A network port in Linux is nothing but a number that identifies one side of a connection between two systems. All networked devices use port numbers to determine to which process a message should be delivered. The domain name and IP address are like a street address, and port numbers are like room numbers.

Tutorial details
Difficulty level	Easy
Root privileges	Yes
Requirements	Linux
Est. reading time	2 minutes

Popular port numbers in Linux

• HTTP – TCP 80
• HTTPS – TCP 443
• POP3 – TCP 110
• SMTP – TCP 25
• SSH – TCP 22
• DNS/DOMAIN – TCP/UDP 53

Use the cat command or grep command/egrep command to query port numbers as follows:
cat /etc/services
grep -w 80 /etc/services
egrep -w ’53/(tcp|udp)’ /etc/services

How to check if a port is in use on Linux

The procedure is as follows:

1. Open the terminal application on Linux.
2. Type any one of the following command to check if a port is in use on Linux
   sudo lsof -i -P -n | grep LISTEN
   sudo netstat -tulpn | grep LISTEN
   sudo netstat -tulpn | grep :443
   sudo ss -tulpn | grep LISTEN
   sudo ss -tulpn | grep ‘:22’
3. Search for the TCP or UDP port description in /etc/services file on Linux:
   grep -E -w ‘PORT_NUMBER_HERE/(tcp|udp)’ /etc/services

Let us see some examples and sample commands in details.

How can you find out which process is listening on a port on Linux

Type the ss command or netstat command to see if a TCP port 443 is in use on Linux?
sudo netstat -tulpn | grep :443
sudo ss -tulpn | grep :443
If a port is open, you should see the output as follows:

The port 443 is in use and opened by nginx service. Where,

• -t : Display TCP sockets/port
• -u : Show UDP sockets/port
• -l : See only listening sockets i.e. open port
• -p : Also display process name that opened port/socket
• -n : View addresses and port numbers in numerical format. Do not use DNS to resolve names.

Getting a list of all open port in production

Simply run:
sudo lsof -i -P -n | grep LISTEN
sudo ss -tulpn
sudo netstat -tulpn
Sample outputs:

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How To Check and Use Serial Ports Under Linux

Display Detected System’s Serial Support Under Linux

Simple run the dmesg command:
$ dmesg | grep tty

A note about USB based serial ports

USB to RS232 adapter cable connects from the USB port of a computer to a device. Such serial to USB adapter connects to devices for configuration or programming under Linux. Here is what the dmesg command displays when you enter USB adapter:
$ dmesg
## use grep command/egrep command to filter out USB devices ##
$ dmesg | grep -i serial
$ dmesg | grep -i FTDI

Listing and using USB serial ports on Linux

Here is how it looks when I attached USB device:
$ sudo dmesg | more

Use the setserial command to check and use serial ports

The setserial is a program designed to set and/or report the configuration information associated with a serial port. This information includes what I/O port and IRQ a particular serial port is using, and whether or not the break key should be interpreted as the Secure Attention Key, and so on.

Debian/Ubuntu Linux install setserial using the apt-get command/apt command

$ sudo apt install setserial

CentOS/RHEL/Oracle Linux install setserial using the yum command

$ sudo yum install setserial
Fedora Linux user try the dnf command:
$ sudo dnf install setserial

Using setserial to list serial ports and devices

Now we installed required package. Open the termial and then type the following setserial command:
$ setserial -g /dev/ttyS[0123]
If you get an error/warning that reads as “Permission denied,” try running the command as the root user. For example, I am running it using the sudo command/su command:
$ sudo setserial -g /dev/ttyS[0123]

The setserial with -g option help to find out what physical serial ports your Linux box has.

Listing or displaying USB serial ports on Linux

Try:
$ sudo setserial -g /dev/ttyUSB[01]

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Linux serial console programs

Once serial ports identified you can configure Linux box and use serial ports using various utilities:

1. minicom– The best friendly serial communication program for controlling modems and connecting to dump devices
2. wvidial or other GUI dial up networking program – a PPP dialer with built-in intelligence.
3. Screen Command: Set Baud Rate [ Terminal Communication ]
4. getty / agetty – agetty opens a tty port, prompts for a login name and invokes the /bin/login command.
5. grub / lilo configuration – To configure serial port as the system console

Cocnlusion

You learned how to list or check serial ports including USB based adptor on Linux.

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Comments on this entry are closed.

I don’t know if this is the place, but I’m looking for information that is similar. I have 2 machines setup through a null modem. I want to read through the com port from one machine to the other. The sending machine is running Windows sending through COM1: and the receiving computer is Linux Red Hat. Any help on this would be appreciated.

You can use pyserial ,its a python package,can be used to read and write in serial ports

another simple serial console program not mentioned is gtkterm.

I want to advise you that probably there is an error in the last sentence “seserial with -g option help to find out what physical serial ports your Linux box has.”; isn’t it setserial?

I have 4 standard serial ports defined, /dev/ttyS0-4, but I added a PCI-E 16 port serial adapter, and I have not found a way to connect anything to the upper 16 ports.
How do you create the virtual links? what options do you give setserial to define the new ports?

it dosn’t work 🙁 I can’t connect to my ASA using debian.

