To check whether a port in Linux is open, 1. Use nc -zv [host] [port] to quickly test connectivity; 2. Use nmap -p [port] to obtain detailed status; 3. Use ss -tuln or netstat -tuln to view the local listening port. These methods are suitable for simple testing, comprehensive scanning and local service inspection, respectively.
Checking whether a port in the Linux system is open is a common operation when troubleshooting network problems, debugging services, or configuring firewalls. The most direct way is to use some command line tools to detect the local or remote port status.

1. Quickly test port connectivity using nc
(Netcat)
nc
is a very light and powerful network debugging tool that can be used to quickly determine whether a port is open.
Usage example:

nc -zv [host] [port]
For example, if you want to check whether the 80 port of this machine is open:
nc -zv localhost 80
If you see a message similar to succeeded!
it means that the port is open; otherwise it may be filtered or closed.

Note: If
nc
is not installed, you can install it through a package manager, such as runningsudo apt install netcat
on Ubuntu.
2. Use nmap
for detailed scanning
If you need more detailed port information, or want to batch scan multiple ports, nmap
is a good choice.
Basic commands:
nmap -p [port] [host]
For example, check the 22 port of the remote host 192.168.1.100:
nmap -p 22 192.168.1.100
The output result will clearly show that the port is open, closed, or filtered.
- If you just want to scan the machine, change the IP to
127.0.0.1
- Multiple ports can be specified at the same time, such as
-p 22,80,443
Tip: Some systems do not install nmap by default, so they can be installed using
sudo apt install nmap
or the corresponding system package management command.
3. Use ss
or netstat
to view the local listening port
If you want to confirm which ports are listening on the machine, instead of connecting to the external host, you can use the following commands:
Use the ss
command:
ss -tuln | grep [port]
For example, check whether there is any listening 3306:
ss -tuln | grep 3306
Or use netstat
(if installed):
netstat -tuln | grep 3306
These two commands will list all TCP/UDP listening ports, and can quickly locate whether a specific port is running with grep
.
Basically these are the methods. Different tools can be selected in different scenarios: use nc
for simple test, use nmap
for comprehensive scan, and use ss
or netstat
for local service status. These commands are combined enough to meet most port detection requirements.
The above is the detailed content of how to check if a port is open in linux. For more information, please follow other related articles on the PHP Chinese website!

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