How to check opened/closed ports on my computer?
@Justgivemeaname: nmap is a tool to check for open ports on another host. If you can run netstat on a machine, it’s much faster and reliable to use it.
@DavidFoerster: Didn’t know about netstat , so I learned that. It says in the link that it should be used from another host, though. Thanks!
8 Answers 8
There’s a few parameters to netstat that are useful for this :
- -l or —listening shows only the sockets currently listening for incoming connection.
- -a or —all shows all sockets currently in use.
- -t or —tcp shows the tcp sockets.
- -u or —udp shows the udp sockets.
- -n or —numeric shows the hosts and ports as numbers, instead of resolving in dns and looking in /etc/services.
You use a mix of these to get what you want. To know which port numbers are currently in use, use one of these:
netstat -atn # For tcp netstat -aun # For udp netstat -atun # For both In the output all port mentioned are in use either listening for incoming connection or connected to a peer** all others are closed. TCP and UDP ports are 16 bits wide (they go from 1-65535)
** They can also be connecting/disconnecting from the peer.
If it shows some process its used. Its closed(not used) if there is no output.
Another alternative command line easy to use to find out which process is using a port:
lsof -n -i4TCP:$PORT | grep LISTEN I added the next function in my .bash_profile,
and now run «pslisten 5060» to see who is grabing my SIP port.
It’s work with Apple Mac OS X too.
Is the port status «LISTENING» indicated that the port is opened?
Yes. It means that some service is listening to that port on your computer for incoming connection i.e. this port is open for establishing new connections.
Any port that are not shown in the output indicated that it’s closed?
Yes. Remember netstat -a will show all active (listening) and passive (non-listening) connections i.e. the ports that are acting as both server (some services are listening to these ports for connections from a different machine/process) and established (connections are established on these ports regardless of the fact the host/a service can be a server or client)
All TCP and UDP ports belong to a category called sockets and there are a whole lot of those. To view socket info you can check man ss .
Thanks. you wrote that -a means server and established. Does «server» means ports that are being listened at by some services? Does «established» mean ports where there are existing connections regardless of it is a client or server’s port? Then what kinds of ports does -a not show?
I don’t think the -a option means «all active» sockets; it just means «all». netstat shows all active sockets by default, but leaves out the passive sockets (open, listening). By using the -a option both active and passive sockets are shown.
@heemayl The second part of your answer is still not correct. A TCP socket in the «listening» state can never be a connection; it is not connected to anything, it is only listening. Listening TCP sockets are also called passive sockets because of this. If a client attempts to connect to a (listening) socket on a server, a new socket will be created on the server to establish a connection with the client. A socket which is part of an established connection is called an active socket.
Another option is ss. It’s much easier to use.
The below command will only output a list of current listening sockets.
root@server:~# ss -l Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port u_dgr UNCONN 0 0 * 23353 * 23352 u_dgr UNCONN 0 0 * 568 * 362 u_dgr UNCONN 0 0 * 14836 * 14837 u_dgr UNCONN 0 0 * 20446 * 369 u_dgr UNCONN 0 0 * 22877 * 369 u_dgr UNCONN 0 0 * 504 * 347 u_dgr UNCONN 0 0 * 16298 * 369 u_dgr UNCONN 0 0 * 23343 * 369 u_dgr UNCONN 0 0 * 24125 * 369 u_dgr UNCONN 0 0 * 24617 * 369 u_dgr UNCONN 0 0 * 23352 * 23353 u_dgr UNCONN 0 0 * 23334 * 369 u_dgr UNCONN 0 0 * 17113 * 369 u_dgr UNCONN 0 0 * 16957 * 369 u_dgr UNCONN 0 0 * 14793 * 362 u_dgr UNCONN 0 0 * 23345 * 362 u_dgr UNCONN 0 0 * 24070 * 369 udp UNCONN 0 0 *:sunrpc *:* udp UNCONN 0 0 *:981 *:* udp UNCONN 0 0 . sunrpc . * udp UNCONN 0 0 . 981 . * tcp LISTEN 0 128 127.0.0.1:85 *:* tcp LISTEN 0 128 *:ssh *:* tcp LISTEN 0 128 *:3128 *:* tcp LISTEN 0 100 127.0.0.1:smtp *:* tcp LISTEN 0 128 *:8006 *:* tcp LISTEN 0 128 *:sunrpc *:* tcp LISTEN 0 128 . ssh . * tcp LISTEN 0 100 ::1:smtp . * tcp LISTEN 0 128 . sunrpc . * Checking Open and Listening Ports on Linux Using netstat and ss
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One step in securing a Linux computer system is identifying which ports are active. Your system’s active ports give you information about which outside applications may be connected to your system. You can also discover if you are unintentionally exposing an application or service to the internet, like a MySQL database. There are several Linux tools that help you discover which ports are in use and identify both ends of active communications. This guide introduces three common tools you can use with links to guides that dive deeper into each tool.
What is a Port in Computer Networking?
Service names and port numbers are used to distinguish between different services that run over transport protocols. Common transport protocols are TCP, UDP, DCCP, and SCTP. These protocols enable communication between applications by establishing a connection and ensuring data is transmitted successfully. Well-known port assignments, such as HTTP at port 80 over TCP and UDP, are listed at the IANA Service Name and Transport Protocol Port Number Registry. These port assignments help distinguish different types of network traffic across the same connection.
How to Check Which Linux Ports Are in Use?
Three tools to help you check ports in use on a Linux system are:
- netstat: This tool shows your server’s network status.