Daemon is a program called getty used by UNIX/Linux systems. There are many different versions of getty. The first modem detected is /dev/ttys0 or /dev/ttys1. To create a link to the devices file so that you can view modem information. Type ln -s /dev/ttys1 /dev/modem.
It is essential to set the modem to AUTO ANSWER using getty.

download minicom from alioth.debian.org/projects/minicom.

create a modem config file by entering minicom -s from the root command line
select the options you want to configure using AT commands
connect to the modem by typing in minicom

Thanks very useful..save my time 🙂

Hi,everybody
I have a serial PCI card with two ports but neither of them is working, on Linux fedora 16. I have tried using “gtkterm” and “minicom” but unfortunately I can’t make a serial connection to my Cisco lab. Here is my output when I issue the command: “setserial -g /dev/ttyS[0123]”.

/dev/ttyS0, UART: 16550A, Port: 0xec00, IRQ: 16
/dev/ttyS1, UART: 16550A, Port: 0xe880, IRQ: 16
/dev/ttyS2, UART: unknown, Port: 0x03e8, IRQ: 4
/dev/ttyS3, UART: unknown, Port: 0x02e8, IRQ: 3

also I want to mention that when I try to connet throught the terminal it generates a message like so: ” Cannot open /dev/ttyS0: Permission denied”.
any help please.
abdel

Check the permissions on /dev/ttyS0… Likely problem is it’s owned by root, and is part of the uucp group. Add yourself to the uucp group, and all should be better. I’m curious to know if this is enough for you as I am having problems communicating with a serial port on Linux as well. I got past the permissions stuff, but it is still not working.

Thanks Tom,
I actually I tried but it’s does not work, still says “permission denied”. I would appreciate if you explain to me step by step the procedure how to do it, I’m knew to Linux.
Best Regards
Abdel

1) ls -la /dev/ttyS0 — This will report the permissions on the port. Typically, this will say crw-rw—- 1 root uucp. The “c” in the string indicates that it’s a “character device” the first “rw” indicates that the owner has read/write permission. The second “rw” means that the group has read/write permissions. The “—” indicates that the rest of the world has no permissions. After the “—-“, “root” indicates that the owner of the device is root, and that the group id is “uucp”. So, you either need to be root, or a member of the uucp group to have read/write access to the port.

2) To add yourself to the uucp group (to get read/write access), you need to edit the file /etc/group, and add your user name to the line that begins with “uucp:” User names are separated by commas, so add a comma, then your user name to the end of the line. You will need root permissions to edit this file.

3) log out, and log back in because group associations are made at login time, and are not queried on each request.

Thanks Tom,
here is my output from the group file I have added my username to it but does not work.

I will just give up.Thanks a lot for your help I relay appreciate it.

Your entry in /etc/group is incorrect. If you read my previous post, I said to add your user id to the line that _begins_ with uucp. You added uucp to the group of users that can access your account. You want an entry in the /etc/group file that looks something like:

The critical element is that it’s the line that begins with uucp

It works. I just add my username to ” dialout ” and it worked fine. Iwould like to thank you for you time and support.

Best Regards
Abdel

hi,
I have connected the SIM300(gsm model) to my pc running ubuntu. I need a code to send a AT command to the GSM module and it should read and display the reply.
Please let me know steps.
I tried so many methods and codes, but no use…

i need C code(program).

all my ttyS* shows unknown, earlier it was working fine, now it doesnt recognise

setserial -g /dev/ttyS*
/dev/ttyS0, UART: unknown, Port: 0x03f8, IRQ: 4
/dev/ttyS1, UART: unknown, Port: 0x02f8, IRQ: 3
/dev/ttyS2, UART: unknown, Port: 0x03e8, IRQ: 4
/dev/ttyS3, UART: unknown, Port: 0x02e8, IRQ: 3

#cat /etc/redhat-release
CentOS Linux release 6.0 (Final)

#dmesg | grep tty
console [tty0] enabled

can u pls help me

tried spending time in google two days, still no success, tried connecting a brand new device too, still no success.

Sorry, but that second sentence is not necessarily true. I have a PC with two dual com port adapters. Windows sees them as COM1 -> COM4 (there is no built in COM port). Linux sees them as /dev/ttyS4->/dev/ttyS7.

/dev/ttyS0-/devttyS3 are apparently reserved, exist whether or not a physical port exists, and /dev/ttyS0 for certain is “special” in that it is used by the default terminal window:

I think minicom still superior to other console utility.

Hi All tried all of the above and still get permission denied . HELP
Kevin

/bpq$ setserial -g /dev/ttyS[012345657

/bpq$ setserial -g /dev/ttyS[012345657]

sudo setserial -g /dev/ttyS1

Hiiiiiiiii
Please help me
i am install minicom and use serial modem sim300 v7.03 but there is problem to type AT cmd on minicom console…….

Edgeports are very easy to use the firmware is on Ubuntu they are from 2 ports to 16+ I bought my last two of them on ebay.
Jim

Hi all..
I got problem to connect with my USB serial port,
i already create
ln -s /dev/ttyUS0

and check dmesg | grep tty
[ 0.000000] console [tty0] enabled
[ 7.722004] systemd[1]: Created slice system-getty.slice.
[ 3659.524452] usb 1-1.1: FTDI USB Serial Device converter now attached to ttyUSB0
[ 4850.183339] ftdi_sio ttyUSB0: FTDI USB Serial Device converter now disconnected from ttyUSB0
[ 4857.907567] usb 1-1.1: FTDI USB Serial Device converter now attached to ttyUSB0
when running SDNC program and select com1 will shown “port is already in use”

hi, have are you\
i have a problem with machine xnc . old version sistem operating linux,
pc removed battery cmos . but change battery do it so machine different data serial port linux dont recognized machine.
how can you slove this problem
your help wil be welcome

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