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How to Find Open Port in Linux?
Before explaining how to list all open network ports in Linux, we will briefly discuss ports in computer networking. In computer networking and software terms, a port is a logical entity representing a network application. A port is a term used to recognize a network service by a number.
The port serves as a communication endpoint for the Linux operating system to identify a specific process or application. A port is a 16-bit (0 to 65535) number that distinguishes one running network application from others. We can classify these port numbers into three categories,
Well-Known Ports (0 to 1023)
Registered ports (1024 to 49151)
Ephemeral ports (49152 to 65535)
Well-known ports
Port numbers 0 to 1023 are designated for standard TCP/IP server-side applications and are known as well-known ports. Knowing well-known ports allows a client application to identify the corresponding server application processes on a peering server host. For example, port 80 is generally bound to the HTTP server application and port 443 to HTTPS.
Registered ports
Registered ports are mapped to a specific service but are not part of a well-known port. Such a port generally represents a particular server application and is commonly accepted by all systems. For example, MySQL Database Server typically uses port 3306
Ephemeral ports
Like server network applications, client network programs also need ports to communicate so that the server can response back to the client application on that specific port. Ephemeral ports are dynamic ports allocated to the client program at runtime, with no pre-reservation requirement.
How to find ports and associated services on your machine:
/etc/services file in Linux keep mapping of services and its corresponding ports. You can view /etc/services to check if a specific port is known to the system. Below command will know ports to your system.
Example
$ cat /etc/services
Output
You will get the output like this.
winrm 47001/tcp # Windows Remote Management Service jvl-mactalk 47100/udp # Configuration of motors conneced to industrial ethernet dbbrowse 47557/tcp # Databeam Corporation dbbrowse 47557/udp # Databeam Corporation directplaysrvr 47624/tcp # Direct Play Server directplaysrvr 47624/udp # Direct Play Server ap 47806/tcp # ALC Protocol ap 47806/udp # ALC Protocol bacnet 47808/tcp # Building Automation and Control Networks bacnet 47808/udp # Building Automation and Control Networks nimcontroller 48000/tcp # Nimbus Controller nimcontroller 48000/udp # Nimbus Controller nimspooler 48001/tcp # Nimbus Spooler nimspooler 48001/udp # Nimbus Spooler nimhub 48002/tcp # Nimbus Hub nimhub 48002/udp # Nimbus Hub nimgtw 48003/tcp # Nimbus Gateway nimgtw 48003/udp # Nimbus Gateway
Find the open ports on the Linux system
There are ways to find the open ports on the system.
By using the netstat tool
By using ss tool
By using the lsof command
Let us check all the ways one by one
By using the netstat tool
netstat is a commonly used command to fetch network-related information from the system. To list all open ports on the system, we can use netstat command. The port can be TCP or UDP ports in different states. For example, below netstat command with lntu switches can list all tcp and udp ports where the state of the port is listening
Example
$ netstat -lntu
Option –l − It will list only listening sockets
Option –n − It will display the port number in numeric
Option t − It will list TCP ports
Option u − It will list UDP ports
Output
You will get the following output
[root@localhost /]# netstat -lntu Active Internet connections (only servers) 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 0 127.0.0.1:25 0.0.0.0:* LISTEN
Decoding output of this command is not that difficult. While the first column shows that it's a TCP port or UDP, Local Address and Foreign Address columns tell us about the server application's IP&Port is and the client application’s IP&Port correspondingly.
0.0.0.0:22 => Server is running on the local machine, and 0.0.0.0 is the reference of all the IPs that is assigned to the local machine, 22 is the port number
In the case of the listening state, Foreign Address is not applicable, as any client (0.0.0.0) with any ephemeral port (*) can connect to the listening port.
By Using SS tool
It is another tool to investigate sockets and is the best alternative to netstat command.
It will also list all open ports on the system.
Example
$ ss – lntu
Output
[root@localhost /]# ss -lntu Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port udp UNCONN 0 0 127.0.0.1:323 *:* udp UNCONN 0 0 [::1]:323 [::]:* tcp LISTEN 0 128 *:22 *:* tcp LISTEN 0 100 127.0.0.1:25 *:* tcp LISTEN 0 128 [::]:22 [::]:* tcp LISTEN 0 100 [::1]:25 [::]:* [root@localhost /]#
By using lsof command
Let us run the following command to check open TCP and UDP ports using the lsof −
lsof -i -P -n | grep LISTEN
-i − Look for listing ports
-P − Inhibits the conversion of port numbers to port names for network files. It may make lsof run a little faster. It is also useful when port name lookup is not working properly.
-n − Do not use DNS name
| grep LISTEN − Again only show ports in LISTEN state using the grep command as filter.
[root@localhost /]# lsof -i -P -n | grep LISTEN sshd 997 root 3u IPv4 16531 0t0 TCP *:22 (LISTEN) sshd 997 root 4u IPv6 16543 0t0 TCP *:22 (LISTEN) master 1242 root 13u IPv4 17202 0t0 TCP 127.0.0.1:25 (LISTEN) master 1242 root 14u IPv6 17203 0t0 TCP [::1]:25 (LISTEN)
Conclusion
On Linux, there are multiple methods for listing open ports. I recommend trying out all of the commands mentioned earlier. Linux experts commonly use the netstat command to find open network ports.
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