- ss: You can view socket statistics with the ss tool. For example, ss allows you to monitor TCP, UDP, and UNIX sockets.
- lsof: This Linux utility lists open files. Since everything on a Linux system can be considered a file, lsof provides a lot of information on your entire system.
While all three tools help you learn how to check if a port is open in Linux, each program has its own advantages and disadvantages. See the following examples to identify which tool is the best fit for your purpose.
Using netstat
The netstat tool is great for inspecting the following areas of your Linux system:
- Unix sockets and network connections
- Routing tables
- Network interfaces
- Network protocols
- Multicast group membership
Running netstat without any options displays all open sockets and network connections. While this checks if a port is open in Linux, it can generate a lot of output. You can control the output using netstat’s command-line options. For example, to view the PID and program name for a system’s listening TCP connections, run netstat with the following command-line options:
The output resembles the following:
Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name tcp 0 0 0.0.0.0:http-alt 0.0.0.0:* LISTEN 381070/monitorix-ht tcp 0 0 localhost:domain 0.0.0.0:* LISTEN 553/systemd-resolve tcp 0 0 0.0.0.0:ssh 0.0.0.0:* LISTEN 2145/sshd: /usr/sbi tcp 0 0 localhost:33060 0.0.0.0:* LISTEN 9638/mysqld tcp 0 0 localhost:mysql 0.0.0.0:* LISTEN 9638/mysqld tcp6 0 0 [::]:http [::]:* LISTEN 10997/apache2 tcp6 0 0 [::]:ssh [::]:* LISTEN 2145/sshd: /usr/sbiTo learn how to install netstat, interpret its output, and view common command line options, see our Inspecting Network Information with netstat guide.
Using ss
Another way to have Linux check ports is via the ss tool. ss was created to improve upon netstat and provides more functionality. It allows you to monitor TCP, UDP, and UNIX sockets. A socket enables programs to communicate with each other across a network and is comprised of an IP address and a port number.
Running the ss with no options displays TCP, UDP, and UNIX sockets. Similar to netstat, this unrestricted list can get quite big on busy machines, so it is useful to restrict the ss command’s output by using command-line options. For example, to view all listening and non-listening TCP sockets issue the following command:
The output resembles the following:
State Recv-Q Send-Q Local Address:Port Peer Address:Port Process LISTEN 0 4096 0.0.0.0:http-alt 0.0.0.0:* LISTEN 0 4096 127.0.0.53%lo:domain 0.0.0.0:* LISTEN 0 128 0.0.0.0:ssh 0.0.0.0:* LISTEN 0 70 127.0.0.1:33060 0.0.0.0:* LISTEN 0 151 127.0.0.1:mysql 0.0.0.0:* ESTAB 0 0 192.0.2.0:ssh 192.0.2.1:51617 TIME-WAIT 0 0 192.0.2.0:ssh 192.0.2.2:60630 TIME-WAIT 0 0 192.0.2.0:ssh 192.0.2.3:51312 TIME-WAIT 0 0 127.0.0.1:http-alt 127.0.0.1:52456 TIME-WAIT 0 0 192.0.2.0:ssh 192.0.2.4:44364 ESTAB 0 0 192.0.2.0:ssh 192.0.2.5:51718 LISTEN 0 511 *:http *:* LISTEN 0 128 [::]:ssh [::]:*Using just the -l parameter tells ss to list all Linux’s listening ports, which are omitted by default, making it easier to check for listening ports in Linux.
To take a deeper dive into the ss tool, read our Learning to Use the ss Tool to its Full Potential guide. This guide provides commands specific to each protocol, commands to view general statistics about a system’s current connections, and ways to filter your output.
Using lsof
Since everything on a Linux system can be considered a file, the lsof tool can report on many aspects of a system, including open network interfaces and network connections. By default, it will list open ports in Linux. The lsof tool is preinstalled on many Linux distributions, so you may consider using it before a tool you need to install.
While one of the most frequent uses of lsof is determining which program listens to a given TCP port, one unique feature of the lsof tool is repeat mode*. This mode allows you to run the lsof command continuously on a timed interval. When inspecting your system to find information about which ports are in use, lsof can return information about which user and processes are using a specific port. For example, when working with a local development environment you may want to find which localhost ports are currently in use. Use the following command to retrieve this information:
The output returns a similar response:
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME Adobe\x20 932 jdoe 14u IPv4 0x3dab8c45775e6b5b 0t0 TCP localhost:15292 (LISTEN) Code\x20H 38254 jdoe 81u IPv4 0x3dab8c45922118fb 0t0 TCP localhost:49336 (LISTEN) VBoxHeadl 49798 jdoe 15u IPv4 0x3dab8c45a01fcf1b 0t0 TCP localhost:rockwell-csp2 (LISTEN) Google 55001 jdoe 37u IPv4 0x3dab8c457579acbb 0t0 TCP localhost:51706->localhost:bmc_patroldb (ESTABLISHED) hugo 57981 jdoe 8041u IPv4 0x3dab8c45a423853b 0t0 TCP localhost:bmc_patroldb (LISTEN) hugo 57981 jdoe 8042u IPv4 0x3dab8c45a3a8e2db 0t0 TCP localhost:bmc_patroldb->localhost:51706 (ESTABLISHED)lsof is a powerful diagnostic tool capable of a significant number of ways that you can combine its command line options to troubleshoot various issues. To learn more about the lsof command read our How to List Open Files with lsof guide. This guide provides information about command-line options, the anatomy of the lsof output, and filtering your output with regular expressions.
This page was originally published on Thursday, February 25, 2021.