Linux Check Which Ports are in Use: A Guide to Port Checking in Linux

Are you a Linux user who wants to know which ports are currently in use on your system? Whether you’re troubleshooting network connectivity issues or just curious about the services running on your machine, being able to check which ports are in use is a valuable skill. In this comprehensive guide, we will explore various methods to check which ports are in use in Linux.

Introduction

Checking which ports are in use is essential for network administrators, system administrators, and security professionals. By knowing which ports are open and being used, you can identify potential security vulnerabilities, troubleshoot network issues, and ensure that the necessary services are running.

Method 1: Using the netstat command with the -tuln option

The netstat command is a powerful tool for monitoring network connections and listening ports on a Linux system. By using the -tuln option, you can display all TCP and UDP listening ports along with their associated processes.

To check which ports are in use using the netstat command, open a terminal and type the following command:

netstat -tuln

This will display a list of all TCP and UDP ports that are currently in use on your system. The output will include the local address, foreign address, state, and the process ID (PID) of the associated process.

Method 2: Using the ss command with the -tuln option

The ss command is another useful tool for checking which ports are in use in Linux. It provides more detailed information than the netstat command and is generally faster and more efficient.

To use the ss command to check which ports are in use, open a terminal and type the following command:

ss -tuln

This will display a list of all TCP and UDP listening ports along with their associated processes. The output will include the local address, foreign address, state, and the process ID (PID) of the associated process.

Method 3: Using the lsof command with the -i option

The lsof command is a versatile tool for listing open files and processes on a Linux system. By using the -i option, you can filter the output to display only network-related information.

To check which ports are in use using the lsof command, open a terminal and type the following command:

lsof -i

This will display a list of all open network connections and listening ports on your system. The output will include the process ID (PID), user, local address, foreign address, and the associated process name.

Method 4: Using the fuser command with the -n tcp option

The fuser command is a simple yet powerful tool for identifying processes that are using specific files or sockets. By using the -n tcp option, you can filter the output to display only TCP connections.

To check which TCP ports are in use using the fuser command, open a terminal and type the following command:

fuser -n tcp

This will display a list of all TCP ports that are currently in use on your system. The output will include the process ID (PID) of the associated process.

Method 5: Using the nmap command with the -p option

The nmap command is a powerful network scanning tool that can be used to discover open ports on remote systems. By using the -p option, you can specify a range of ports to scan.

To check which ports are in use using the nmap command, open a terminal and type the following command:

nmap -p 1-65535 localhost

This will scan all ports from 1 to 65535 on your local machine and display the open ports. The output will include the port number, state, and the associated service.

Method 6: Using the sockstat command

The sockstat command is a BSD-based tool that can be used to display information about open sockets on a Linux system. It provides a concise and easy-to-read output.

To check which ports are in use using the sockstat command, open a terminal and type the following command:

sockstat

This will display a list of all open sockets on your system, including the local and foreign addresses, state, and the associated process ID (PID).

Method 7: Using the iptables command with the -L option

The iptables command is a powerful firewall management tool that can also be used to check which ports are in use. By using the -L option, you can display the current firewall rules.

To check which ports are in use using the iptables command, open a terminal and type the following command:

iptables -L

This will display the current firewall rules, including any open ports that are allowed through the firewall.

Method 8: Using the cat command to read the /proc/net/tcp file

The /proc/net/tcp file contains information about all TCP connections on a Linux system. By using the cat command, you can read the contents of this file and extract the information you need.

To check which TCP ports are in use using the cat command, open a terminal and type the following command:

cat /proc/net/tcp

This will display the contents of the /proc/net/tcp file, which includes information about all TCP connections on your system. You can use tools like grep or awk to filter the output and display only the information you need.

Method 9: Using the cat command to read the /proc/net/udp file

The /proc/net/udp file contains information about all UDP connections on a Linux system. By using the cat command, you can read the contents of this file and extract the information you need.

To check which UDP ports are in use using the cat command, open a terminal and type the following command:

cat /proc/net/udp

This will display the contents of the /proc/net/udp file, which includes information about all UDP connections on your system. You can use tools like grep or awk to filter the output and display only the information you need.

Method 10: Using the cat command to read the /proc/net/raw file

The /proc/net/raw file contains information about all raw sockets on a Linux system. By using the cat command, you can read the contents of this file and extract the information you need.

To check which raw ports are in use using the cat command, open a terminal and type the following command:

cat /proc/net/raw

This will display the contents of the /proc/net/raw file, which includes information about all raw sockets on your system. You can use tools like grep or awk to filter the output and display only the information you need.

Method 11: Using the cat command to read the /proc/net/tcp6 file

The /proc/net/tcp6 file contains information about all IPv6 TCP connections on a Linux system. By using the cat command, you can read the contents of this file and extract the information you need.

To check which IPv6 TCP ports are in use using the cat command, open a terminal and type the following command:

cat /proc/net/tcp6

This will display the contents of the /proc/net/tcp6 file, which includes information about all IPv6 TCP connections on your system. You can use tools like grep or awk to filter the output and display only the information you need.

