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

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

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I want to install webserver-apache on a Linux platform which uses port no.80 but I am not sure whether is port is open or not , and being used by some other application or not.

  1. Output of grep 80 /etc/services is:

http 80/tcp www www-http #World Wide web Http

http 80/udp www www-http #Hypertext transfer protocol

2.netstat -an | grep 80 | more :

It gives some IP's one of which is IP:80 TIME_WAIT

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


sudo netstat -anp | grep ':80 '

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

How to check if a certain port is open and unused?, It will attempt to connect to your IP address, on the specific port, and let you know if it is open or closed to the outside world. Local issues too? Since there are so many thousands of common port numbers, the easiest approach is to remember the ranges. Which will tell you if a given port is reserved or not. Thanks to Google, you can also look up which services use a specific port in no time at all. Find Open Ports In Windows


This can be achieved using the nc command as follows:

# nc -z IP PORT

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.

$ nc -w 2 -z 216.58.196.174 81

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

How can I find out if a port is opened or not?, netstat -l. Will list ports in use if you have permissions to check. Trying is the only way to see if a port is open (not being blocked by a firewall etc)  Probably the simplest way to see which port is used by which process is to use the trusty command prompt. Click the Start button, type cmd, then right-click “Command Prompt” when it shows up in the search results. Click “Run as administrator.” Once you’re in the elevated command prompt, enter the following command:


Try piping lsof into grep and searching for a port number:

lsof|grep <port>

If nothing shows up that means the port is not in use

You can kill a process on a specific port using

kill -9 <pid>

Where pid is the process id obtained from the first command.

How to check if port is open and unused?, By default, Windows and the network router open some ports used for Internet and a new page displays all of the application-specific ports open on the router​. In network technology, ports divide into two classes. One is a physical port, such as USB port. The second is the logical port, which generally refers to the port in the TCP/IP protocol. These port will cause some virus and harm your PC. As a result, there is necessary to check if the port is open. Determine What Ports are Being Used in Windows 10


       netstat -tln | tail -n +3 | awk '{ print $4 }'  

this one displays bind addresses of tcp listening endpoint. 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 detail:

netstat -tln - all listening tcp ports

tail -n +3 - cut of the header of netstat

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 than that's the specific IP of the network car/network interface.

How to See the Ports That Are Blocked on a Router, To view the list of open ports: Open the command line. See this article for instructions. Enter the command. netstat -a. Press Enter on the keyboard. The list of  One of the checks done in a network scan by Acunetix Vulnerability Scanner is a TCP and UDP port scan. Any open ports detected during the scan will be reported as shown in the screenshot. In this particular scan, these ports have been detected as being open on the server: 80, 1027, 135, 1457, 3389, 139, 8443.


As part of a script you would probably want to use something like this:

resp=`netstat -tunl | grep ":80 "`
if [ -z "$resp" ]; then
    echo "80 Port is free"
else
    echo "80 Port is not free"
fi

How to view the list of open ports in Windows, A port is a number identifying a certain type of network traffic, as I've already explained. Unused software is often overlooked and not updated, which makes While an open port scanner is used for checking the external IP  If you receive “ Press any key to continue ” prompt, this means that the port is open and responding to telnet. If you receive “ Could not open connection ” or a blank screen with blinking cursor, this means the port is closed.


5 Best Open Port Scanners and Checkers 2020, If Show Remote Text is checked, the test will open up the port and read data for approximately 10 Leave Source Port blank or pick another unused port. To see​  Open port checking is only possible if your computer is accessible by external/public IP address. It is worth considering that if your computer is connected to the Internet, through the router, the test results are exactly to the router. Testing the port status for the computer inside a subnet is only possible with port forwarding.


System Monitoring, Usually, if you want to see all the used and listening ports on your computer, you'​d computer had an open connection to a remote IP address on TCP port 80,  Option One: View Port Use Along with Process Names. First, you’ll need to open the Command Prompt in administrator mode. Hit Start, and then type “command” into the search box. When you see “Command Prompt” appear in the results, right-click it and choose “Run as administrator.”


Quickly Find Local Open Ports, To check if a port is open If the window hangs while saying "Connecting " and then goes away, If the window instantly goes away, the port is probably not accessible. If the Note: We can specify the IP address of an internal computer. To scan Nmap ports on a remote system, enter the following in the terminal: sudo nmap 192.168.0.1 Replace the IP address with the IP address of the system you’re testing. This is the basic format for Nmap, and it will return information about the ports on that system.