Method 12: Using the cat command to read the /proc/net/udp6 file

The /proc/net/udp6 file contains information about all IPv6 UDP connections on a Linux system. By using the cat command, you can read the contents of this file and extract the information you need.

To check which IPv6 UDP ports are in use using the cat command, open a terminal and type the following command:

cat /proc/net/udp6

This will display the contents of the /proc/net/udp6 file, which includes information about all IPv6 UDP connections on your system. You can use tools like grep or awk to filter the output and display only the information you need.

Method 13: Using the cat command to read the /proc/net/raw6 file

The /proc/net/raw6 file contains information about all IPv6 raw sockets on a Linux system. By using the cat command, you can read the contents of this file and extract the information you need.

To check which IPv6 raw ports are in use using the cat command, open a terminal and type the following command:

cat /proc/net/raw6

This will display the contents of the /proc/net/raw6 file, which includes information about all IPv6 raw sockets on your system. You can use tools like grep or awk to filter the output and display only the information you need.

Method 14: Using the cat command to read the /etc/services file

The /etc/services file is a system file that maps well-known port numbers to their corresponding services. By using the cat command, you can read the contents of this file and find out which services are associated with specific port numbers.

To check which services are associated with specific port numbers using the cat command, open a terminal and type the following command:

cat /etc/services

This will display the contents of the /etc/services file, which includes a list of well-known port numbers and their corresponding services. You can use tools like grep or awk to filter the output and display only the information you need.

Method 15: Using the lsof command with the -i :port_number option

The lsof command can also be used to check which ports are in use by specifying a specific port number. By using the -i :port_number option, you can filter the output to display only the processes using the specified port.

To check which processes are using a specific port using the lsof command, open a terminal and type the following command:

lsof -i :port_number

This will display the processes that are using the specified port. The output will include the process ID (PID), user, local address, foreign address, and the associated process name.

Method 16: Using the netstat command with the -anp option

The netstat command can also be used to check which ports are in use by specifying the -anp option. This will display all listening and non-listening ports along with their associated processes.

To check which ports are in use using the netstat command, open a terminal and type the following command:

netstat -anp

This will display a list of all listening and non-listening ports on your system, along with their associated processes. The output will include the local address, foreign address, state, and the process ID (PID) of the associated process.

Method 17: Using the ss command with the -anp option

The ss command can also be used to check which ports are in use by specifying the -anp option. This will display all listening and non-listening ports along with their associated processes.

To check which ports are in use using the ss command, open a terminal and type the following command:

ss -anp

This will display a list of all listening and non-listening ports on your system, along with their associated processes. The output will include the local address, foreign address, state, and the process ID (PID) of the associated process.

Method 18: Using the fuser command with the -n udp option

The fuser command can also be used to check which UDP ports are in use by specifying the -n udp option. This will display all UDP ports that are currently in use on your system.

To check which UDP ports are in use using the fuser command, open a terminal and type the following command:

fuser -n udp

This will display a list of all UDP ports that are currently in use on your system. The output will include the process ID (PID) of the associated process.

Method 19: Using the fuser command with the -n tcp6 option

The fuser command can also be used to check which IPv6 TCP ports are in use by specifying the -n tcp6 option. This will display all IPv6 TCP ports that are currently in use on your system.

To check which IPv6 TCP ports are in use using the fuser command, open a terminal and type the following command:

fuser -n tcp6

This will display a list of all IPv6 TCP ports that are currently in use on your system. The output will include the process ID (PID) of the associated process.

Method 20: Using the fuser command with the -n udp6 option

The fuser command can also be used to check which IPv6 UDP ports are in use by specifying the -n udp6 option. This will display all IPv6 UDP ports that are currently in use on your system.

To check which IPv6 UDP ports are in use using the fuser command, open a terminal and type the following command:

fuser -n udp6

This will display a list of all IPv6 UDP ports that are currently in use on your system. The output will include the process ID (PID) of the associated process.

Conclusion

Checking which ports are in use is an important task for Linux users, whether for troubleshooting network issues or ensuring the security of their systems. In this guide, we have explored various methods to check which ports are in use in Linux, including commands like netstat, ss, lsof, fuser, nmap, sockstat, iptables, and cat. Each method provides a different level of detail and can be used in different scenarios. By familiarizing yourself with these methods, you can effectively monitor and manage the ports on your Linux system.

FAQs

Q: Can I check which ports are in use on remote systems?

A: Yes, you can use tools like nmap to scan remote systems and check which ports are open and in use.

Q: How can I close a specific port that is currently in use?

A: To close a specific port, you need to identify the process that is using the port and terminate that process. You can use tools like lsof or fuser to find the process ID (PID) and then use the kill command to terminate the process.

Q: Are there any graphical tools available for checking which ports are in use?

A: Yes, there are several graphical tools available for checking which ports are in use, such as netstat GUI, ss GUI, and lsof GUI. These tools provide a more user-friendly interface for monitoring and managing ports on a Linux system.